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.

5 files changed, 23 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 810c3219e4f..79fe0c98c6f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3697,6 +3697,8 @@ AC_SUBST_FILE([module_env_snippet_27])
 module_env_snippet_25="$srcdir/src/module-env-25.h"
 module_env_snippet_26="$srcdir/src/module-env-26.h"
 module_env_snippet_27="$srcdir/src/module-env-27.h"
+emacs_major_version="${PACKAGE_VERSION%%.*}"
+AC_SUBST(emacs_major_version)
 
 ### Use -lpng if available, unless '--with-png=no'.
 HAVE_PNG=no
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
diff --git a/etc/NEWS b/etc/NEWS
index cf6f4fea3e9..9b32d720b62 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1933,6 +1933,9 @@ convert between timespec structures and Emacs Lisp time values.
 'extract_big_integer' to create and extract arbitrary-size integer
 values.
 
+** emacs-module.h now defines a macro EMACS_MAJOR_VERSION that expands
+to the major version of the latest Emacs supported by the header.
+
 
 * Changes in Emacs 27.1 on Non-Free Operating Systems
 
diff --git a/src/emacs-module.h.in b/src/emacs-module.h.in
index fbc62a61ef4..9955e30eb7a 100644
--- a/src/emacs-module.h.in
+++ b/src/emacs-module.h.in
@@ -32,6 +32,8 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include <gmp.h>
 #endif
 
+#define EMACS_MAJOR_VERSION @emacs_major_version@
+
 #if defined __cplusplus && __cplusplus >= 201103L
 # define EMACS_NOEXCEPT noexcept
 #else
diff --git a/test/data/emacs-module/mod-test.c b/test/data/emacs-module/mod-test.c
index b7007bd80ff..a9154fa167d 100644
--- a/test/data/emacs-module/mod-test.c
+++ b/test/data/emacs-module/mod-test.c
@@ -445,6 +445,11 @@ bind_function (emacs_env *env, const char *name, emacs_value Sfun)
 int
 emacs_module_init (struct emacs_runtime *ert)
 {
+  /* Check that EMACS_MAJOR_VERSION is defined and an integral
+     constant.  */
+  char dummy[EMACS_MAJOR_VERSION];
+  assert (27 <= sizeof dummy);
+
   if (ert->size < sizeof *ert)
     {
       fprintf (stderr, "Runtime size of runtime structure (%"pT" bytes) "