gpgme/doc/gpgme.texi

\input texinfo                  @c -*- Texinfo -*-
@setfilename gpgme.info
@settitle The `GnuPG Made Easy' Reference Manual

@dircategory GNU Libraries
@direntry
* @acronym{GPGME}: (gpgme).          Adding support for cryptography to your program.
@end direntry

@include version.texi

@c Unify some of the indices.
@syncodeindex tp fn
@syncodeindex pg fn

@ifinfo
This file documents the @acronym{GPGME} library.

This is Edition @value{EDITION}, last updated @value{UPDATED}, of
@cite{The `GnuPG Made Easy' Reference Manual}, for Version
@value{VERSION}.

Copyright @copyright{} 2002 g10 Code GmbH.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software Needs Free Documentation'' and
``GNU Lesser General Public License'', the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.

@end ifinfo

@iftex
@shorttitlepage The `GnuPG Made Easy' Reference Manual
@end iftex
@titlepage
@center @titlefont{The `GnuPG Made Easy'}
@sp 1
@center @titlefont{Reference Manual}
@sp 6
@center Edition @value{EDITION}
@sp 1
@center last updated @value{UPDATED}
@sp 1
@center for version @value{VERSION}
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 2002 g10 Code GmbH.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software Needs Free Documentation'' and
``GNU Lesser General Public License'', the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.
@end titlepage
@page

@ifnottex
@node Top
@top Main Menu
This is Edition @value{EDITION}, last updated @value{UPDATED}, of
@cite{The `GnuPG Made Easy' Reference Manual}, for Version
@value{VERSION} of the @acronym{GPGME} library.
@end ifnottex

@menu
* Introduction::                  How to use this manual.
* Preparation::                   What you should do before using the library.
* Protocols and Engines::         Supported crypto protocols.
* Error Handling::                Error numbers and their meanings.
* Exchanging Data::               Passing data to and from @acronym{GPGME}.
* Contexts::                      Handling @acronym{GPGME} contexts.

Appendices

* Copying::                       The GNU General Public License says how you
                                  can copy and share `GnuPG Made Easy'.
* Free Documentation License::    This manual is under the GNU Free
                                  Documentation License.

Indices

* Concept Index::                 Index of concepts and programs.
* Function and Data Index::       Index of functions, variables and data types.


@detailmenu
 --- The Detailed Node Listing ---

Introduction

* Getting Started::               Purpose of the manual, and how to use it.
* Features::                      Reasons to install and use @acronym{GPGME}.
* Overview::                      Basic architecture of the @acronym{GPGME} library.

Preparation

* Header::                        What header file you need to include.
* Building the Source::           Compiler options to be used.
* Using Automake::                Compiler options to be used the easy way.
* Library Version Check::         Getting and verifying the library version.
* Multi Threading::               How @acronym{GPGME} can be used in an MT environment.

Protocols and Engines

* Engine Version Check::          Verifying the engine version.
* Engine Information::            Obtaining more information about the engines.
* OpenPGP::                       Support for the OpenPGP protocol.
* Cryptographic Message Syntax::  Support for the CMS.

Error Handling

* Error Values::                  A list of all error values used.
* Error Strings::                 How to get a descriptive string from a value.

Exchanging Data 

* Creating Data Buffers::         Creating new data buffers.
* Destroying Data Buffers::       Releasing data buffers.
* Manipulating Data Buffers::     Operations on data buffers.

Creating Data Buffers

* Memory Based Data Buffers::     Creating memory based data buffers.
* File Based Data Buffers::       Creating file based data buffers.
* Callback Based Data Buffers::   Creating callback based data buffers.

Contexts

* Creating Contexts::             Creating new @acronym{GPGME} contexts.
* Destroying Contexts::           Releasing @acronym{GPGME} contexts.
* Context Attributes::            Setting properties of a context.
* Key Management::                Managing keys with @acronym{GPGME}.
* Trust Item Management::         Managing trust items with @acronym{GPGME}.
* Crypto Operations::             Using a context for cryptography.
* Run Control::                   Controlling how operations are run.

Context Attributes

* Protocol Selection::            Selecting the protocol used by a context.
* @acronym{ASCII} Armor::                   Requesting @acronym{ASCII} armored output.
* Text Mode::                     Choosing canonical text mode.
* Included Certificates::         Including a number of certificates.
* Key Listing Mode::              Selecting key listing mode.
* Passphrase Callback::           Getting the passphrase from the user.
* Progress Meter Callback::       Being informed about the progress.

Key Management

* Listing Keys::                  Browsing the list of available keys.
* Information About Keys::        Requesting detailed information about keys.
* Key Signatures::                Listing the signatures on a key.
* Manipulating Keys::             Operations on keys.
* Generating Keys::               Creating new key pairs.
* Exporting Keys::                Retrieving key data from the key ring.
* Importing Keys::                Adding keys to the key ring.
* Deleting Keys::                 Removing keys from the key ring.

Trust Item Management

* Listing Trust Items::           Browsing the list of available trust items.
* Information About Trust Items:: Requesting detailed information about trust items.
* Manipulating Trust Items::      Operations on trust items.

Crypto Operations

* Decrypt::                       Decrypting a ciphertext.
* Verify::                        Verifying a signature.
* Decrypt and Verify::            Decrypting a signed ciphertext.
* Sign::                          Creating a signature.
* Encrypt::                       Encrypting a plaintext.
* Detailed Results::              How to obtain more info about the operation.

Sign

* Selecting Signers::             How to choose the keys to sign with.
* Creating a Signature::          How to create a signature.

Encrypt

* Selecting Recipients::          How to choose the recipients.
* Encrypting a Plaintext::        How to encrypt a plaintext.

Run Control

* Waiting For Completion::        Waiting until an operation is completed.
* Using External Event Loops::    Advanced control over what happens when.

Using External Event Loops

* I/O Callback Interface::        How I/O callbacks are registered.
* Registering I/O Callbacks::     How to use I/O callbacks for a context.
* I/O Callback Example::          An example how to use I/O callbacks.
* I/O Callback Example GTK+::     How to integrate @acronym{GPGME} in GTK+.
* I/O Callback Example GDK::      How to integrate @acronym{GPGME} in GDK.

@end detailmenu
@end menu

@node Introduction
@chapter Introduction

`GnuPG Made Easy' (@acronym{GPGME}) is a C language library that
allows to add support for cryptography to a program.  It is designed
to make access to crypto engines like GnuPG or GpgSM easier for
applications.  @acronym{GPGME} provides a high-level crypto API for
encryption, decryption, signing, signature verification and key
management.

@acronym{GPGME} uses GnuPG and GpgSM as its backends to support
OpenPGP and the Cryptographic Message Syntax (CMS).

@menu
* Getting Started::               Purpose of the manual, and how to use it.
* Features::                      Reasons to install and use @acronym{GPGME}.
* Overview::                      Basic architecture of the @acronym{GPGME} library.
@end menu


@node Getting Started
@section Getting Started

This manual documents the @acronym{GPGME} library programming
interface.  All functions and data types provided by the library are
explained.

The reader is assumed to possess basic knowledge about cryptography in
general, and public key cryptography in particular.  The underlying
cryptographic engines that are used by the library are not explained,
but where necessary, special features or requirements by an engine are
mentioned as far as they are relevant to @acronym{GPGME} or its users.

This manual can be used in several ways.  If read from the beginning
to the end, it gives a good introduction into the library and how it
can be used in an application.  Forward references are included where
necessary.  Later on, the manual can be used as a reference manual to
get just the information needed about any particular interface of the
library.  Experienced programmers might want to start looking at the
examples at the end of the manual, and then only read up those parts
of the interface which are unclear.


@node Features
@section Features

@acronym{GPGME} has a couple of advantages over other libraries doing
a similar job, and over implementing support for GnuPG or other crypto
engines into your application directly.

@table @asis
@item it's free software
Anybody can use, modify, and redistribute it under the terms of the GNU
General Public License (@pxref{Copying}).

@item it's flexible
@acronym{GPGME} provides transparent support for several cryptographic
protocols by different engines.  Currently, @acronym{GPGME} supports
the OpenPGP protocol using GnuPG as the backend, and the Cryptographic
Message Syntax using GpgSM as the backend.

@item it's easy
@acronym{GPGME} hides the differences between the protocols and
engines from the programmer behind an easy-to-use interface.  This way
the programmer can focus on the other parts of the program, and still
integrate strong cryptography in his application.  Once support for
@acronym{GPGME} has been added to a program, it is easy to add support
for other crypto protocols once @acronym{GPGME} backends provide them.
@end table


@node Overview
@section Overview

@acronym{GPGME} provides a data abstraction that is used to pass data
to the crypto engine, and receive returned data from it.  Data can be
read from memory or from files, but it can also be provided by a
callback function.

The actual cryptographic operations are always set within a context.
A context provides configuration parameters that define the behaviour
of all operations performed within it.  Only one operation per context
is allowed at any time, but when one operation is finished, you can
run the next operation in the same context.  There can be more than
one context, and all can run different operations at the same time.

Furthermore, @acronym{GPGME} has rich key management facilities
including listing keys, querying their attributes, generating,
importing, exporting and deleting keys, and acquiring information
about the trust path.

With some precautions, @acronym{GPGME} can be used in a multi-threaded
environment, although it is not completely thread safe and thus needs
the support of the application.


@node Preparation
@chapter Preparation

To use @acronym{GPGME}, you have to perform some changes to your
sources and the build system.  The necessary changes are small and
explained in the following sections.  At the end of this chapter, it
is described how the library is initialized, and how the requirements
of the library are verified.

@menu
* Header::                        What header file you need to include.
* Building the Source::           Compiler options to be used.
* Using Automake::                Compiler options to be used the easy way.
* Library Version Check::         Getting and verifying the library version.
* Multi Threading::               How @acronym{GPGME} can be used in an MT environment.
@end menu


@node Header
@section Header
@cindex header file
@cindex include file

All interfaces (data types and functions) of the library are defined
in the header file `gpgme.h'.  You must include this in all programs
using the library, either directly or through some other header file,
like this:

@example
#include <gpgme.h>
@end example

The name space of @acronym{GPGME} is @code{gpgme_*} for function
names, @code{Gpgme*} for data types and @code{GPGME_*} for other
symbols.  Symbols internal to @acronym{GPGME} take the form
@code{_gpgme_*}.

Because @acronym{GPGME} links to the Assuan library, linking to
@acronym{GPGME} will also use the @code{assuan_*} and @code{_assuan_*}
name space indirectly.


@node Building the Source
@section Building the Source
@cindex compiler options
@cindex compiler flags

If you want to compile a source file including the `gpgme.h' header
file, you must make sure that the compiler can find it in the
directory hierarchy.  This is accomplished by adding the path to the
directory in which the header file is located to the compilers include
file search path (via the @option{-I} option).

However, the path to the include file is determined at the time the
source is configured.  To solve this problem, gpgme ships with a small
helper program @command{gpgme-config} that knows about the path to the
include file and other configuration options.  The options that need
to be added to the compiler invocation at compile time are output by
the @option{--cflags} option to @command{gpgme-config}.  The following
example shows how it can be used at the command line:

@example
gcc -c foo.c `gpgme-config --cflags`
@end example

Adding the output of @samp{gpgme-config --cflags} to the compilers
command line will ensure that the compiler can find the @acronym{GPGME} header
file.

A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files.  For this to work,
the path to the library files has to be added to the library search
path (via the @option{-L} option).  For this, the option
@option{--libs} to @command{gpgme-config} can be used.  For
convenience, this option also outputs all other options that are
required to link the program with @acronym{GPGME} (in particular, the
@samp{-lgpgme} option).  The example shows how to link @file{foo.o}
with the @acronym{GPGME} library to a program @command{foo}.

@example
gcc -o foo foo.o `gpgme-config --libs`
@end example

Of course you can also combine both examples to a single command by
specifying both options to @command{gpgme-config}:

@example
gcc -o foo foo.c `gpgme-config --cflags --libs`
@end example


@node Using Automake
@section Using Automake
@cindex automake
@cindex autoconf

It is much easier if you use GNU Automake instead writing your own
Makefiles.  If you do that you don't have to worry about finding and
invoking the @command{gpgme-config} script at all.  @acronym{GPGME}
provides an extension to Automake that does all the work for you.

@c A simple macro for optional variables.
@macro ovar{varname}
@r{[}@var{\varname\}@r{]}
@end macro
@defmac AM_PATH_GPGME (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
Check whether @acronym{GPGME} (at least version @var{minimum-version},
if given) exists on the host system.  If it is found, execute
@var{action-if-found}, otherwise do @var{action-if-not-found}, if
given.

Additionally, the function defines @code{GPGME_CFLAGS} to the flags
needed for compilation of the program to find the @file{gpgme.h}
header file, and @code{GPGME_LIBS} to the linker flags needed to link
the program to the @acronym{GPGME} library.
@end defmac

You can use the defined Autoconf variables like this in your
@file{Makefile.am}:

@example
AM_CPPFLAGS = $(GPGME_CFLAGS)
LDADD = $(GPGME_LIBS)
@end example


@node Library Version Check
@section Library Version Check
@cindex version check, of the library

@deftypefun {const char *} gpgme_check_version (@w{const char *@var{required_version}})
The function @code{gpgme_check_version} has three purposes.  It can be
used to retrieve the version number of the library.  In addition it
can verify that the version number is higher than a certain required
version number.  In either case, the function initializes some
sub-systems, and for this reason alone it must be invoked early in
your program, before you make use of the other functions in
@acronym{GPGME}.

If @var{required_version} is @code{NULL}, the function returns a
pointer to a statically allocated string containing the version number
of the library.

If @var{required_version} is not @code{NULL}, it should point to a
string containing a version number, and the function checks that the
version of the library is at least as high as the version number
provided.  In this case, the function returns a pointer to a
statically allocated string containing the version number of the
library.  If @var{REQUIRED_VERSION} is not a valid version number, or
if the version requirement is not met, the function returns
@code{NULL}.

If you use a version of a library that is backwards compatible with
older releases, but contains additional interfaces which your program
uses, this function provides a run-time check if the necessary
features are provided by the installed version of the library.
@end deftypefun


@node Multi Threading
@section Multi Threading
@cindex thread-safeness
@cindex multi-threading

The @acronym{GPGME} library is not entirely thread-safe, but it can
still be used in a multi-threaded environment if some care is taken.
If the following requirements are met, there should be no race
conditions to worry about:

@itemize @bullet
@item
@acronym{GPGME} supports the thread libraries pthread and GNU Pth.
The support for this has to be enabled at compile time.
@acronym{GPGME} will automatically detect the location in which the
thread libraries are installed and activate the support for them.

Support for other thread libraries is very easy to add.  Please
contact us if you have the need.

@item
If you link your program dynamically to @acronym{GPGME} and your
supported thread library, @acronym{GPGME} will automatically detect
the presence of this library and activate its use.  If you link to
both pthread and GNU Pth, @acronym{GPGME} will use the pthread
support.  This feature requires weak symbol support.

@item
If you link your program statically to @acronym{GPGME}, there is
currently no easy way to make sure that @acronym{GPGME} detects the
presence of the thread library.  This will be solved in a future
version.

@item
The function @code{gpgme_check_version} must be called before any
other function in the library, because it initializes the thread
support subsystem in @acronym{GPGME}.  To achieve this in all
generality, it is necessary to synchronize the call to this function
with all other calls to functions in the library, using the
synchronization mechanisms available in your thread library.
Otherwise, specific compiler or CPU memory cache optimizations could
lead to the situation where a thread is started and uses
@acronym{GPGME} before the effects of the initialization are visible
for this thread.  It doesn't even suffice to call
@code{gpgme_check_version} before creating this other
thread@footnote{In SMP systems the new thread could be started on
another CPU before the effects of the initialization are seen by that
CPU's memory cache.  Not doing proper synchronization here leads to
the same problems the double-checked locking idiom has.  You might
find that if you don't do proper synchronization, it still works in
most configurations.  Don't let this fool you.  Someday it might lead
to subtle bugs when someone tries it on a DEC Alpha or an SMP
machine.}.

For example, if you are using POSIX threads, each thread that wants to
call functions in @acronym{GPGME} could call the following function
before any function in the library:

@example
#include <pthread.h>

void
initialize_gpgme (void)
@{
  static int gpgme_init;
  static pthread_mutext_t gpgme_init_lock = PTHREAD_MUTEX_INITIALIZER;

  pthread_mutex_lock (&gpgme_init_lock);
  if (!gpgme_init)
    @{
      gpgme_check_version ();
      gpgme_init = 1;
    @}
  pthread_mutex_unlock (&gpgme_init_lock);
@}
@end example

@item
Any @code{GpgmeData}, @code{GpgmeCtx} and @code{GpgmeRecipients}
object must only be accessed by one thread at a time.  If multiple
threads want to deal with the same object, the caller has to make sure
that operations on that object are fully synchronized.

@item
Only one thread at any time is allowed to call @code{gpgme_wait}.  If
multiple threads call this function, the caller must make sure that
all invocations are fully synchronized.  It is safe to start
asynchronous operations while a thread is running in gpgme_wait.
@end itemize


@node Protocols and Engines
@chapter Protocols and Engines
@cindex protocol
@cindex engine
@cindex crypto engine
@cindex backend
@cindex crypto backend

@acronym{GPGME} supports several cryptographic protocols, however, it
does not implement them.  Rather it uses backends (also called
engines) which implement the protocol.  @acronym{GPGME} uses
inter-process communication to pass data back and forth between the
application and the backend, but the details of the communication
protocol and invocation of the backends is completely hidden by the
interface.  All complexity is handled by @acronym{GPGME}.  Where an
exchange of information between the application and the backend is
necessary, @acronym{GPGME} provides the necessary callback function
hooks and further interfaces.

@deftp {Data type} {enum GpgmeProtocol}
@tindex GpgmeProtocol
The @code{GpgmeProtocol} type specifies the set of possible protocol
values that are supported by @acronym{GPGME}.  The following protocols
are supported:

@table @code
@item GPGME_PROTOCOL_OpenPGP
This specifies the OpenPGP protocol.
@item GPGME_PROTOCOL_CMS
This specifies the Cryptographic Message Syntax.
@end table
@end deftp


@deftypefun const char *gpgme_get_protocol_name (@w{GpgmeProtocol @var{protocol}})
The function @code{gpgme_get_protocol_name} returns a statically
allocated string describing the protocol @var{protocol}, or
@code{NULL} if the protocol number is not valid.
@end deftypefun

@menu
* Engine Version Check::          Verifying the engine version.
* Engine Information::            Obtaining more information about the engines.
* OpenPGP::                       Support for the OpenPGP protocol.
* Cryptographic Message Syntax::  Support for the CMS.
@end menu


@node Engine Version Check
@section Engine Version Check
@cindex version check, of the engines

@deftypefun GpgmeError gpgme_engine_check_version (@w{GpgmeProtocol @var{protocol}})
The function @code{gpgme_engine_check_version} verifies that the
engine implementing the protocol @var{PROTOCOL} is installed in the
expected path and meets the version requirement of @acronym{GPGME}.

This function returns @code{GPGME_No_Error} if the engine is available
and @code{GPGME_Invalid_Engine} if it is not.
@end deftypefun


@node Engine Information
@section Engine Information
@cindex engine, information about

@deftp {Data type} {GpgmeEngineInfo}
@tindex GpgmeProtocol
The @code{GpgmeEngineInfo} type specifies a pointer to a structure
describing a crypto backend engine.  The structure contains the
following elements:

@table @code
@item GpgmeEngineInfo next
This is a pointer to the next engine info structure in the linked
list, or @code{NULL} if this is the last element.

@item GpgmeProtocol protocol
This is the protocol for which the crypo engine is used.  You can
convert this to a string with @code{gpgme_get_protocol_name} for
printing.

@item const char *file_name
This is a string holding the file name of the executable of the crypto
engine.  Currently, it is never @code{NULL}, but using @code{NULL} is
reserved for future use, so always check before you use it.

@item const char *version
This is a string containing the version number of the crypto engine.
It might be @code{NULL} if the version number can not be determined,
for example because the executable doesn't exist or is invalid.

@item const char *req_version
This is a string containing the minimum required version number of the
crypto engine for @acronym{GPGME} to work correctly.  This is the
version number that @code{gpgme_engine_check_version} verifies
against.  Currently, it is never @code{NULL}, but using @code{NULL} is
reserved for future use, so always check before you use it.
@end table
@end deftp

@deftypefun GpgmeError gpgme_get_engine_info (GpgmeEngineInfo *info)
The function @code{gpgme_get_engine_info} returns a linked list of
engine info structures in @var{info}.  Each info structure describes
one configured crypto backend engine.

The memory for the info structures is allocated the first time this
function is invoked, and must not be freed by the caller.

This function returns @code{GPGME_No_Error} if successful, and
@code{GPGME_Out_Of_Core} if not enough memory is available for the
operation.
@end deftypefun

Here is the example how you can provide more diagnostics if you
receive an error message which indicates that the crypto engine is
invalid.

@example
GpgmeCtx ctx;
GpgmeError err;

[...]

if (err == GPGME_Invalid_Engine)
  @{
    GpgmeEngineInfo info;
    err = gpgme_get_engine_info (&info);
    if (!err)
      @{
        while (info && info->protocol != gpgme_get_protocol (ctx))
          info = info->next;
        if (!info)
          fprintf (stderr, "GPGME compiled without support for protocol %s",
                   gpgme_get_protocol_name (info->protocol));
        else if (info->path && !info->version)
          fprintf (stderr, "Engine %s not installed properly",
                   info->path);
        else if (info->path && info->version && info->req_version)
          fprintf (stderr, "Engine %s version %s installed, "
                   "but at least version %s required", info->path,
                   info->version, info->req_version);
        else
          fprintf (stderr, "Unknown problem with engine for protocol %s",
                   gpgme_get_protocol_name (info->protocol));
      @}
  @}
@end example


@node OpenPGP
@section OpenPGP
@cindex OpenPGP
@cindex GnuPG
@cindex protocol, GnuPG
@cindex engine, GnuPG

OpenPGP is implemented by GnuPG, the @acronym{GNU} Privacy Guard.
This is the first protocol that was supported by @acronym{GPGME}.

The OpenPGP protocol is specified by @code{GPGME_PROTOCOL_OpenPGP}.


@node Cryptographic Message Syntax
@section Cryptographic Message Syntax
@cindex CMS
@cindex cryptographic message syntax
@cindex GpgSM
@cindex protocol, CMS
@cindex engine, GpgSM
@cindex S/MIME
@cindex protocol, S/MIME

@acronym{CMS} is implemented by GpgSM, the S/MIME implementation for
GnuPG.

The @acronym{CMS} protocol is specified by @code{GPGME_PROTOCOL_CMS}.


@node Error Handling
@chapter Error Handling
@cindex error handling

Many functions in @acronym{GPGME} can return an error if they fail.
For this reason, the application should always catch the error
condition and take appropriate measures, for example by releasing the
resources and passing the error up to the caller, or by displaying a
descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the
operation, but the result of an operation that failed properly.  For
example, if you try to decrypt a tempered message, the decryption will
fail.  Another error value actually means that the end of a data
buffer or list has been reached.  The following descriptions explain
what each error message means in general.  Some error values have
specific meanings if returned by a specific function.  Such cases are
described in the documentation of those functions.

@menu
* Error Values::                  A list of all error values used.
* Error Strings::                 How to get a descriptive string from a value.
@end menu


@node Error Values
@section Error Values
@cindex error values, list of

@deftp {Data type} {enum GpgmeError}
@tindex GpgmeError
The @code{GpgmeError} type specifies the set of all error values that
are used by @acronym{GPGME}.  Possible values are:

@table @code
@item GPGME_EOF
This value indicates the end of a list, buffer or file.

@item GPGME_No_Error
This value indicates success.  The value of this error is @code{0}.

@item GPGME_General_Error
This value means that something went wrong, but either there is not
enough information about the problem to return a more useful error
value, or there is no separate error value for this type of problem.

@item GPGME_Out_Of_Core
This value means that an out-of-memory condition occurred.

@item GPGME_Invalid_Value
This value means that some user provided data was out of range.  This
can also refer to objects.  For example, if an empty @code{GpgmeData}
object was expected, but one containing data was provided, this error
value is returned.

@item GPGME_Busy
This value is returned if you try to start a new operation in a
context that is already busy with some earlier operation which was not
cancelled or finished yet.

@item GPGME_No_Request
This value is in some sense the opposite of @code{GPGME_Busy}.  There
is no pending operation, but it is required for the function to
succeed.

@item GPGME_Exec_Error
This value means that an error occurred when trying to spawn a child
process.

@item GPGME_Too_Many_Procs
This value means that there are too many active backend processes.

@item GPGME_Pipe_Error
This value means that the creation of a pipe failed.

@item GPGME_No_Recipients 
This value means that no valid recipients for a message have been set.

@item GPGME_Invalid_Recipients 
This value means that some, but not all, recipients for a message have
been invalid.

@item GPGME_No_Data
This value means that a @code{GpgmeData} object which was expected to
have content was found empty.

@item GPGME_Conflict
This value means that a conflict of some sort occurred.

@item GPGME_Not_Implemented
This value indicates that the specific function (or operation) is not
implemented.  This error should never happen.  It can only occur if
you use certain values or configuration options which do not work,
but for which we think that they should work at some later time.

@item GPGME_Read_Error
This value means that an I/O read operation failed.

@item GPGME_Write_Error
This value means that an I/O write operation failed.

@item GPGME_Invalid_Type
This value means that a user provided object was of a wrong or
incompatible type.  Usually this refers to the type of a
@code{GpgmeData} object.

@item GPGME_Invalid_Mode
This value means that a @code{GpgmeData} object has an incorrect mode
of operation (for example, doesn't support output although it is
attempted to use it as an output buffer).

@item GPGME_File_Error
This value means that a file I/O operation failed.  The value of
@var{errno} contains the system error value.

@item GPGME_Decryption_Failed
This value indicates that a decryption operation was unsuccessful.

@item GPGME_No_Passphrase
This value means that the user did not provide a passphrase when
requested.

@item GPGME_Canceled
This value means that the operation was canceled.

@item GPGME_Invalid_Key
This value means that a key was invalid.

@item GPGME_Invalid_Engine
This value means that the engine that implements the desired protocol
is currently not available.  This can either be because the sources
were configured to exclude support for this engine, or because the
engine is not installed properly.
@end table
@end deftp


@node Error Strings
@section Error Strings
@cindex error values, printing of
@cindex error strings

@deftypefun {const char *} gpgme_strerror (@w{GpgmeError @var{err}})
The function @code{gpgme_strerror} returns a pointer to a statically
allocated string containing a description of the error with the error
value @var{err}.  This string can be used to output a diagnostic
message to the user.

The following example illustrates the use of @code{gpgme_strerror}:

@example
GpgmeCtx ctx;
GpgmeError err = gpgme_new (&ctx);
if (err)
  @{
    fprintf (stderr, "%s: creating GpgME context failed: %s\n",
             argv[0], gpgme_strerror (err));
    exit (1);
  @}
@end example
@end deftypefun


@node Exchanging Data
@chapter Exchanging Data
@cindex data, exchanging

A lot of data has to be exchanged between the user and the crypto
engine, like plaintext messages, ciphertext, signatures and
information about the keys.  The technical details about exchanging
the data information are completely abstracted by @acronym{GPGME}.
The user provides and receives the data via @code{GpgmeData} objects,
regardless of the communication protocol between @acronym{GPGME} and
the crypto engine in use.

@deftp {Data type} {GpgmeData}
The @code{GpgmeData} type is a handle for a container for generic
data, which is used by @acronym{GPGME} to exchange data with the user.
@end deftp

@menu
* Creating Data Buffers::         Creating new data buffers.
* Destroying Data Buffers::       Releasing data buffers.
* Manipulating Data Buffers::     Operations on data buffers.
@end menu


@node Creating Data Buffers
@section Creating Data Buffers
@cindex data buffer, creation

Data objects can be based on memory, files, or callback functions
provided by the user.  Not all operations are supported by all
objects.


@menu
* Memory Based Data Buffers::     Creating memory based data buffers.
* File Based Data Buffers::       Creating file based data buffers.
* Callback Based Data Buffers::   Creating callback based data buffers.
@end menu


@node Memory Based Data Buffers
@subsection Memory Based Data Buffers

Memory based data objects store all data in allocated memory.  This is
convenient, but only practical for an amount of data that is a
fraction of the available physical memory.  The data has to be copied
from its source and to its destination, which can often be avoided by
using one of the other data object 

@deftypefun GpgmeError gpgme_data_new (@w{GpgmeData *@var{dh}})
The function @code{gpgme_data_new} creates a new @code{GpgmeData}
object and returns a handle for it in @var{dh}.  The data object is
memory based and initially empty.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} is not a
valid pointer, and @code{GPGME_Out_Of_Core} if not enough memory is
available.
@end deftypefun

@deftypefun GpgmeError gpgme_data_new_from_mem (@w{GpgmeData *@var{dh}}, @w{const char *@var{buffer}}, @w{size_t @var{size}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_mem} creates a new
@code{GpgmeData} object and fills it with @var{size} bytes starting
from @var{buffer}.

If @var{copy} is not zero, a private copy of the data is made.  If
@var{copy} is zero, the data is taken from the specified buffer as
needed, and the user has to ensure that the buffer remains valid for
the whole life span of the data object.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} or
@var{buffer} is not a valid pointer, and @code{GPGME_Out_Of_Core} if
not enough memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_data_new_from_file (@w{GpgmeData *@var{dh}}, @w{const char *@var{filename}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_file} creates a new
@code{GpgmeData} object and fills it with the content of the file
@var{filename}.

If @var{copy} is not zero, the whole file is read in at initialization
time and the file is not used anymore after that.  This is the only
mode supported currently.  Later, a value of zero for @var{copy} might
cause all reads to be delayed until the data is needed, but this is
not yet implemented.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} or
@var{filename} is not a valid pointer, @code{GPGME_File_Error} if an
I/O operation fails, @code{GPGME_Not_Implemented} if @var{code} is
zero, and @code{GPGME_Out_Of_Core} if not enough memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_data_new_from_filepart (@w{GpgmeData *@var{dh}}, @w{const char *@var{filename}}, @w{FILE *@var{fp}}, @w{off_t @var{offset}}, @w{size_t @var{length}})
The function @code{gpgme_data_new_from_filepart} creates a new
@code{GpgmeData} object and fills it with a part of the file specified
by @var{filename} or @var{fp}.

Exactly one of @var{filename} and @var{fp} must be non-zero, the other
must be zero.  The argument that is not zero specifies the file from
which @var{length} bytes are read into the data object, starting from
@var{offset}.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} and
exactly one of @var{filename} and @var{fp} is not a valid pointer,
@code{GPGME_File_Error} if an I/O operation fails, and
@code{GPGME_Out_Of_Core} if not enough memory is available.
@end deftypefun


@node File Based Data Buffers
@subsection File Based Data Buffers

File based data objects operate directly on file descriptors or
streams.  Only a small amount of data is stored in core at any time,
so the size of the data objects is not limited by @acronym{GPGME}.

@deftypefun GpgmeError gpgme_data_new_from_fd (@w{GpgmeData *@var{dh}}, @w{int @var{fd}})
The function @code{gpgme_data_new_from_fd} creates a new
@code{GpgmeData} object and uses the file descriptor @var{fd} to read
from (if used as an input data object) and write to (if used as an
output data object).

When using the data object as an input buffer, the function might read
a bit more from the file descriptor than is actually needed by the
crypto engine in the desired operation because of internal buffering.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_data_new_from_stream (@w{GpgmeData *@var{dh}}, @w{FILE *@var{stream}})
The function @code{gpgme_data_new_from_stream} creates a new
@code{GpgmeData} object and uses the I/O stream @var{stream} to read
from (if used as an input data object) and write to (if used as an
output data object).

When using the data object as an input buffer, the function might read
a bit more from the stream than is actually needed by the crypto
engine in the desired operation because of internal buffering.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun


@node Callback Based Data Buffers
@subsection Callback Based Data Buffers

If neither memory nor file based data objects are a good fit for your
application, you can implement the functions a data object provides
yourself and create a data object from these callback functions.

@deftp {Data type} {ssize_t (*GpgmeDataReadCb) (@w{void *@var{handle}}, @w{void @var{*buffer}}, @w{size_t @var{size}})}
@tindex GpgmeDataReadCb
The @code{GpgmeDataReadCb} type is the type of functions which
@acronym{GPGME} calls if it wants to read data from a user-implemented
data object.  The function should read up to @var{size} bytes from the
current read position into the space starting at @var{buffer}.  The
@var{handle} is provided by the user at data object creation time.

The function should return the number of bytes read, 0 on EOF, and -1
on error.  If an error occurs, @var{errno} should be set to describe
the type of the error.
@end deftp

@deftp {Data type} {ssize_t (*GpgmeDataWriteCb) (@w{void *@var{handle}}, @w{const void @var{*buffer}}, @w{size_t @var{size}})}
@tindex GpgmeDataWriteCb
The @code{GpgmeDataWriteCb} type is the type of functions which
@acronym{GPGME} calls if it wants to write data to a user-implemented
data object.  The function should write up to @var{size} bytes to the
current write position from the space starting at @var{buffer}.  The
@var{handle} is provided by the user at data object creation time.

The function should return the number of bytes written, and -1 on
error.  If an error occurs, @var{errno} should be set to describe the
type of the error.
@end deftp

@deftp {Data type} {off_t (*GpgmeDataSeekCb) (@w{void *@var{handle}}, @w{off_t @var{offset}}, @w{int @var{whence}})}
@tindex GpgmeDataSeekCb
The @code{GpgmeDataSeekCb} type is the type of functions which
@acronym{GPGME} calls if it wants to change the current read/write
position in a user-implemented data object, just like the @code{lseek}
function.

The function should return the new read/write position, and -1 on
error.  If an error occurs, @var{errno} should be set to describe the
type of the error.
@end deftp

@deftp {Data type} {void (*GpgmeDataReleaseCb) (@w{void *@var{handle}})}
@tindex GpgmeDataReleaseCb
The @code{GpgmeDataReleaseCb} type is the type of functions which
@acronym{GPGME} calls if it wants to destroy a user-implemented data
object.  The @var{handle} is provided by the user at data object
creation time.
@end deftp

@deftp {Data type} {struct GpgmeDataCbs}
This structure is used to store the data callback interface functions
described above.  It has the following members:

@table @code
@item GpgmeDataReadCb read
This is the function called by @acronym{GPGME} to read data from the
data object.  It is only required for input data object.

@item GpgmeDataWriteCb write
This is the function called by @acronym{GPGME} to write data to the
data object.  It is only required for output data object.

@item GpgmeDataSeekCb seek
This is the function called by @acronym{GPGME} to change the current
read/write pointer in the data object (if available).  It is optional.

@item GpgmeDataReleaseCb release
This is the function called by @acronym{GPGME} to release a data
object.  It is optional.
@end table
@end deftp

@deftypefun GpgmeError gpgme_data_new_from_cbs (@w{GpgmeData *@var{dh}}, @w{struct GpgmeDataCbs *@var{cbs}}, @w{void *@var{handle}})
The function @code{gpgme_data_new_from_cbs} creates a new
@code{GpgmeData} object and uses the user-provided callback functions
to operate on the data object.

The handle @var{handle} is passed as first argument to the callback
functions.  This can be used to identify this data object.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun

The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun GpgmeError gpgme_data_new_with_read_cb (@w{GpgmeData *@var{dh}}, @w{int (*@var{readfunc})} (@w{void *@var{hook}}, @w{char *@var{buffer}}, @w{size_t @var{count}}, @w{size_t *@var{nread}}), @w{void *@var{hook_value}})
The function @code{gpgme_data_new_with_read_cb} creates a new
@code{GpgmeData} object and uses the callback function @var{readfunc}
to retrieve the data on demand.  As the callback function can supply
the data in any way it wants, this is the most flexible data type
@acronym{GPGME} provides.  However, it can not be used to write data.

The callback function receives @var{hook_value} as its first argument
whenever it is invoked.  It should return up to @var{count} bytes in
@var{buffer}, and return the number of bytes actually read in
@var{nread}.  It may return @code{0} in @var{nread} if no data is
currently available.  To indicate @code{EOF} the function should
return with an error code of @code{-1} and set @var{nread} to
@code{0}.  The callback function may support to reset its internal
read pointer if it is invoked with @var{buffer} and @var{nread} being
@code{NULL} and @var{count} being @code{0}.

The function returns @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} or
@var{readfunc} is not a valid pointer, and @code{GPGME_Out_Of_Core} if
not enough memory is available.
@end deftypefun


@node Destroying Data Buffers
@section Destroying Data Buffers
@cindex data buffer, destruction

@deftypefun void gpgme_data_release (@w{GpgmeData @var{dh}})
The function @code{gpgme_data_release} destroys the data object with
the handle @var{dh}.  It releases all associated resources that were
not provided by the user in the first place.
@end deftypefun

@deftypefun {char *} gpgme_data_release_and_get_mem (@w{GpgmeData @var{dh}}, @w{size_t *@var{length}})
The function @code{gpgme_data_release_and_get_mem} is like
@code{gpgme_data_release}, except that it returns the data buffer and
its length that was provided by the object.

The user has to release the buffer with @code{free}.  In case the user
provided the data buffer in non-copy mode, a copy will be made for
this purpose.

In case an error returns, or there is no suitable data buffer that can
be returned to the user, the function will return @code{NULL}.
@end deftypefun


@node Manipulating Data Buffers
@section Manipulating Data Buffers
@cindex data buffere, manipulation

@deftypefun ssize_t gpgme_data_read (@w{GpgmeData @var{dh}}, @w{void *@var{buffer}}, @w{size_t @var{length}})
The function @code{gpgme_data_read} reads up to @var{length} bytes
from the data object with the handle @var{dh} into the space starting
at @var{buffer}.

If no error occurs, the actual amount read is returned.  If the end of
the data object is reached, the function returns @code{GPGME_EOF} and
sets @var{nread} to zero.

In all other cases, the function returns -1 and sets @var{errno}.
@end deftypefun

@deftypefun ssize_t gpgme_data_write (@w{GpgmeData @var{dh}}, @w{const void *@var{buffer}}, @w{size_t @var{size}})
The function @code{gpgme_data_write} writes up to @var{size} bytes
starting from @var{buffer} into the data object with the handle
@var{dh} at the current write position.

The function returns the number of bytes actually written, or -1 if an
error occurs.  If an error occurs, @var{errno} is set.
@end deftypefun

/* Set the current position from where the next read or write starts
   in the data object with the handle DH to OFFSET, relativ to
   WHENCE.  */
off_t gpgme_data_seek (GpgmeData dh, off_t offset, int whence);

@deftypefun off_t gpgme_data_seek (@w{GpgmeData @var{dh}}, @w{off_t *@var{offset}}, @w{int @var{whence}})
The function @code{gpgme_data_seek} changes the current read/write
position.

The @var{whence} argument specifies how the @var{offset} should be
interpreted.  It must be one of the following symbolic constants:

@table @code
@item SEEK_SET
Specifies that @var{whence} is a count of characters from the
beginning of the data object.

@item SEEK_CUR
Specifies that @var{whence} is a count of characters from the current
file position.  This count may be positive or negative.

@item SEEK_END
Specifies that @var{whence} is a count of characters from the end of
the data object.  A negative count specifies a position within the
current extent of the data object; a positive count specifies a
position past the current end.  If you set the position past the
current end, and actually write data, you will extend the data object
with zeros up to that position.
@end table

If successful, the function returns the resulting file position,
measured in bytes from the beginning of the data object.  You can use
this feature together with @code{SEEK_CUR} to read the current
read/write position.

If the function fails, -1 is returned and @var{errno} is set.
@end deftypefun

The following function is deprecated and should not be used.  It will
be removed in a future version of @acronym{GPGME}.

@deftypefun GpgmeError gpgme_data_rewind (@w{GpgmeData @var{dh}})
The function @code{gpgme_data_rewind} is equivalent to:

@example
  return (gpgme_data_seek (dh, 0, SEEK_SET) == -1)
    ? mk_error (File_Error) : 0;
@end example
@end deftypefun

@c
@c  GpgmeDataEncoding
@c
@deftp {Data type} {enum GpgmeDataEncoding}
@tindex GpgmeDataEncoding
The @code{GpgmeDataEncoding} type specifies the encoding of a
@code{GpgmeData} object.  This encoding is useful to give the backend
a hint on the type of data.  The following data types are available:

@table @code
@item GPGME_DATA_ENCODING_NONE
This specifies that the encoding is not known.  This is the default
for a new data object.  The backend will try its best to detect the
encoding automatically.

@item GPGME_DATA_ENCODING_BINARY
This specifies that the data is encoding in binary form; i.e. there is
no special encoding.

@item GPGME_DATA_ENCODING_BASE64
This specifies that the data is encoded using the Base-64 encoding
scheme as used by @acronym{MIME} and other protocols.

@item GPGME_DATA_ENCODING_ARMOR
This specifies that the data is encoded in an armored form as used by
OpenPGP and PEM.
@end table
@end deftp

@deftypefun GpgmeDataEncoding gpgme_data_get_encoding (@w{GpgmeData @var{dh}})
The function @code{gpgme_data_get_encoding} returns the encoding of
the data object with the handle @var{dh}.  If @var{dh} is not a valid
pointer (e.g. @code{NULL}) @code{GPGME_DATA_ENCODING_NONE} is
returned.
@end deftypefun

@deftypefun GpgmeError gpgme_data_set_encoding (@w{GpgmeData @var{dh}, GpgmeDataEncoding @var{enc}})
The function @code{gpgme_data_set_encoding} changes the encoding of
the data object with the handle @var{dh} to @var{enc}.
@end deftypefun


@c
@c    Chapter Contexts
@c 
@node Contexts
@chapter Contexts
@cindex context

All cryptographic operations in @acronym{GPGME} are performed within a
context, which contains the internal state of the operation as well as
configuration parameters.  By using several contexts you can run
several cryptographic operations in parallel, with different
configuration.

@deftp {Data type} {GpgmeCtx}
The @code{GpgmeCtx} type is a handle for a @acronym{GPGME} context,
which is used to hold the configuration, status and result of
cryptographic operations.
@end deftp

@menu
* Creating Contexts::             Creating new @acronym{GPGME} contexts.
* Destroying Contexts::           Releasing @acronym{GPGME} contexts.
* Context Attributes::            Setting properties of a context.
* Key Management::                Managing keys with @acronym{GPGME}.
* Trust Item Management::         Managing trust items with @acronym{GPGME}.
* Crypto Operations::             Using a context for cryptography.
* Run Control::                   Controlling how operations are run.
@end menu


@node Creating Contexts
@section Creating Contexts
@cindex context, creation

@deftypefun GpgmeError gpgme_new (@w{GpgmeCtx *@var{ctx}})
The function @code{gpgme_data_new} creates a new @code{GpgmeCtx}
object and returns a handle for it in @var{ctx}.

The function returns @code{GPGME_No_Error} if the context was
successfully created, @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, and @code{GPGME_Out_Of_Core} if not enough memory is
available.
@end deftypefun


@node Destroying Contexts
@section Destroying Contexts
@cindex context, destruction

@deftypefun void gpgme_release (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_release} destroys the context with the handle
@var{ctx} and releases all associated resources.
@end deftypefun


@node Context Attributes
@section Context Attributes
@cindex context, attributes

@menu
* Protocol Selection::            Selecting the protocol used by a context.
* @acronym{ASCII} Armor::                   Requesting @acronym{ASCII} armored output.
* Text Mode::                     Choosing canonical text mode.
* Included Certificates::       Including a number of certificates.
* Key Listing Mode::              Selecting key listing mode.
* Passphrase Callback::           Getting the passphrase from the user.
* Progress Meter Callback::       Being informed about the progress.
@end menu


@node Protocol Selection
@subsection Protocol Selection
@cindex context, selecting protocol
@cindex protocol, selecting

@deftypefun GpgmeError gpgme_set_protocol (@w{GpgmeCtx @var{ctx}}, @w{GpgmeProtocol @var{proto}})
The function @code{gpgme_set_protocol} sets the protocol used within
the context @var{ctx} to @var{proto}.  All crypto operations will be
performed by the crypto engine configured for that protocol.
@xref{Protocols and Engines}.

Setting the protocol with @code{gpgme_set_protocol} does not check if
the crypto engine for that protocol is available and installed
correctly.  @xref{Engine Version Check}.

The function returns @code{GPGME_No_Error} if the protocol could be
set successfully, and @code{GPGME_Invalid_Value} if @var{protocol} is
not a valid protocol.
@end deftypefun

@deftypefun GpgmeProtocol gpgme_get_protocol (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_get_protocol} retrieves the protocol currently
use with the context @var{ctx}.
@end deftypefun

@node @acronym{ASCII} Armor
@subsection @acronym{ASCII} Armor
@cindex context, armor mode
@cindex @acronym{ASCII} armor
@cindex armor mode

@deftypefun void gpgme_set_armor (@w{GpgmeCtx @var{ctx}}, @w{int @var{yes}})
The function @code{gpgme_set_armor} specifies if the output should be
@acronym{ASCII} armored.  By default, output is not @acronym{ASCII}
armored.

@acronym{ASCII} armored output is disabled if @var{yes} is zero, and
enabled otherwise.
@end deftypefun

@deftypefun int gpgme_get_armor (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_get_armor} returns 1 if the output is
@acronym{ASCII} armored, and @code{0} if it is not, or if @var{ctx} is
not a valid pointer.
@end deftypefun


@node Text Mode
@subsection Text Mode
@cindex context, text mode
@cindex text mode
@cindex canonical text mode

@deftypefun void gpgme_set_textmode (@w{GpgmeCtx @var{ctx}}, @w{int @var{yes}})
The function @code{gpgme_set_textmode} specifies if canonical text mode
should be used.  By default, text mode is not used.

Text mode is for example used for the RFC2015 signatures; note that
the updated RFC 3156 mandates that the mail user agent does some
preparations so that text mode is not needed anymore.

This option is only relevant to the OpenPGP crypto engine, and ignored
by all other engines.

Canonical text mode is disabled if @var{yes} is zero, and enabled
otherwise.
@end deftypefun

@deftypefun int gpgme_get_textmode (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_get_textmode} returns 1 if canonical text
mode is enabled, and @code{0} if it is not, or if @var{ctx} is not a
valid pointer.
@end deftypefun


@node Included Certificates
@subsection Included Certificates
@cindex certificates, included

@deftypefun void gpgme_set_include_certs (@w{GpgmeCtx @var{ctx}}, @w{int @var{nr_of_certs}})
The function @code{gpgme_set_include_certs} specifies how many
certificates should be included in an S/MIME signed message.  By
default, only the sender's certificate is included.  The possible
values of @var{nr_of_certs} are:

@table @code
@item -2
Include all certificates except the root certificate.
@item -1
Include all certificates.
@item 0
Include no certificates.
@item 1
Include the sender's certificate only.
@item n
Include the first n certificates of the certificates path, starting
from the sender's certificate.  The number @code{n} must be positive.
@end table

Values of @var{nr_of_certs} smaller than -2 are undefined.

This option is only relevant to the CMS crypto engine, and ignored
by all other engines.
@end deftypefun

@deftypefun int gpgme_get_include_certs (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_get_include_certs} returns the number of
certificates to include into an S/MIME signed message.
@end deftypefun


@node Key Listing Mode
@subsection Key Listing Mode
@cindex key listing mode
@cindex key listing, mode of

@deftypefun void gpgme_set_keylist_mode (@w{GpgmeCtx @var{ctx}}, @w{int @var{mode}})
The function @code{gpgme_set_keylist_mode} changes the default
behaviour of the key listing functions.  The value in @var{mode} is a
bitwise-or combination of one or multiple of the following bit values:

@table @code
@item GPGME_KEYLIST_MODE_LOCAL
The @code{GPGME_KEYLIST_MODE_LOCAL} symbol specifies that the local
keyring should be searched for keys in the keylisting operation.  This
is the default.

@item GPGME_KEYLIST_MODE_EXTERN
The @code{GPGME_KEYLIST_MODE_EXTERN} symbol specifies that an external
source should be should be searched for keys in the keylisting
operation.  The type of external source is dependant on the crypto
engine used.  For example, it can be a remote keyserver or LDAP
certificate server.
@end table

At least one of @code{GPGME_KEYLIST_MODE_LOCAL} and
@code{GPGME_KEYLIST_MODE_EXTERN} must be specified.  For future binary
compatibility, you should get the current mode with
@code{gpgme_get_keylist_mode} and modify it by setting or clearing the
appropriate bits, and then using that calulcated value in the
@code{gpgme_set_keylisting_mode} operation.  This will leave all other
bits in the mode value intact (in particular those that are not used
in the current version of the library).

The function returns @code{GPGME_No_Error} if the mode could be set
correctly, and @code{GPGME_Invalid_Value} if @var{ctx} is not a valid
pointer or @var{mode} is not a valid mode.
@end deftypefun


@deftypefun int gpgme_get_keylist_mode (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_get_keylist_mode} returns the current key
listing mode of the context @var{ctx}.  This value can then be
modified and used in a subsequent @code{gpgme_set_keylist_mode}
operation to only affect the desired bits (and leave all others
intact).

The function returns 0 if @var{ctx} is not a valid pointer, and the
current mode otherwise.  Note that 0 is not a valid mode value.
@end deftypefun


@node Passphrase Callback
@subsection Passphrase Callback
@cindex callback, passphrase
@cindex passphrase callback

@deftp {Data type} {GpgmeError (*GpgmePassphraseCb)(void *@var{hook}, const char *@var{desc}, void **@var{r_hd}, const char **@var{result})}
@tindex GpgmePassphraseCb
The @code{GpgmePassphraseCb} type is the type of functions usable as
passphrase callback function.

The string @var{desc} contains a text usable to be displayed to the
user of the application.  The function should return a passphrase for
the context when invoked with @var{desc} not being @code{NULL} in
*@var{result}.

The user may store information about the resources associated with the
returned passphrase in @var{*r_hd}.  When the passphrase is no longer
needed by @acronym{GPGME}, the passphrase callback function will be
called with @var{desc} being @var{NULL}, and @var{r_hd} being the same
as at the first invocation.

If an error occurs, return the corresponding @code{GpgmeError} value.
You can use @code{GPGME_Canceled} to abort the operation.  Otherwise,
return @code{0}.
@end deftp

@deftypefun void gpgme_set_passphrase_cb (@w{GpgmeCtx @var{ctx}}, @w{GpgmePassphraseCb @var{passfunc}}, @w{void *@var{hook_value}})
The function @code{gpgme_set_passphrase_cb} sets the function that is
used when a passphrase needs to be provided by the user to
@var{passfunc}.  The function @var{passfunc} needs to implemented by
the user, and whenever it is called, it is called with its first
argument being @var{hook_value}.  By default, no passphrase callback
function is set.

Not all crypto engines require this callback to retrieve the
passphrase.  It is better if the engine retrieves the passphrase from
a trusted agent (a daemon process), rather than having each user to
implement their own passphrase query.

The user can disable the use of a passphrase callback function by
calling @code{gpgme_set_passphrase_cb} with @var{passfunc} being
@code{NULL}.
@end deftypefun

@deftypefun void gpgme_get_passphrase_cb (@w{GpgmeCtx @var{ctx}}, @w{GpgmePassphraseCb *@var{passfunc}}, @w{void **@var{hook_value}})
The function @code{gpgme_get_passphrase_cb} returns the function that
is used when a passphrase needs to be provided by the user in
@var{*passfunc}, and the first argument for this function in
@var{*hook_value}.  If no passphrase callback is set, or @var{ctx} is
not a valid pointer, @code{NULL} is returned in both variables.

@var{passfunc} or @var{hook_value} can be @code{NULL}.  In this case,
the corresponding value will not be returned.
@end deftypefun


@node Progress Meter Callback
@subsection Progress Meter Callback
@cindex callback, progress meter
@cindex progress meter callback

@deftp {Data type} {const char *(*GpgmeProgressCb)(void *@var{hook}, const char *@var{what}, int @var{type}, int @var{current}, int @var{total})}
@tindex GpgmeProgressCb
The @code{GpgmeProgressCb} type is the type of functions usable as
progress callback function.

The arguments are specific to the crypto engine.  More information
about the progress information returned from the GnuPG engine can be
found in the GnuPG source code in the file @file{doc/DETAILS} in the
section PROGRESS.
@end deftp

@deftypefun void gpgme_set_progress_cb (@w{GpgmeCtx @var{ctx}}, @w{GpgmeProgressCb @var{progfunc}}, @w{void *@var{hook_value}})
The function @code{gpgme_set_progress_cb} sets the function that is
used when progress information about a cryptographic operation is
available.  The function @var{progfunc} needs to implemented by the
user, and whenever it is called, it is called with its first argument
being @var{hook_value}.  By default, no progress callback function
is set.

Setting a callback function allows an interactive program to display
progress information about a long operation to the user.

The user can disable the use of a progress callback function by
calling @code{gpgme_set_progress_cb} with @var{progfunc} being
@code{NULL}.
@end deftypefun

@deftypefun void gpgme_get_progress_cb (@w{GpgmeCtx @var{ctx}}, @w{GpgmeProgressCb *@var{progfunc}}, @w{void **@var{hook_value}})
The function @code{gpgme_get_progress_cb} returns the function that is
used to inform the user about the progress made in @var{*progfunc},
and the first argument for this function in @var{*hook_value}.  If no
progress callback is set, or @var{ctx} is not a valid pointer,
@code{NULL} is returned in both variables.

@var{progfunc} or @var{hook_value} can be @code{NULL}.  In this case,
the corresponding value will not be returned.
@end deftypefun


@node Key Management
@section Key Management
@cindex key management

Some of the cryptographic operations require that recipients or
signers are specified.  This is always done by specifying the
respective keys that should be used for the operation.  The following
section describes how such keys can be selected and manipulated.

@deftp {Data type} GpgmeKey
The @code{GpgmeKey} type is a handle for a public or secret key, and
is used to select the key for operations involving it.

A key can contain several user IDs and sub keys.
@end deftp

@menu
* Listing Keys::                  Browsing the list of available keys.
* Information About Keys::        Requesting detailed information about keys.
* Key Signatures::                Listing the signatures on a key.
* Manipulating Keys::             Operations on keys.
* Generating Keys::               Creating new key pairs.
* Exporting Keys::                Retrieving key data from the key ring.
* Importing Keys::                Adding keys to the key ring.
* Deleting Keys::                 Removing keys from the key ring.
@end menu


@node Listing Keys
@subsection Listing Keys
@cindex listing keys
@cindex key listing
@cindex key listing, start
@cindex key ring, list
@cindex key ring, search

@deftypefun GpgmeError gpgme_op_keylist_start (@w{GpgmeCtx @var{ctx}}, @w{const char *@var{pattern}}, @w{int @var{secret_only}})
The function @code{gpgme_op_keylist_start} initiates a key listing
operation inside the context @var{ctx}.  It sets everything up so that
subsequent invocations of @code{gpgme_op_keylist_next} return the keys
in the list.

If @var{pattern} is @code{NULL}, all available keys are returned.
Otherwise, @var{pattern} contains an engine specific expression that
is used to limit the list to all keys matching the pattern.

If @var{secret_only} is not @code{0}, the list is restricted to secret
keys only.

The context will be busy until either all keys are received (and
@code{gpgme_op_keylist_next} returns @code{GPGME_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, and passes through any errors that are reported by the
crypto engine support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_keylist_ext_start (@w{GpgmeCtx @var{ctx}}, @w{const char *@var{pattern}[]}, @w{int @var{secret_only}}, @w{int @var{reserved}})
The function @code{gpgme_op_keylist_ext_start} initiates an extended
key listing operation inside the context @var{ctx}.  It sets
everything up so that subsequent invocations of
@code{gpgme_op_keylist_next} return the keys in the list.

If @var{pattern} or @var{*pattern} is @code{NULL}, all available keys
are returned.  Otherwise, @var{pattern} is a @code{NULL} terminated
array of strings that are used to limit the list to all keys matching
at least one of the patterns verbatim.

If @var{secret_only} is not @code{0}, the list is restricted to secret
keys only.

The value of @var{reserved} must be @code{0}.

The context will be busy until either all keys are received (and
@code{gpgme_op_keylist_next} returns @code{GPGME_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, and passes through any errors that are reported by the
crypto engine support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_keylist_next (@w{GpgmeCtx @var{ctx}}, @w{GpgmeKey *@var{r_key}})
The function @code{gpgme_op_keylist_next} returns the next key in the
list created by a previous @code{gpgme_op_keylist_start} operation in
the context @var{ctx}.  The key will have one reference for the user.
@xref{Manipulating Keys}.

This is the only way to get at @code{GpgmeKey} objects in
@acronym{GPGME}.

If the last key in the list has already been returned,
@code{gpgme_op_keylist_next} returns @code{GPGME_EOF}.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} or
@var{r_key} is not a valid pointer, @code{GPGME_No_Request} if there
is no pending operation, @code{GPGME_Out_Of_Core} if there is not
enough memory for the operation.
@end deftypefun

@deftypefun GpgmeError gpgme_op_keylist_end (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_op_keylist_next} ends a pending key list
operation in the context @var{ctx}.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, @code{GPGME_No_Request} if there is no pending
operation, @code{GPGME_Out_Of_Core} if at some time during the
operation there was not enough memory available.
@end deftypefun

The following example illustrates how all keys containing a certain
string (@code{g10code}) can be listed with their key ID and the name
and e-mail address of the main user ID:

@example
GpgmeCtx ctx;
GpgmeError err = gpgme_new (&ctx);

if (!err)
  @{
    err = gpgme_op_keylist_start (ctx, "g10code", 0);
    while (!err && (err = gpgme_op_keylist_next (ctx, &key)) != GPGME_EOF)
      @{
        printf ("%s: %s <%s>\n",
                gpgme_key_get_string_attr (key, GPGME_ATTR_KEYID, 0, 0),
	        gpgme_key_get_string_attr (key, GPGME_ATTR_NAME, 0, 0),
	        gpgme_key_get_string_attr (key, GPGME_ATTR_EMAIL, 0, 0));
        gpgme_key_release (key);
      @}
    gpgme_release (ctx);
  @}
if (err)
  @{
    fprintf (stderr, "%s: can not list keys: %s\n",
             argv[0], gpgme_strerror (err));
    exit (1);
  @}
@end example

In a simple program, for which a blocking operation is acceptable, the
following function can be used to retrieve a single key.

@deftypefun GpgmeError gpgme_get_key (@w{GpgmeCtx @var{ctx}}, @w{const char *@var{fpr}}, @w{GpgmeKey *@var{r_key}}, @w{int @var{secret}}, @w{int @var{force_update}})
The function @code{gpgme_get_key} gets the key with the fingerprint
(or key ID) @var{fpr} from the key cache or from the crypto backend
and return it in @var{r_key}.  If @var{force_update} is true, force a
refresh of the key from the crypto backend and replace the key in the
cache, if any.  If @var{secret} is true, get the secret key.

If the @code{GPGME_KEYLIST_MODE_SIGS} mode is active, the key will be
retrieved with the key signatures (and updated if necessary).

The function returns @code{GPGME_Invalid_Value} if @var{ctx} or
@var{r_key} is not a valid pointer, @code{GPGME_Busy} if there is a
pending operation, @code{GPGME_Invalid_Key} if @var{fpr} is not a
fingerprint or key ID, @code{GPGME_Out_Of_Core} if at some time during
the operation there was not enough memory available.
@end deftypefun


@node Information About Keys
@subsection Information About Keys
@cindex key, information about
@cindex key, attributes
@cindex attributes, of a key

@deftypefun {char *} gpgme_key_get_as_xml (@w{GpgmeKey @var{key}})
The function @code{gpgme_key_get_as_xml} returns a string in
@acronym{XML} format describing the key @var{key}.  The user has to
release the string with @code{free}.

The function returns @code{NULL} if @var{key} is not a valid pointer,
or there is not enough memory available.
@end deftypefun

@deftp {Data type} GpgmeAttr
The @code{GpgmeAttr} type is used to specify a key or trust item
attribute.  The following attributes are defined:

@table @code
@item GPGME_ATTR_KEYID
This is the key ID of a sub key.  It is representable as a string.

For trust items, the trust item refers to the key with this ID.

@item GPGME_ATTR_FPR
This is the fingerprint of a sub key.  It is representable as a
string.

@item GPGME_ATTR_ALGO
This is the crypto algorithm for which the sub key can be used.  It
is representable as a string and as a number.  The numbers correspond
to the @code{enum gcry_pk_algos} values in the gcrypt library.

@item GPGME_ATTR_LEN
This is the key length of a sub key.  It is representable as a
number.

@item GPGME_ATTR_CREATED
This is the timestamp at creation time of a sub key.  It is
representable as a number.

@item GPGME_ATTR_EXPIRE
This is the expiration time of a sub key.  It is representable as a
number.

@item GPGME_ATTR_OTRUST
XXX FIXME  (also for trust items)

@item GPGME_ATTR_USERID
This is a user ID.  There can be more than one user IDs in a
@var{GpgmeKey} object.  The first one (with index 0) is the primary
user ID.  The user ID is representable as a number.

For trust items, this is the user ID associated with this trust item.

@item GPGME_ATTR_NAME
This is the name belonging to a user ID.  It is representable as a string.

@item GPGME_ATTR_EMAIL
This is the email address belonging to a user ID.  It is representable
as a string.

@item GPGME_ATTR_COMMENT
This is the comment belonging to a user ID.  It is representable as a
string.

@item GPGME_ATTR_VALIDITY
This is the validity belonging to a user ID.  It is representable as a
string and as a number.  See below for a list of available validities.

For trust items, this is the validity that is associated with this
trust item.

@item GPGME_ATTR_UID_REVOKED
This specifies if a user ID is revoked.  It is representable as a
number, and is @code{1} if the user ID is revoked, and @code{0}
otherwise.

@item GPGME_ATTR_UID_INVALID
This specifies if a user ID is invalid.  It is representable as a
number, and is @code{1} if the user ID is invalid, and @code{0}
otherwise.

@item GPGME_ATTR_LEVEL
This is the trust level of a trust item.

@item GPGME_ATTR_TYPE
This returns information about the type of key.  For the string function
this will eother be "PGP" or "X.509".  The integer function returns 0
for PGP and 1 for X.509.  It is also used for the type of a trust item.

@item GPGME_ATTR_IS_SECRET
This specifies if the key is a secret key.  It is representable as a
number, and is @code{1} if the key is revoked, and @code{0} otherwise.

@item GPGME_ATTR_KEY_REVOKED
This specifies if a sub key is revoked.  It is representable as a
number, and is @code{1} if the key is revoked, and @code{0} otherwise.

@item GPGME_ATTR_KEY_INVALID
This specifies if a sub key is invalid.  It is representable as a
number, and is @code{1} if the key is invalid, and @code{0} otherwise.

@item GPGME_ATTR_KEY_EXPIRED
This specifies if a sub key is expired.  It is representable as a
number, and is @code{1} if the key is expired, and @code{0} otherwise.

@item GPGME_ATTR_KEY_DISABLED
This specifies if a sub key is disabled.  It is representable as a
number, and is @code{1} if the key is disabled, and @code{0} otherwise.

@item GPGME_ATTR_KEY_CAPS
This is a description of the capabilities of a sub key.  It is
representable as a string.  The string contains the letter ``e'' if
the key can be used for encryption, ``s'' if the key can be used for
signatures, and ``c'' if the key can be used for certifications.

@item GPGME_ATTR_CAN_ENCRYPT
This specifies if a sub key can be used for encryption.  It is
representable as a number, and is @code{1} if the sub key can be used
for encryption, and @code{0} otherwise.

@item GPGME_ATTR_CAN_SIGN
This specifies if a sub key can be used for signatures.  It is
representable as a number, and is @code{1} if the sub key can be used
for signatures, and @code{0} otherwise.

@item GPGME_ATTR_CAN_CERTIFY
This specifies if a sub key can be used for certifications.  It is
representable as a number, and is @code{1} if the sub key can be used
for certifications, and @code{0} otherwise.

@item GPGME_ATTR_SERIAL
The X.509 issuer serial attribute of the key.  It is representable as
a string.

@item GPGME_ATTR_ISSUE
The X.509 issuer name attribute of the key.  It is representable as a
string.

@item GPGME_ATTR_CHAINID
The X.509 chain ID can be used to build the certification chain.  It
is representable as a string.
@end table
@end deftp

@deftp {Data type} GpgmeValidity
The @code{GpgmeValidity} type is used to specify the validity of a user ID
in a key.  The following validities are defined:

@table @code
@item GPGME_VALIDITY_UNKNOWN
The user ID is of unknown validity.  The string representation of this
validity is ``?''.

@item GPGME_VALIDITY_UNDEFINED
The validity of the user ID is undefined.  The string representation of this
validity is ``q''.

@item GPGME_VALIDITY_NEVER
The user ID is never valid.  The string representation of this
validity is ``n''.

@item GPGME_VALIDITY_MARGINAL
The user ID is marginally valid.  The string representation of this
validity is ``m''.

@item GPGME_VALIDITY_FULL
The user ID is fully valid.  The string representation of this
validity is ``f''.

@item GPGME_VALIDITY_ULTIMATE
The user ID is ultimately valid.  The string representation of this
validity is ``u''.
@end table
@end deftp

@deftypefun {const char *} gpgme_key_get_string_attr (@w{GpgmeKey @var{key}}, @w{GpgmeAttr @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_get_string_attr} returns the value of the
string-representable attribute @var{what} of key @var{key}.  If the
attribute is an attribute of a sub key or an user ID, @var{idx}
specifies the sub key or user ID of which the attribute value is
returned.  The argument @var{reserved} is reserved for later use and
should be @code{NULL}.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun {unsigned long} gpgme_key_get_ulong_attr (@w{GpgmeKey @var{key}}, @w{GpgmeAttr @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_get_ulong_attr} returns the value of the
number-representable attribute @var{what} of key @var{key}.  If the
attribute is an attribute of a sub key or an user ID, @var{idx}
specifies the sub key or user ID of which the attribute value is
returned.  The argument @var{reserved} is reserved for later use and
should be @code{NULL}.

The function returns @code{0} if the attribute can't be returned as a
number, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun


@node Key Signatures
@subsection Key Signatures
@cindex key, signatures
@cindex signatures, on a key

The signatures on a key are only available if the key was retrieved
via a listing operation with the @code{GPGME_KEYLIST_MODE_SIGS} mode
enabled, because it is expensive to retrieve all signatures of a key.

So, before using the below interfaces to retrieve the signatures on a
key, you have to make sure that the key was listed with signatures
enabled.  One convenient, but blocking, way to do this is to use the
function @code{gpgme_get_key}.

@deftp {Data type} GpgmeAttr
The @code{GpgmeAttr} type is used to specify a key signature
attribute.  The following attributes are defined:

@table @code
@item GPGME_ATTR_KEYID
This is the key ID of the key which was used for the signature.  It is
representable as a string.

@item GPGME_ATTR_ALGO
This is the crypto algorithm used to create the signature.  It is
representable as a string and as a number.  The numbers correspond to
the @code{enum gcry_pk_algos} values in the gcrypt library.

@item GPGME_ATTR_CREATED
This is the timestamp at creation time of the signature.  It is
representable as a number.

@item GPGME_ATTR_EXPIRE
This is the expiration time of the signature.  It is representable as
a number.

@item GPGME_ATTR_USERID
This is the user ID associated with the signing key.  The user ID is
representable as a number.

@item GPGME_ATTR_NAME
This is the name belonging to a user ID.  It is representable as a string.

@item GPGME_ATTR_EMAIL
This is the email address belonging to a user ID.  It is representable
as a string.

@item GPGME_ATTR_COMMENT
This is the comment belonging to a user ID.  It is representable as a
string.

@item GPGME_ATTR_KEY_REVOKED
This specifies if a key signature is a revocation signature.  It is
representable as a number, and is @code{1} if the key is revoked, and
@code{0} otherwise.

@c @item GPGME_ATTR_KEY_EXPIRED
@c This specifies if a key signature is expired.  It is representable as
@c a number, and is @code{1} if the key is revoked, and @code{0}
@c otherwise.
@c
@item GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature.  It is
representable as a number.  The meaning is specific to the crypto
engine.

@item GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature.  It is
representable as a number.  The meaning is specific to the crypto
engine.

@item GPGME_ATTR_SIG_STATUS
This is the same value as returned by @code{gpgme_get_sig_status}.
@end table
@end deftp

@deftypefun {const char *} gpgme_key_sig_get_string_attr (@w{GpgmeKey @var{key}}, @w{int @var{uid_idx}}, @w{GpgmeAttr @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_sig_get_string_attr} returns the value of
the string-representable attribute @var{what} of the signature
@var{idx} on the user ID @var{uid_idx} in the key @var{key}.  The
argument @var{reserved} is reserved for later use and should be
@code{NULL}.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{uid_idx} or @var{idx}
out of range, or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun {unsigned long} gpgme_key_sig_get_ulong_attr (@w{GpgmeKey @var{key}}, @w{int @var{uid_idx}}, @w{GpgmeAttr @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_sig_get_ulong_attr} returns the value of
the number-representable attribute @var{what} of the signature
@var{idx} on the user ID @var{uid_idx} in the key @var{key}.  The
argument @var{reserved} is reserved for later use and should be
@code{NULL}.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{uid_idx} or @var{idx}
out of range, or @var{reserved} not @code{NULL}.
@end deftypefun


@node Manipulating Keys
@subsection Manipulating Keys
@cindex key, manipulation

@deftypefun void gpgme_key_ref (@w{GpgmeKey @var{key}})
The function @code{gpgme_key_ref} acquires an additional reference for
the key @var{key}.
@end deftypefun

@deftypefun void gpgme_key_unref (@w{GpgmeKey @var{key}})
@deftypefunx void gpgme_key_release (@w{GpgmeKey @var{key}})
The function @code{gpgme_key_ref} releases a reference for the key
@var{key}.  If this was the last reference, the key will be destroyed
and all resources associated to it will be released.

The function @code{gpgme_key_release} is an alias for
@code{gpgme_key_unref}.
@end deftypefun


@node Generating Keys
@subsection Generating Keys
@cindex key, creation
@cindex key ring, add

@deftypefun GpgmeError gpgme_op_genkey (@w{GpgmeCtx @var{ctx}}, @w{const char *@var{parms}}, @w{GpgmeData @var{pubkey}}, @w{GpgmeData @var{seckey}}, @w{char **@var{fpr}})
The function @code{gpgme_op_genkey} generates a new key pair in the
context @var{ctx} and puts it into the standard key ring if both
@var{pubkey} and @var{seckey} are @code{NULL}.  In this case the
function returns immediately after starting the operation, and does
not wait for it to complete.  If @var{pubkey} is not @code{NULL} it
should be the handle for an empty (newly created) data object, and
upon successful completion the data object will contain the public
key.  If @var{seckey} is not @code{NULL} it should be the handle for
an empty (newly created) data object, and upon successful completion
the data object will contain the secret key.

Note that not all crypto engines support this interface equally.
GnuPG does not support @var{pubkey} and @var{subkey}, they should be
both @code{NULL}, and the key pair will be added to the standard key
ring.  GpgSM does only support @var{pubkey}, the secret key will be
stored by @command{gpg-agent}.  GpgSM expects @var{pubkey} being not
@code{NULL}.

The argument @var{parms} specifies parameters for the key in an XML
string.  The details about the format of @var{parms} are specific to
the crypto engine used by @var{ctx}.  Here is an example for GnuPG as
the crypto engine:

@example
<GnupgKeyParms format="internal">
Key-Type: DSA
Key-Length: 1024
Subkey-Type: ELG-E
Subkey-Length: 1024
Name-Real: Joe Tester
Name-Comment: with stupid passphrase
Name-Email: joe@@foo.bar
Expire-Date: 0
Passphrase: abc
</GnupgKeyParms>
@end example

Here is an example for GpgSM as the crypto engine:
@example
<GnupgKeyParms format="internal">
Key-Type: RSA
Key-Length: 1024
Name-DN: C=de,O=g10 code,OU=Testlab,CN=Joe 2 Tester
Name-Email: joe@@foo.bar
</GnupgKeyParms>
@end example

Strings should be given in UTF-8 encoding.  The only format supported
for now is ``internal''.  The content of the @code{GnupgKeyParms}
container is passed verbatim to GnuPG.  Control statements are not
allowed.

If @var{fpr} is not a null pointer, the function succeeds, and the
crypto engine supports it, *@var{fpr} will contain a string with the
fingerprint of the key, allocated with @code{malloc}.  If both a
primary and a sub key was generated, the fingerprint of the primary
key will be returned.  If the crypto engine does not provide the
fingerprint, *@var{fpr} will be a null pointer.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, @code{GPGME_Invalid_Value} if @var{parms} is not
a valid XML string, @code{GPGME_Not_Supported} if @var{pubkey} or
@var{seckey} is not valid, and @code{GPGME_General_Error} if no key
was created by the backend.
@end deftypefun

@deftypefun GpgmeError gpgme_op_genkey_start (@w{GpgmeCtx @var{ctx}}, @w{const char *@var{parms}}, @w{GpgmeData @var{pubkey}}, @w{GpgmeData @var{seckey}})
The function @code{gpgme_op_genkey_start} initiates a
@code{gpgme_op_genkey} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, @code{GPGME_Invalid_Value} if @var{parms} is not
a valid XML string, and @code{GPGME_Not_Supported} if @var{pubkey} or
@var{seckey} is not @code{NULL}.
@end deftypefun


@node Exporting Keys
@subsection Exporting Keys
@cindex key, export
@cindex key ring, export from

@deftypefun GpgmeError gpgme_op_export (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{recipients}}, @w{GpgmeData @var{keydata}})
The function @code{gpgme_op_export} extracts the public keys of the
user IDs in @var{recipients} and returns them in the data buffer
@var{keydata}.  The type of the public keys returned is determined by
the @acronym{ASCII} armor attribute set for the context @var{ctx}.

The function returns @code{GPGME_No_Error} if the operation completed
successfully, @code{GPGME_Invalid_Value} if @var{recipients} is
@code{NULL} or @var{keydata} is not a valid empty data buffer, and
passes through any errors that are reported by the crypto engine
support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_export_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{recipients}}, @w{GpgmeData @var{keydata}})
The function @code{gpgme_op_export_start} initiates a
@code{gpgme_op_export} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, and @code{GPGME_Invalid_Value} if
@var{recipients} is @code{NULL} or @var{keydata} is not a valid empty
data buffer.
@end deftypefun


@node Importing Keys
@subsection Importing Keys
@cindex key, import
@cindex key ring, import to

@deftypefun GpgmeError gpgme_op_import (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{keydata}})
The function @code{gpgme_op_import} adds the keys in the data buffer
@var{keydata} to the key ring of the crypto engine used by @var{ctx}.
The format of @var{keydata} can be @var{ASCII} armored, for example,
but the details are specific to the crypto engine.

More information about the import is available with
@code{gpgme_get_op_info}.  @xref{Detailed Results}.

The function returns @code{GPGME_No_Error} if the import was completed
successfully, @code{GPGME_Invalid_Value} if @var{keydata} if @var{ctx}
or @var{keydata} is not a valid pointer, and @code{GPGME_No_Data} if
@var{keydata} is an empty data buffer.
@end deftypefun

@deftypefun GpgmeError gpgme_op_import_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{keydata}})
The function @code{gpgme_op_import_start} initiates a
@code{gpgme_op_import} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the import could be
started successfully, @code{GPGME_Invalid_Value} if @var{keydata} if
@var{ctx} or @var{keydata} is not a valid pointer, and
@code{GPGME_No_Data} if @var{keydata} is an empty data buffer.
@end deftypefun

@deftypefun GpgmeError gpgme_op_import_ext (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{keydata}}, @w{int *@var{nr}})
The function @code{gpgme_op_import_ext} is like
@code{gpgme_op_import}, but also returns the number of processed keys
in @var{nr}.  This is the same as the @code{count} information in the
detailed results available with @code{gpgme_get_op_info}.
@end deftypefun


@node Deleting Keys
@subsection Deleting Keys
@cindex key, delete
@cindex key ring, delete from

@deftypefun GpgmeError gpgme_op_delete (@w{GpgmeCtx @var{ctx}}, @w{const GpgmeKey @var{key}}, @w{int @var{allow_secret}})
The function @code{gpgme_op_delete} deletes the key @var{key} from the
key ring of the crypto engine used by @var{ctx}.  If
@var{allow_secret} is @code{0}, only public keys are deleted,
otherwise secret keys are deleted as well, if that is supported.

The function returns @code{GPGME_No_Error} if the key was deleted
successfully, @code{GPGME_Invalid_Value} if @var{ctx} or @var{key} is
not a valid pointer, @code{GPGME_Invalid_Key} if @var{key} could not
be found in the keyring, and @code{GPGME_Conflict} if the secret key
for @var{key} is available, but @var{allow_secret} is zero.
@end deftypefun

@deftypefun GpgmeError gpgme_op_delete_start (@w{GpgmeCtx @var{ctx}}, @w{const GpgmeKey @var{key}}, @w{int @var{allow_secret}})
The function @code{gpgme_op_delete_start} initiates a
@code{gpgme_op_delete} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation was
started successfully, and @code{GPGME_Invalid_Value} if @var{ctx} or
@var{key} is not a valid pointer.
@end deftypefun


@node Trust Item Management
@section Trust Item Management
@cindex trust item

@strong{Caution:} The trust items interface is experimental.

@deftp {Data type} GpgmeTrustItem
The @code{GpgmeTrustItem} type is a handle for a trust item.
@end deftp

@menu
* Listing Trust Items::           Browsing the list of available trust items.
* Information About Trust Items:: Requesting detailed information about trust items.
* Manipulating Trust Items::      Operations on trust items.
@end menu


@node Listing Trust Items
@subsection Listing Trust Items
@cindex trust item list

@deftypefun GpgmeError gpgme_op_trustlist_start (@w{GpgmeCtx @var{ctx}}, @w{const char *@var{pattern}}, @w{int @var{max_level}})
The function @code{gpgme_op_trustlist_start} initiates a trust item
listing operation inside the context @var{ctx}.  It sets everything up
so that subsequent invocations of @code{gpgme_op_trustlist_next} return
the trust items in the list.

The string @var{pattern} contains an engine specific expression that
is used to limit the list to all trust items matching the pattern.  It
can not be the empty string.

The argument @var{max_level} is currently ignored.

The context will be busy until either all trust items are received
(and @code{gpgme_op_trustlist_next} returns @code{GPGME_EOF}), or
@code{gpgme_op_trustlist_end} is called to finish the operation.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, and passes through any errors that are reported by the
crypto engine support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_trustlist_next (@w{GpgmeCtx @var{ctx}}, @w{GpgmeTrustItem *@var{r_item}})
The function @code{gpgme_op_trustlist_next} returns the next trust
item in the list created by a previous @code{gpgme_op_trustlist_start}
operation in the context @var{ctx}.  The trust item can be destroyed
with @code{gpgme_trust_item_release}.  @xref{Manipulating Trust Items}.

This is the only way to get at @code{GpgmeTrustItem} objects in
@acronym{GPGME}.

If the last trust item in the list has already been returned,
@code{gpgme_op_trustlist_next} returns @code{GPGME_EOF}.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} or
@var{r_item} is not a valid pointer, @code{GPGME_No_Request} if there
is no pending operation, @code{GPGME_Out_Of_Core} if there is not
enough memory for the operation.
@end deftypefun

@deftypefun GpgmeError gpgme_op_trustlist_end (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_op_trustlist_next} ends a pending key list
operation in the context @var{ctx}.

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, @code{GPGME_No_Request} if there is no pending
operation, @code{GPGME_Out_Of_Core} if at some time during the
operation there was not enough memory available.
@end deftypefun


@node Information About Trust Items
@subsection Information About Trust Items
@cindex trust item, information about
@cindex trust item, attributes
@cindex attributes, of a trust item

Trust items have attributes which can be queried using the interfaces
below.  The attribute identifiers are shared with those for key
attributes.  @xref{Information About Keys}.

@deftypefun {const char *} gpgme_trust_item_get_string_attr (@w{GpgmeTrustItem @var{item}}, @w{GpgmeAttr @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_trust_item_get_string_attr} returns the value
of the string-representable attribute @var{what} of trust item
@var{item}.  The arguments @var{idx} and @var{reserved} are reserved
for later use and should be @code{0} and @code{NULL} respectively.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun int gpgme_trust_item_get_int_attr (@w{GpgmeTrustItem @var{item}}, @w{GpgmeAttr @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_trust_item_get_int_attr} returns the value of
the number-representable attribute @var{what} of trust item
@var{item}.  If the attribute occurs more than once in the trust item,
the index is specified by @var{idx}.  However, currently no such
attribute exists, so @var{idx} should be @code{0}.  The argument
@var{reserved} is reserved for later use and should be @code{NULL}.

The function returns @code{0} if the attribute can't be returned as a
number, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun


@node Manipulating Trust Items
@subsection Manipulating Trust Items
@cindex trust item, manipulation

@deftypefun void gpgme_trust_item_release (@w{GpgmeTrustItem @var{item}})
The function @code{gpgme_trust_item_release} destroys a
@code{GpgmeTrustItem} object and releases all associated resources.
@end deftypefun

@node Crypto Operations
@section Crypto Operations
@cindex cryptographic operation

@menu
* Decrypt::                       Decrypting a ciphertext.
* Verify::                        Verifying a signature.
* Decrypt and Verify::            Decrypting a signed ciphertext.
* Sign::                          Creating a signature.
* Encrypt::                       Encrypting a plaintext.
* Detailed Results::              How to obtain more info about the operation.
@end menu


@node Decrypt
@subsection Decrypt
@cindex decryption
@cindex cryptographic operation, decryption

@deftypefun GpgmeError gpgme_op_decrypt (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @var{plain}})
The function @code{gpgme_op_decrypt} decrypts the ciphertext in the
data object @var{cipher} and stores it into the data object
@var{plain}.

The function returns @code{GPGME_No_Error} if the ciphertext could be
decrypted successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{cipher} or @var{plain} is not a valid pointer,
@code{GPGME_No_Data} if @var{cipher} does not contain any data to
decrypt, @code{GPGME_Decryption_Failed} if @var{cipher} is not a valid
cipher text, @code{GPGME_No_Passphrase} if the passphrase for the
secret key could not be retrieved, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_decrypt_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @var{plain}})
The function @code{gpgme_op_decrypt_start} initiates a
@code{gpgme_op_decrypt} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, and @code{GPGME_Invalid_Value} if @var{cipher}
or @var{plain} is not a valid pointer.
@end deftypefun


@node Verify
@subsection Verify
@cindex verification
@cindex signature, verification
@cindex cryptographic operation, verification
@cindex cryptographic operation, signature check
@cindex signature, status

@deftp {Data type} {enum GpgmeSigStat}
@tindex GpgmeSigStat
The @code{GpgmeSigStat} type holds the result of a signature check, or
the combined result of all signatures.  The following results are
possible:

@table @code
@item GPGME_SIG_STAT_NONE
This status should not occur in normal operation.

@item GPGME_SIG_STAT_GOOD
This status indicates that the signature is valid.  For the combined
result this status means that all signatures are valid.

@item GPGME_SIG_STAT_GOOD_EXP
This status indicates that the signature is valid but expired.  For
the combined result this status means that all signatures are valid
and expired.

@item GPGME_SIG_STAT_GOOD_EXPKEY
This status indicates that the signature is valid but the key used to
verify the signature has expired.  For the combined result this status
means that all signatures are valid and all keys are expired.

@item GPGME_SIG_STAT_BAD
This status indicates that the signature is invalid.  For the combined
result this status means that all signatures are invalid.

@item GPGME_SIG_STAT_NOKEY
This status indicates that the signature could not be verified due to
a missing key.  For the combined result this status means that all
signatures could not be checked due to missing keys.

@item GPGME_SIG_STAT_NOSIG
This status indicates that the signature data provided was not a real
signature.

@item GPGME_SIG_STAT_ERROR
This status indicates that there was some other error which prevented
the signature verification.

@item GPGME_SIG_STAT_DIFF
For the combined result this status means that at least two signatures
have a different status.  You can get each key's status with
@code{gpgme_get_sig_status}.
@end table
@end deftp


@deftypefun GpgmeError gpgme_op_verify (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{sig}}, @w{GpgmeData @var{signed_text}}, @w{GpgmeData @var{plain}})
The function @code{gpgme_op_verify} verifies that the signature in the
data object @var{sig} is a valid signature.  If @var{sig} is a
detached signature, then the signed text should be provided in
@var{signed_text} and @var{plain} should be a null pointer.
Otherwise, if @var{sig} is a normal (or cleartext) signature,
@var{signed_text} should be a null pointer and @var{plain} should be a
writable data object that will contain the plaintext after successful
verification.

The results of the individual signature verifications can be retrieved
with @code{gpgme_get_sig_status} and @code{gpgme_get_sig_key}.

The function returns @code{GPGME_No_Error} if the operation could be
completed successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{sig}, @var{plain} or @var{r_stat} is not a valid pointer,
@code{GPGME_No_Data} if @var{sig} does not contain any data to verify,
and passes through any errors that are reported by the crypto engine
support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_verify_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{sig}}, @w{GpgmeData @var{signed_text}}, @w{GpgmeData @var{plain}})
The function @code{gpgme_op_verify_start} initiates a
@code{gpgme_op_verify} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{sig}, @var{plain} or @var{r_stat} is not a valid pointer, and
@code{GPGME_No_Data} if @var{sig} or @var{plain} does not contain any
data to verify.
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_status (@w{GpgmeCtx @var{ctx}}, @w{int @var{idx}}, @w{GpgmeSigStat *@var{r_stat}}, @w{time_t *@var{r_created}})
The function @code{gpgme_get_sig_status} receives information about a
signature after the @code{gpgme_op_verify} or
@code{gpgme_op_verify_decrypt} operation.  A single detached signature
can contain signatures by more than one key.  The @var{idx} specifies
which signature's information should be retrieved, starting from
@var{0}.

The status of the signature will be returned in @var{r_stat} if it is
not @code{NULL}.  The creation time stamp of the signature will be
returned in @var{r_created} if it is not @var{NULL}.

The function returns a statically allocated string that contains the
fingerprint of the key which signed the plaintext, or @code{NULL} if
@var{ctx} is not a valid pointer, the operation is still pending, or
no verification could be performed.
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_string_attr (@w{GpgmeCtx @var{ctx}}, @w{int @var{idx}}, @w{GpgmeAttr @var{what}}, @w{int @var{whatidx}})
This function is similar to @code{gpgme_get_sig_status} but may be used
to retrieve more detailed information.  @var{ctx} should be the context
used for the last signature verification, @var{idx} is used to enumerate
over all signatures starting with @code{0} and @var{whatidx} should be
@code{0} unless otherwise stated.

The following values may be used for @var{what}:
@table @code
@item GPGME_ATTR_FPR
Return the fingerprint of the key used to create the signature.

@item GPGME_ATTR_ERRTOK
Return a token with a more detailed error description.  A @var{whatidx}
of @code{0} returns an error token associated with validity calculation,
a value of @code{1} return an error token related to the certificate
checking.

@end table
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_ulong_attr (@w{GpgmeCtx @var{ctx}}, @w{int @var{idx}}, @w{GpgmeAttr @var{waht}}, @w{int @var{whatidx}})
This function is similar to @code{gpgme_get_sig_string_attr} but used
for attributes which can be represented by an @code{unsigned long} data
type.  @var{ctx} should be the context used for the last signature
verification, @var{idx} is used to enumerate over all signatures
starting with @code{0} and @var{whatidx} should be @code{0} unless
otherwise stated.

The following values may be used for @var{what}:
@table @code
@item GPGME_ATTR_CREATED
Return the creation time of the signature in seconds since Epoch.  This
is the same value as returned by @code{gpgme_get_sig_status}.

@item GPGME_ATTR_EXPIRE
Return the expiration time of the signature in seconds since Epoch. 

@item GPGME_ATTR_VALIDITY
Returns the validity of the key used to create the signature.  This is a
shortcut function which avoids an extra key lookup.  The value returned
is one of @code{GPGME_VALIDITY_UNKNOWN}, @code{GPGME_VALIDITY_NEVER},
@code{GPGME_VALIDITY_MARGINAL} or @code{GPGME_VALIDITY_FULL}.

@item GPGME_ATTR_SIG_STATUS
This is the same value as returned by @code{gpgme_get_sig_status}.

@item GPGME_ATTR_SIG_SUMMARY
This returns a bit vector giving a summary of the signature status.
Itprovides an easy interface to a defined semantic of the signature
status.  Checking just one bit is sufficient to see whether a signature
is valid without any restrictions.

The defined bits are:
  @table @code
  @item GPGME_SIGSUM_VALID
  The signature is fully valid.

  @item GPGME_SIGSUM_GREEN
  The signature is good but one might want to display some extra
  information.  Check the other bits.

  @item GPGME_SIGSUM_RED
  The signature is bad. It might be useful to checkother bits and
  display moe information, i.e. a revoked certificate might not render a
  signature invalid when the message was received prior to the cause for
  the revocation.

  @item GPGME_SIGSUM_KEY_REVOKED
  The key or at least one certificate has been revoked.

  @item GPGME_SIGSUM_KEY_EXPIRED
  The key or one of the certificates has expired. It is probably a good
  idea to display the date of the expiration.

  @item GPGME_SIGSUM_SIG_EXPIRED
  The signature has expired.

  @item GPGME_SIGSUM_KEY_MISSING
  Can't verifydue to a missing key o certificate.

  @item GPGME_SIGSUM_CRL_MISSING
  The CRL (or an equivalent mechanism) is not available. 

  @item GPGME_SIGSUM_CRL_TOO_OLD
  Available CRL is too old.

  @item GPGME_SIGSUM_BAD_POLICY
  A policy requirement was not met. 

  @item GPGME_SIGSUM_SYS_ERROR
  A system error occured. 

  @end table

@end table
@end deftypefun


@deftypefun {const char *} gpgme_get_sig_key (@w{GpgmeCtx @var{ctx}}, @w{int @var{idx}}, @w{GpgmeKey *@var{r_key}})
The function @code{gpgme_get_sig_status} receives a @code{GpgmeKey}
object for the key which was used to verify the signature after the
@code{gpgme_op_verify} or @code{gpgme_op_verify_decrypt} operation.  A
single detached signature can contain signatures by more than one key.
The @var{idx} specifies which signature's information should be
retrieved, starting from @var{0}.  The key will have on reference for
the user.

The function is a convenient way to retrieve the keys belonging to the
fingerprints returned by @code{gpgme_get_sig_status}.

The function returns @code{GPGME_No_Error} if the key could be
returned, @code{GPGME_Invalid_Value} if @var{r_key} is not a valid
pointer, @code{GPGME_Invalid_Key} if the fingerprint is not valid,
@code{GPGME_EOF} if @var{idx} is too large, or some other error value
if a problem occurred requesting the key.
@end deftypefun

@deftypefun {char *} gpgme_get_notation (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_get_notation} can be used to retrieve
notation data from the last signature check in the context @var{ctx}.

If there is notation data available from the last signature check,
this function may be used to return this notation data as a string.
The string is an XML representation of that data embedded in a
<notation> container.  The user has to release the string with
@code{free}.

The function returns a string if the notation data is available or
@code{NULL} if there is no such data available.
@end deftypefun


@node Decrypt and Verify
@subsection Decrypt and Verify
@cindex decryption and verification
@cindex verification and decryption
@cindex signature check
@cindex cryptographic operation, decryption and verification

@deftypefun GpgmeError gpgme_op_decrypt_verify (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @var{plain}})
The function @code{gpgme_op_decrypt_verify} decrypts the ciphertext in
the data object @var{cipher} and stores it into the data object
@var{plain}.  If @var{cipher} contains signatures, they will be
verified.

After the operation completed, @code{gpgme_op_get_sig_status} and
@code{gpgme_op_get_sig_key} can be used to retrieve more information
about the signatures.

The function returns @code{GPGME_No_Error} if the ciphertext could be
decrypted successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{cipher}, @var{plain} or @var{r_stat} is not a valid pointer,
@code{GPGME_No_Data} if @var{cipher} does not contain any data to
decrypt, @code{GPGME_Decryption_Failed} if @var{cipher} is not a valid
cipher text, @code{GPGME_No_Passphrase} if the passphrase for the
secret key could not be retrieved, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_decrypt_verify (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @var{plain}})
The function @code{gpgme_op_decrypt_verify_start} initiates a
@code{gpgme_op_decrypt_verify} operation.  It can be completed by
calling @code{gpgme_wait} on the context.  @xref{Waiting For
Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{cipher}, @var{plain} or @var{r_stat} is not a valid pointer, and
@code{GPGME_No_Data} if @var{cipher} does not contain any data to
decrypt.
@end deftypefun


@node Sign
@subsection Sign
@cindex signature, creation
@cindex sign
@cindex cryptographic operation, signing

A signature can contain signatures by one or more keys.  The set of
keys used to create a signatures is contained in a context, and is
applied to all following signing operations in this context (until the
set is changed).

@menu
* Selecting Signers::             How to choose the keys to sign with.
* Creating a Signature::          How to create a signature.
@end menu


@node Selecting Signers
@subsubsection Selecting Signers
@cindex signature, selecting signers
@cindex signers, selecting

@deftypefun void gpgme_signers_clear (@w{GpgmeCtx @var{ctx}})
The function @code{gpgme_signers_clear} releases a reference for each
key on the signers list and removes the list of signers from the
context @var{ctx}.

Every context starts with an empty list.
@end deftypefun

@deftypefun GpgmeError gpgme_signers_add (@w{GpgmeCtx @var{ctx}}, @w{const GpgmeKey @var{key}})
The function @code{gpgme_signers_add} adds the key @var{key} to the
list of signers in the context @var{ctx}.

Calling this function acquires an additional reference for the key.
@end deftypefun

@deftypefun GpgmeKey gpgme_signers_enum (@w{const GpgmeCtx @var{ctx}}, @w{int @var{seq}})
The function @code{gpgme_signers_enum} returns the @var{seq}th key in
the list of signers in the context @var{ctx}.  An additional reference
is acquired for the user.

If @var{seq} is out of range, @code{NULL} is returned.
@end deftypefun


@node Creating a Signature
@subsubsection Creating a Signature

@deftp {Data type} {enum GpgmeSigMode}
@tindex GpgmeSigMode
The @code{GpgmeSigMode} type is used to specify the desired type of a
signature.  The following modes are available:

@table @code
@item GPGME_SIG_MODE_NORMAL
A normal signature is made, the output includes the plaintext and the
signature.

@item GPGME_SIG_MODE_DETACH
A detached signature is made.

@item GPGME_SIG_MODE_CLEAR
A clear text signature is made.  The @acronym{ASCII} armor and text
mode settings of the context are ignored.
@end table
@end deftp

@deftypefun GpgmeError gpgme_op_sign (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{sig}}, @w{GpgmeSigMode @var{mode}})
The function @code{gpgme_op_sign} creates a signature for the text in
the data object @var{plain} and returns it in the data object
@var{sig}.  The type of the signature created is determined by the
@acronym{ASCII} armor and text mode attributes set for the context
@var{ctx} and the requested signature mode @var{mode}.

More information about the signatures is available with
@code{gpgme_get_op_info}.  @xref{Detailed Results}.

If an S/MIME signed message is created using the CMS crypto engine,
the number of certificates to include in the message can be specified
with @code{gpgme_set_include_certs}.  @xref{Included Certificates}.

The function returns @code{GPGME_No_Error} if the signature could be
created successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{plain} or @var{sig} is not a valid pointer, @code{GPGME_No_Data}
if the signature could not be created, @code{GPGME_No_Passphrase} if
the passphrase for the secret key could not be retrieved, and passes
through any errors that are reported by the crypto engine support
routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_sign_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{sig}}, @w{GpgmeSigMode @var{mode}})
The function @code{gpgme_op_sign_start} initiates a
@code{gpgme_op_sign} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, and @code{GPGME_Invalid_Value} if @var{ctx},
@var{plain} or @var{sig} is not a valid pointer.
@end deftypefun


@node Encrypt
@subsection Encrypt
@cindex encryption
@cindex cryptographic operation, encryption

One plaintext can be encrypted for several recipients at the same
time.  The list of recipients is created independently of any context,
and then passed to the encryption operation.

@menu
* Selecting Recipients::          How to choose the recipients.
* Encrypting a Plaintext::        How to encrypt a plaintext.
@end menu


@node Selecting Recipients
@subsubsection Selecting Recipients
@cindex encryption, selecting recipients
@cindex recipients

@deftp {Data type} GpgmeRecipients
The @code{GpgmeRecipients} type is a handle for a set of recipients
that can be used in an encryption process.
@end deftp

@deftypefun GpgmeError gpgme_recipients_new (@w{GpgmeRecipients *@var{r_rset}})
The function @code{gpgme_recipients_new} creates a new, empty set of
recipients and returns a handle for it in @var{r_rset}.

The function returns @code{GPGME_No_Error} if the recipient set could
be created successfully, and @code{GPGME_Out_Of_Core} if not enough
memory was available.
@end deftypefun

@deftypefun void gpgme_recipients_release (@w{GpgmeRecipients @var{rset}})
The function @code{gpgme_recipients_release} destroys the set of
recipients @var{rset} and releases all associated resources.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_add_name (@w{GpgmeRecipients @var{rset}}, @w{const char *@var{name}})
The function @code{gpgme_recipients_add_name} adds the recipient
@var{name} to the set of recipients @var{rset}.  This is equivalent to
@code{gpgme_recipients_add_name_with_validity} with a validity of
@code{GPGME_VALIDITY_UNKNOWN}.

The function returns @code{GPGME_No_Error} if the recipient was added
successfully, @code{GPGME_Invalid_Value} if @var{rset} or @var{name}
is not a valid pointer, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_add_name_with_validity (@w{GpgmeRecipients @var{rset}}, @w{const char *@var{name}}, @w{GpgmeValidity @var{val}})
The function @code{gpgme_recipients_add_name_with_validity} adds the
recipient @var{name} with the validity @var{val} to the set of
recipients @var{rset}.  If the validity is not known, the function
@code{gpgme_recipients_add_name} can be used.
@xref{Information About Keys}, for the possible values for @var{val}.

The function returns @code{GPGME_No_Error} if the recipient was added
successfully, @code{GPGME_Invalid_Value} if @var{rset} or @var{name}
is not a valid pointer, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun

@deftypefun {unsigned int} gpgme_recipients_count (@w{const @var{GpgmeRecipients rset}})
The function @code{gpgme_recipients_count} returns the number of
recipients in the set @var{rset}.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_enum_open (@w{const GpgmeRecipients @var{rset}}, @w{void **@var{iter}})
The function @code{gpgme_recipients_enum_open} creates a new iterator
@var{iter} that can be used to walk through the set of recipients in
@var{rset}, using @code{gpgme_recipients_enum_read}.

If the iterator is not needed anymore, it can be closed with
@code{gpgme_recipients_enum_close}.

The function returns @code{GPGME_No_Error} if the enumerator was
successfully created and @code{GPGME_Invalid_Value} if @var{rset} or
@var{iter} is not a valid pointer.
@end deftypefun

@deftypefun {const char *} gpgme_recipients_enum_read (@w{const GpgmeRecipients @var{rset}}, @w{void **@var{iter}})
The function @code{gpgme_recipients_enum_read} returns a string
containing the name of the next recipient in the set @var{rset} for
the iterator @var{iter}.  The string is valid as long as @var{rset} is
valid or the function is called the next time with the same recipient
set and iterator, whatever is earlier.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_enum_close (@w{const GpgmeRecipients @var{rset}}, @w{void **@var{iter}})
The function @code{gpgme_recipients_enum_close} releases the iterator
@var{iter} for the recipient set @var{rset}.
@end deftypefun


@node Encrypting a Plaintext
@subsubsection Encrypting a Plaintext

@deftypefun GpgmeError gpgme_op_encrypt (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{rset}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{cipher}})
The function @code{gpgme_op_encrypt} encrypts the plaintext in the data
object @var{plain} for the recipients @var{rset} and stores the
ciphertext in the data object @var{cipher}.  The type of the
ciphertext created is determined by the @acronym{ASCII} armor and text
mode attributes set for the context @var{ctx}.

If @code{GPGME_Invalid_Recipients} is returned, some recipients in
@var{rset} are invalid, but not all.  In this case the plaintext is
encrypted for all valid recipients and returned in @var{cipher}.  More
information about the invalid recipients is available with
@code{gpgme_get_op_info}.  @xref{Detailed Results}.

If @var{recp} is @code{NULL}, symmetric rather than public key
encryption is performed.  Symmetrically encrypted cipher text can be
deciphered with @code{gpgme_op_decrypt}.  Note that in this case the
crypto backend needs to retrieve a passphrase from the user.
Symmetric encryption is currently only supported for the OpenPGP
crypto backend.

The function returns @code{GPGME_No_Error} if the ciphertext could be
created successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{rset}, @var{plain} or @var{cipher} is not a valid pointer,
@code{GPGME_No_Recipients} if @var{rset} does not contain any valid
recipients, @code{GPGME_Invalid_Recipients} if @var{rset} contains
some invalid recipients, @code{GPGME_No_Passphrase} if the passphrase
for the secret key could not be retrieved, and passes through any
errors that are reported by the crypto engine support routines.
@end deftypefun

@deftypefun GpgmeError gpgme_op_encrypt_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{rset}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{cipher}})
The function @code{gpgme_op_encrypt_start} initiates a
@code{gpgme_op_encrypt} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{rset}, @var{plain} or @var{cipher} is not a valid pointer, and
@code{GPGME_No_Recipients} if @var{rset} does not contain any valid
recipients.
@end deftypefun


@deftypefun GpgmeError gpgme_op_encrypt_sign (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{rset}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{cipher}})
The function @code{gpgme_op_encrypt_sign} does a combined encrypt and
sign operation.  It is used like @code{gpgme_op_encrypt}, but the
ciphertext also contains signatures for the signers listed in
@var{ctx}.

The combined encrypt and sign operation is currently only available
for the OpenPGP crypto engine.
@end deftypefun

@deftypefun GpgmeError gpgme_op_encrypt_sign_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{rset}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{cipher}})
The function @code{gpgme_op_encrypt_sign_start} initiates a
@code{gpgme_op_encrypt_sign} operation.  It can be completed by
calling @code{gpgme_wait} on the context.  @xref{Waiting For
Completion}.

The function returns @code{GPGME_No_Error} if the operation could be
started successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{rset}, @var{plain} or @var{cipher} is not a valid pointer, and
@code{GPGME_No_Recipients} if @var{rset} does not contain any valid
recipients.
@end deftypefun


@node Detailed Results
@subsection Detailed Results
@cindex cryptographic operation, detailed results

@deftypefun {char *} gpgme_get_op_info (@w{GpgmeCtx @var{ctx}}, @w{int @var{reserved}})
The function @code{gpgme_get_op_info} retrieves more information about
the last crypto operation.

The function returns a string in the XML format.  The user has to
release the string with @code{free}.

Here is a sample of the information that might be returned:
@example
<GnupgOperationInfo>
  <signature>
    <detached/> <!-- or cleartext or standard -->
    <algo>17</algo>
    <hashalgo>2</hashalgo>
    <micalg>pgp-sha1</micalg>
    <sigclass>01</sigclass>
    <created>9222222</created>
    <fpr>121212121212121212</fpr>
  </signature>
</GnupgOperationInfo>
@end example

Currently, the only operations that return additional information are
encrypt, sign and import.  @xref{Encrypt}, @xref{Sign},
@xref{Importing Keys}.

The function returns a string or @code{NULL} if no such data is
available.
@end deftypefun


@node Run Control
@section Run Control
@cindex run control
@cindex cryptographic operation, running

@acronym{GPGME} supports running operations synchronously and
asynchronously.  You can use asynchronous operation to set up a
context up to initiating the desired operation, but delay performing
it to a later point.

Furthermore, you can use an external event loop to control exactly
when @acronym{GPGME} runs.  This ensures that @acronym{GPGME} only
runs when necessary and also prevents it from blocking for a long
time.

@menu
* Waiting For Completion::        Waiting until an operation is completed.
* Using External Event Loops::    Advanced control over what happens when.
@end menu


@node Waiting For Completion
@subsection Waiting For Completion
@cindex cryptographic operation, wait for
@cindex wait for completion

@deftypefun GpgmeCtx gpgme_wait (@w{GpgmeCtx @var{ctx}}, @w{GpgmeError *@var{status}}, @w{int @var{hang}})
The function @code{gpgme_wait} continues the pending operation within
the context @var{ctx}.  In particular, it ensures the data exchange
between @acronym{GPGME} and the crypto backend and watches over the
run time status of the backend process.

If @var{hang} is true, the function does not return until the
operation is completed or cancelled.  Otherwise the function will not
block for a long time.

The error status of the finished operation is returned in @var{status}
if @code{gpgme_wait} does not return @code{NULL}.

The @var{ctx} argument can be @code{NULL}.  In that case,
@code{gpgme_wait} waits for any context to complete its operation.

@code{gpgme_wait} can be used only in conjunction with any context
that has a pending operation initiated with one of the
@code{gpgme_op_*_start} functions except @code{gpgme_op_keylist_start}
and @code{gpgme_op_trustlist_start} (for which you should use the
corresponding @code{gpgme_op_*_next} functions).  If @var{ctx} is
@code{NULL}, all of such contexts are waited upon and possibly
returned.  Synchronous operations running in parallel, as well as key
and trust item list operations, do not affect @code{gpgme_wait}.

In a multi-threaded environment, only one thread should ever call
@code{gpgme_wait} at any time, irregardless if @var{ctx} is specified
or not.  This means that all calls to this function should be fully
synchronized by locking primitives.  It is safe to start asynchronous
operations while a thread is running in @code{gpgme_wait}.

The function returns the @var{ctx} of the context which has finished
the operation.  If @var{hang} is false, and the timeout expires,
@code{NULL} is returned and @code{*status} will be set to 0.  If an
error occurs, @code{NULL} is returned and the error is returned in
@code{*status}.
@end deftypefun


@node Using External Event Loops
@subsection Using External Event Loops
@cindex event loop, external

@acronym{GPGME} hides the complexity of the communication between the
library and the crypto engine.  The price of this convenience is that
the calling thread can block arbitrary long waiting for the data
returned by the crypto engine.  In single-threaded programs, in
particular if they are interactive, this is an unwanted side-effect.
OTOH, if @code{gpgme_wait} is used without the @var{hang} option being
enabled, it might be called unnecessarily often, wasting CPU time that
could be used otherwise.

The I/O callback interface described in this section lets the user
take control over what happens when.  @acronym{GPGME} will provide the
user with the file descriptors that should be monitored, and the
callback functions that should be invoked when a file descriptor is
ready for reading or writing.  It is then the user's responsibility to
decide when to check the file descriptors and when to invoke the
callback functions.  Usually this is done in an event loop, that also
checks for events in other parts of the program.  If the callback
functions are only called when the file descriptors are ready,
@acronym{GPGME} will never block.  This gives the user mroe control
over the program flow, and allows to perform other tasks when
@acronym{GPGME} would block otherwise.

By using this advanced mechanism, @acronym{GPGME} can be integrated
smoothly into GUI toolkits like GTK+ even for single-threaded
programs.

@menu
* I/O Callback Interface::        How I/O callbacks are registered.
* Registering I/O Callbacks::     How to use I/O callbacks for a context.
* I/O Callback Example::          An example how to use I/O callbacks.
* I/O Callback Example GTK+::     How to use @acronym{GPGME} with GTK+.
* I/O Callback Example GDK::      How to use @acronym{GPGME} with GDK.
@end menu


@node I/O Callback Interface
@subsubsection I/O Callback Interface

@deftp {Data type} {GpgmeError (*GpgmeIOCb) (@w{void *@var{data}}, @w{int @var{fd}})}
@tindex GpgmeIOCb
The @code{GpgmeIOCb} type is the type of functions which
@acronym{GPGME} wants to register as I/O callback handlers using the
@code{GpgmeRegisterIOCb} functions provided by the user.

@var{data} and @var{fd} are provided by @acronym{GPGME} when the I/O
callback handler is registered, and should be passed through to the
handler when it is invoked by the user because it noticed activity on
the file descriptor @var{fd}.

The callback handler always returns @code{0}, but you should consider
the return value to be reserved for later use.
@end deftp

@deftp {Data type} {GpgmeError (*GpgmeRegisterIOCb) (@w{void *@var{data}}, @w{int @var{fd}}, @w{int @var{dir}}, @w{GpgmeIOCb @var{fnc}}, @w{void *@var{fnc_data}}, @w{void **@var{tag}})}
@tindex GpgmeRegisterIOCb
The @code{GpgmeRegisterIOCb} type is the type of functions which can
be called by @acronym{GPGME} to register an I/O callback funtion
@var{fnc} for the file descriptor @var{fd} with the user.
@var{fnc_data} should be passed as the first argument to @var{fnc}
when the handler is invoked (the second argument should be @var{fd}).
If @var{dir} is 0, @var{fnc} should be called by the user when
@var{fd} is ready for writing.  If @var{dir} is 1, @var{fnc} should be
called when @var{fd} is ready for reading.

@var{data} was provided by the user when registering the
@code{GpgmeRegisterIOCb} function with @acronym{GPGME} and will always
be passed as the first argument when registering a callback function.
For example, the user can use this to determine the event loop to
which the file descriptor should be added.

@acronym{GPGME} will call this function when a crypto operation is
initiated in a context for which the user has registered I/O callback
handler functions with @code{gpgme_set_io_cbs}.  It can also call this
function when it is in an I/O callback handler for a file descriptor
associated to this context.

The user should return a unique handle in @var{tag} identifying this
I/O callback registration, which will be passed to the
@code{GpgmeRegisterIOCb} function without interpretation when the file
descriptor should not be monitored anymore.
@end deftp

@deftp {Data type} {void (*GpgmeRemoveIOCb) (@w{void *@var{tag}})}
The @code{GpgmeRemoveIOCb} type is the type of functions which can be
called by @acronym{GPGME} to remove an I/O callback handler that was
registered before.  @var{tag} is the handle that was returned by the
@code{GpgmeRegisterIOCb} for this I/O callback.

@acronym{GPGME} can call this function when a crypto operation is in
an I/O callback.  It will also call this function when the context is
destroyed while an operation is pending.
@end deftp

@deftp {Data type} {enum GpgmeEventIO}
@tindex GpgmeEventIO
The @code{GpgmeEventIO} type specifies the type of an event that is
reported to the user by @acronym{GPGME} as a consequence of an I/O
operation.  The following events are defined:

@table @code
@item GPGME_EVENT_START
The operation is fully initialized now, and you can start to run the
registered I/O callback handlers now.  Note that registered I/O
callback handlers must not be run before this event is signalled.
@var{type_data} is @code{NULL} and reserved for later use.

@item GPGME_EVENT_DONE
The operation is finished, the last I/O callback for this operation
was removed.  The accompanying @var{type_data} points to a
@code{GpgmeError} variable that contains the status of the operation
that finished.  This event is signalled after the last I/O callback
has been removed.

@item GPGME_EVENT_NEXT_KEY
In a @code{gpgme_op_keylist_start} operation, the next key was
received from the crypto engine.  The accompanying @var{type_data} is
a @code{GpgmeKey} variable that contains the key with one reference
for the user.

@item GPGME_EVENT_NEXT_TRUSTITEM
In a @code{gpgme_op_trustlist_start} operation, the next trust item
was received from the crypto engine.  The accompanying @var{type_data}
is a @code{GpgmeTrustItem} variable that contains the trust item with
one reference for the user.
@end table
@end deftp

@deftp {Data type} {void (*GpgmeEventIOCb) (@w{void *@var{data}}, @w{GpgmeEventIO @var{type}}, @w{void *@var{type_data}})}
The @code{GpgmeEventIOCb} type is the type of functions which can be
called by @acronym{GPGME} to signal an event for an operation running
in a context which has I/O callback functions registered by the user.

@var{data} was provided by the user when registering the
@code{GpgmeEventIOCb} function with @acronym{GPGME} and will always be
passed as the first argument when registering a callback function.
For example, the user can use this to determine the context in which
this event has occured.

@var{type} will specify the type of event that has occured.
@var{type_data} specifies the event further, as described in the above
list of possible @code{GpgmeEventIO} types.

@acronym{GPGME} can call this function in an I/O callback handler.
@end deftp


@node Registering I/O Callbacks
@subsubsection Registering I/O Callbacks

@deftp {Data type} {struct GpgmeIOCbs}
@tindex GpgmeEventIO
This structure is used to store the I/O callback interface functions
described in the previous section.  It has the following members:

@table @code
@item GpgmeRegisterIOCb add
This is the function called by @acronym{GPGME} to register an I/O
callback handler.  It must be specified.

@item void *add_data
This is passed as the first argument to the @code{add} function when
it is called by @acronym{GPGME}.  For example, it can be used to
determine the event loop to which the file descriptor should be added.

@item GpgmeRemoveIOCb remove
This is the function called by @acronym{GPGME} to remove an I/O
callback handler.  It must be specified.

@item GpgmeEventIOCb event
This is the function called by @acronym{GPGME} to signal an event for
an operation.  It is optional, but if you don't specify it, you can
not retrieve the return value of the operation.

@item void *event_data
This is passed as the first argument to the @code{event} function when
it is called by @acronym{GPGME}.  For example, it can be used to
determine the context in which the event has occured.
@end table
@end deftp

@deftypefun void gpgme_set_io_cbs (@w{GpgmeCtx @var{ctx}}, @w{struct GpgmeIOCbs *@var{io_cbs}})
The function @code{gpgme_set_io_cbs} enables the I/O callback
interface for the context @var{ctx}.  The I/O callback functions are
specified by @var{io_cbs}.

If @var{io_cbs}->@code{add} is @code{NULL}, the I/O callback interface
is disabled for the context, and normal operation is restored.
@end deftypefun

@deftypefun void gpgme_get_io_cbs (@w{GpgmeCtx @var{ctx}}, @w{struct GpgmeIOCbs *@var{io_cbs}})
The function @code{gpgme_get_io_cbs} returns the I/O callback
functions set with @code{gpgme_set_io_cbs} in @var{io_cbs}.
@end deftypefun


@node I/O Callback Example
@subsubsection I/O Callback Example

To actually use an external event loop, you have to implement the I/O
callback functions that are used by @acronym{GPGME} to register and
unregister file descriptors.  Furthermore, you have to actually
monitor these file descriptors for activity and call the appropriate
I/O callbacks.

The following example illustrates how to do that.  The example uses
locking to show in which way the the callbacks and the event loop can
run concurrently.  For the event loop, we use a fixed array.  For a
real-world implementation, you should use a dynamically sized
structure because the number of file descriptors needed for a crypto
operation in @acronym{GPGME} is not predictable.

@example
#include <pthread.h>
#include <sys/types.h>
#include <gpgme.h>

/* The following structure holds the result of a crypto operation.  */
struct op_result
@{
  int done;
  GpgmeError err;
@};

/* The following structure holds the data associated with one I/O
callback.  */
struct one_fd
@{
  int fd;
  int dir;
  GpgmeIOCb fnc;
  void *fnc_data;
@};

struct event_loop
@{
  pthread_mutex_t lock;
#define MAX_FDS 32
  /* Unused slots are marked with FD being -1.  */
  struct one_fd fds[MAX_FDS];
@};
@end example

The following functions implement the I/O callback interface.

@example
GpgmeError
add_io_cb (void *data, int fd, int dir, GpgmeIOCb fnc, void *fnc_data,
	   void **r_tag)
@{
  struct event_loop *loop = data;
  struct one_fd *fds = loop->fds;
  int i;

  pthread_mutex_lock (&loop->lock);
  for (i = 0; i < MAX_FDS; i++)
    @{
      if (fds[i].fd == -1)
	@{
	  fds[i].fd = fd;
	  fds[i].dir = dir;
	  fds[i].fnc = fnc;
	  fds[i].fnc_data = fnc_data;
	  break;
	@}
    @}
  pthread_mutex_unlock (&loop->lock);
  if (i == MAX_FDS)
    return GPGME_General_Error;
  *r_tag = &fds[i];
  return 0;
@}

void
remove_io_cb (void *tag)
@{
  struct one_fd *fd = tag;

  pthread_mutex_lock (&loop->lock);
  fd->fd = -1;
  pthread_mutex_unlock (&loop->lock);
@}

void
event_io_cb (void *data, GpgmeEventIO type, void *type_data)
@{
  struct op_result *result = data;
  GpgmeError *err = data;

  /* We don't support list operations here.  */
  if (type == GPGME_EVENT_DONE)
    @{
      result->done = 1;
      result->err = *data;
    @}
@}
@end example

The final missing piece is the event loop, which will be presented
next.  We only support waiting for the success of a single operation.

@example
int
do_select (struct event_loop *loop)
@{
  fd_set rfds;
  fd_set wfds;
  int i, n;
  int any = 0;

  pthread_mutex_lock (&loop->lock);
  FD_ZERO (&rfds);
  FD_ZERO (&wfds);
  for (i = 0; i < FDLIST_MAX; i++)
    if (fdlist[i].fd != -1)
      FD_SET (fdlist[i].fd, fdlist[i].dir ? &rfds : &wfds);
  pthread_mutex_unlock (&loop->unlock);

  do
    @{
      n = select (FD_SETSIZE, &rfds, &wfds, NULL, 0);
    @}
  while (n < 0 && errno == EINTR);

  if (n < 0)
    return n;	/* Error or timeout.  */

  pthread_mutex_lock (&loop->lock);
  for (i = 0; i < FDLIST_MAX && n; i++)
    @{
      if (fdlist[i].fd != -1)
	@{
	  if (FD_ISSET (fdlist[i].fd, fdlist[i].dir ? &rfds : &wfds))
	    @{
	      assert (n);
	      n--;
	      any = 1;
              /* The I/O callback handler can register/remove callbacks,
                 so we have to unlock the file descriptor list.  */
              pthread_mutex_unlock (&loop->lock);
	      (*fdlist[i].fnc) (fdlist[i].fnc_data, fdlist[i].fd);
              pthread_mutex_lock (&loop->lock);
	    @}
	@}
    @}
  pthread_mutex_unlock (&loop->lock);
  return any;
@}

void
wait_for_op (struct event_loop *loop, struct op_result *result)
@{
  int ret;

  do
    @{
      ret = do_select (loop);
    @}
  while (ret >= 0 && !result->done);
  return ret;
@}
@end example

The main function shows how to put it all together.

@example
int
main (int argc, char *argv[])
@{
  struct event_loop loop;
  struct op_result result;
  GpgmeCtx ctx;
  GpgmeError err;
  GpgmeData sig, text;
  GpgmeSigStat status;
  int i;
  struct GpgmeIOCbs io_cbs =
  @{
    add_io_cb,
    &loop,
    remove_io_cb,
    event_io_cb,
    &result
  @};

  /* Initialize the loop structure.  */
  loop.lock = PTHREAD_MUTEX_INITIALIZER;
  for (i = 0; i < MAX_FDS; i++)
    loop->fds[i].fd = -1;

  /* Initialize the result structure.  */
  result.done = 0;

  err = gpgme_data_new_from_file (&sig, "signature", 1);
  if (!err)
    err = gpgme_data_new_from_file (&text, "text", 1);
  if (!err)
    err = gpgme_new (&ctx);
  if (!err)
    @{
       gpgme_set_io_cbs (ctx, &io_cbs);
       err = gpgme_op_verify_start (ctx, sig, text, &status);
    @}
  if (err)
    @{
      fprintf (stderr, "gpgme error: %s\n", gpgme_strerror (err));
      exit (1);
    @}

  wait_for_op (&loop, &result);
  if (!result.done)
    @{
      fprintf (stderr, "select error\n");
      exit (1);
    @}
  if (!result.err)
    @{
      fprintf (stderr, "verification failed: %s\n", gpgme_strerror (result.err));
      exit (1);
    @}
  /* Evaluate STATUS.  */
  @dots{}
  return 0;
@}
@end example


@node I/O Callback Example GTK+
@subsubsection I/O Callback Example GTK+
@cindex GTK+, using @acronym{GPGME} with

The I/O callback interface can be used to integrate @acronym{GPGME}
with the GTK+ event loop.  The following code snippets shows how this
can be done using the appropriate register and remove I/O callback
functions.  In this example, the private data of the register I/O
callback function is unused.  The event notifications is missing
because it does not require any GTK+ specific setup.

@example
#include <gtk/gtk.h>

struct my_gpgme_io_cb
@{
  GpgmeIOCb fnc;
  void *fnc_data;
  guint input_handler_id
@};

void
my_gpgme_io_cb (gpointer data, gint source, GdkInputCondition condition)
@{
  struct my_gpgme_io_cb *iocb = data;
  (*(iocb->fnc)) (iocb->data, source);
@}

void
my_gpgme_remove_io_cb (void *data)
@{
  struct my_gpgme_io_cb *iocb = data;
  gtk_input_remove (data->input_handler_id);
@}

void
my_gpgme_register_io_callback (void *data, int fd, int dir, GpgmeIOCb fnc,
                               void *fnc_data, void **tag)
@{
  struct my_gpgme_io_cb *iocb = g_malloc (sizeof (struct my_gpgme_io_cb));
  iocb->fnc = fnc;
  iocb->data = fnc_data;
  iocb->input_handler_id = gtk_input_add_full (fd, dir
                                                   ? GDK_INPUT_READ
                                                   : GDK_INPUT_WRITE,
                                               my_gpgme_io_callback,
                                               0, iocb, NULL);
  *tag = iocb;
  return 0;
@}
@end example


@node I/O Callback Example GDK
@subsubsection I/O Callback Example GDK
@cindex GDK, using @acronym{GPGME} with

The I/O callback interface can also be used to integrate
@acronym{GPGME} with the GDK event loop.  The following code snippets
shows how this can be done using the appropriate register and remove
I/O callback functions.  In this example, the private data of the
register I/O callback function is unused.  The event notifications is
missing because it does not require any GDK specific setup.

It is very similar to the GTK+ example in the previous section.

@example
#include <gdk/gdk.h>

struct my_gpgme_io_cb
@{
  GpgmeIOCb fnc;
  void *fnc_data;
  gint tag;
@};

void
my_gpgme_io_cb (gpointer data, gint source, GdkInputCondition condition)
@{
  struct my_gpgme_io_cb *iocb = data;
  (*(iocb->fnc)) (iocb->data, source);
@}

void
my_gpgme_remove_io_cb (void *data)
@{
  struct my_gpgme_io_cb *iocb = data;
  gdk_input_remove (data->tag);
@}

void
my_gpgme_register_io_callback (void *data, int fd, int dir, GpgmeIOCb fnc,
                               void *fnc_data, void **tag)
@{
  struct my_gpgme_io_cb *iocb = g_malloc (sizeof (struct my_gpgme_io_cb));
  iocb->fnc = fnc;
  iocb->data = fnc_data;
  iocb->tag = gtk_input_add_full (fd, dir ? GDK_INPUT_READ : GDK_INPUT_WRITE,
                                  my_gpgme_io_callback, iocb, NULL);
  *tag = iocb;
  return 0;
@}
@end example


@include gpl.texi


@include fdl.texi


@node Concept Index
@unnumbered Concept Index

@printindex cp


@node Function and Data Index
@unnumbered Function and Data Index

@printindex fn


@summarycontents
@contents
@bye