move contents of HACKING.md to a doc/HOWTO

reference list of files and removed -Werror from instructions on adding
functions rename HACKING.md to HOWTO.md.

Reviewed-by: Eugene Syromiatnikov <[email protected]>
Reviewed-by: Frederik Wedel-Heinen <[email protected]>
MergeDate: Mon Apr 27 06:14:57 2026
(Merged from https://github.com/openssl/openssl/pull/18003)

1 files changed, 85 insertions, 0 deletions

diff --git a/doc/HOWTO/adding-functions.md b/doc/HOWTO/adding-functions.md
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+ADDING FUNCTIONS to OPENSSL
+===========================
+
+This document describes the way to add custom modifications to OpenSSL
+sources.
+
+If you are adding new C source files
+------------------------------------
+
+Please update the `build.info` files in the directories where you placed the
+C source files, to include a line like this for each new C source file:
+
+-   In `crypto/` or any of its subdirectories (intended for `libcrypto`):
+
+        SOURCE[../libcrypto]={name-of-C-source-file}
+
+-   In `ssl/` or any of its subdirectories (intended for `libssl`):
+
+        SOURCE[../libssl]={name-of-C-source-file}
+
+Do note that the path given as the `SOURCE` attribute must be adapted
+appropriately for the location of the `build.info` file, as it's a relative
+path to where the library itself is built, for example:
+
+-   For `crypto/build.info`, the library path should be `../libcrypto`
+-   For `crypto/evp/build.info`, the library path should be
+    `../../libcrypto`
+-   For `ssl/build.info`, the library path should be `../libssl`
+-   For `ssl/quic/build.info`, the library path should be `../../libssl`
+
+To know more about `build.info` files, please read [doc/internal/man7/build.info.pod].
+For better viewing, consider converting it to HTML or PDF using `pod2html`
+or `pod2pdf`.
+
+Adding new public functions
+---------------------------
+
+If you are adding new public functions to the custom library build, you need to
+either add a prototype in one of the existing OpenSSL header files, or
+provide a new header file and edit.
+
+Only headers in the `include/openssl` subdirectory are considered for public
+functions.  If you're creating a new header file, it must be located in that
+directory.
+
+Functions declared in `include/openssl` header files are assumed to be part
+of the `libcrypto` library unless specified otherwise.  *If your new
+functions are meant for the `libssl` library*, you will need to edit
+[Configurations/unix-Makefile.tmpl] and add the header file name in the
+array `my @sslheaders_tmpl`.
+
+Updating OpenSSL's bookkeeping files
+------------------------------------
+
+OpenSSL has a few bookkeeping files to keep track of exposed functions, most
+importantly `util/libcrypto.num` and `util/libssl.num`.  Any time a new
+public function - as defined above - is added, these files must be updated.
+
+To make such an update, please do the following:
+
+    ./Configure --strict-warnings [your-options]
+    make update
+
+If you plan to submit the changes you made to OpenSSL (see
+[CONTRIBUTING.md]), it's also worth running the following after running
+`make update`, to ensure that documentation has correct format.
+
+    make doc-nits
+
+`make update` ensures that your functions declarations are added to
+`util/libcrypto.num` or `util/libssl.num`.
+It also generates files related to OIDs (in the `crypto/objects/`  folder)
+and error messages.
+
+More details are at
+  [doc/HOWTO/documenting-functions-macros.md](Documenting Functions and Macros)
+
+If a git merge error occurs in one of these generated files, then the
+generated files need to be removed and regenerated using `make update`.
+To aid in this process, the generated files should be committed separately
+so they can be removed easily by reverting that commit.
+
+[doc/internal/man7/build.info.pod]: ../doc/internal/man7/build.info.pod
+[Configurations/unix-Makefile.tmpl]: ../../Configurations/unix-Makefile.tmpl
+[CONTRIBUTING.md]: ../../CONTRIBUTING.md