[PATCH] flex: fix stage1flex build under a C23 default

[Aurelien DESBRIERES <aurelien@hackers.camp>]

lib/malloc.c declares "void *malloc ();" with an empty parameter list
and calls it with one argument. That meant "unspecified" in C89 and
means "none" in C23, so the call is rejected:

  lib/malloc.c:16:15: error: too many arguments to function 'malloc';
                             expected 0, have 1

It is the language mode that decides, not the compiler version: the
same GCC 16 compiles the file with -std=gnu17 and rejects it with
-std=gnu23. GCC 15 made gnu23 the default, so every host from that
release on hits it while GCC 14 does not.

The file is compiled by stage1flex -- the bootstrap scanner flex builds
with the host compiler before it can build itself -- so neither CFLAGS
nor BUILD_CFLAGS reaches that command line and no flag in the recipe
can silence it.

stdlib.h has the right declaration and the file already includes
sys/types.h for size_t. lib/realloc.c includes stdlib.h already and
needs no change.

Signed-off-by: Aurelien DESBRIERES <aurelien@hackers.camp>
---
 ...b-malloc-declare-malloc-via-stdlib.h.patch | 38 +++++++++++++++++++
 meta/recipes-devtools/flex/flex_2.6.4.bb      |  1 +
 2 files changed, 39 insertions(+)
 create mode 100644 meta/recipes-devtools/flex/flex/0001-lib-malloc-declare-malloc-via-stdlib.h.patch

diff --git a/meta/recipes-devtools/flex/flex/0001-lib-malloc-declare-malloc-via-stdlib.h.patch b/meta/recipes-devtools/flex/flex/0001-lib-malloc-declare-malloc-via-stdlib.h.patch
new file mode 100644
index 0000000000..a64d6220a8
--- /dev/null
+++ b/meta/recipes-devtools/flex/flex/0001-lib-malloc-declare-malloc-via-stdlib.h.patch
@@ -0,0 +1,38 @@
+From: Aurelien Desbrieres <aurelien@hackers.camp>
+Date: Sat, 13 Sep 2026 00:00:00 +0200
+Subject: [PATCH] lib/malloc.c: declare malloc via stdlib.h
+
+The gnulib fallback declares "void *malloc ();" with an empty parameter
+list and calls it with one argument. That meant "unspecified" in C89 and
+means "none" in C23, which GCC 14 and later implement by default, so the
+call is rejected:
+
+  lib/malloc.c:16:15: error: too many arguments to function 'malloc';
+                             expected 0, have 1
+
+stdlib.h has the right declaration and the file already includes
+sys/types.h for size_t, so the local one has nothing to add.
+lib/realloc.c includes stdlib.h already and needs no change.
+
+The file is dead code wherever malloc(0) returns non-NULL -- glibc
+included -- since AC_FUNC_MALLOC substitutes rpl_malloc only where it
+does not, but it is compiled regardless and the build stops there.
+
+Upstream-Status: Inappropriate [flex 2.6.4 is the last release, 2017]
+
+Signed-off-by: Aurelien Desbrieres <aurelien@hackers.camp>
+---
+ lib/malloc.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/lib/malloc.c
++++ b/lib/malloc.c
+@@ -3,7 +3,7 @@
+      
+      #include <sys/types.h>
+      
+-     void *malloc ();
++#include <stdlib.h>
+      
+      /* Allocate an N-byte block of memory from the heap.
+         If N is zero, allocate a 1-byte block.  */
diff --git a/meta/recipes-devtools/flex/flex_2.6.4.bb b/meta/recipes-devtools/flex/flex_2.6.4.bb
index 793a935962..96c596764b 100644
--- a/meta/recipes-devtools/flex/flex_2.6.4.bb
+++ b/meta/recipes-devtools/flex/flex_2.6.4.bb
@@ -19,6 +19,7 @@ SRC_URI = "${GITHUB_BASE_URI}/download/v${PV}/flex-${PV}.tar.gz \
            file://check-funcs.patch \
            file://0001-Emit-no-line-directives-if-gen_line_dirs-is-false.patch \
            file://0001-build-tests-add-missing-parser-scanner-dependencies.patch \
+           file://0001-lib-malloc-declare-malloc-via-stdlib.h.patch \
            "
 
 SRC_URI[md5sum] = "2882e3179748cc9f9c23ec593d6adc8d"
-- 
2.55.0