Export major version of latest Emacs supported by emacs-module.h.

This is useful if module authors want to support multiple versions of
emacs-module.h.

* configure.ac (emacs_major_version): Define substitution.

* src/emacs-module.h.in (EMACS_MAJOR_VERSION): Define macro.

* doc/lispref/internals.texi (Module Initialization): Document
EMACS_MAJOR_VERSION preprocessor macro.

* test/data/emacs-module/mod-test.c (emacs_module_init): Verify
behavior of EMACS_MAJOR_VERSION.

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index 5ae71afbef2..cfeb492af40 100644
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -1191,6 +1191,17 @@ grow with new Emacs releases.  Given the version of Emacs, the module
 can use only the parts of the module @acronym{API} that existed in
 that version, since those parts are identical in later versions.
 
+@file{emacs-module.h} defines a preprocessor macro
+@code{EMACS_MAJOR_VERSION}.  It expands to an integer literal which is
+the latest major version of Emacs supported by the header.
+@xref{Version Info}.  Note that the value of
+@code{EMACS_MAJOR_VERSION} is a compile-time constant and does not
+represent the version of Emacs that is currently running and has
+loaded your module.  If you want your module to be compatible with
+various versions of @file{emacs-module.h} as well as various versions
+of Emacs, you can use conditional compilation based on
+@code{EMACS_MAJOR_VERSION}.
+
 We recommend that modules always perform the compatibility
 verification, unless they do their job entirely in the initialization
 function, and don't access any Lisp objects or use any Emacs functions