core: fix build on musl libc (Alpine) - stray #undef _GNU_SOURCE in lib/url.c

[Lt-Flash]

What

Fixes the OpenSIPS core failing to compile on musl libc (Alpine Linux) with a modern GCC. A single stray #undef _GNU_SOURCE in lib/url.c hides the POSIX time prototypes on musl, so the very first core object (lib/url.o) never builds.

Symptom

Building master in a stock Alpine 3.24 container (musl 1.2.x, gcc 15.2.0) dies immediately:

Compiling lib/url.c
lib/../mem/../dprint.h:204:9: error: implicit declaration of function 'ctime_r'; did you mean 'ctime'?
lib/../ut.h:1401:5:  error: implicit declaration of function 'clock_gettime'; did you mean 'lock_get'?
lib/../ut.h:1401:19: error: 'CLOCK_REALTIME' undeclared (first use in this function)
make: *** [Makefile.rules:28: lib/url.o] Error 1

With gcc 14+, -Wimplicit-function-declaration is an error by default, so this is a hard build failure, not a warning. The same tree builds fine on glibc.

Root cause

lib/url.c opens with:

#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#undef _GNU_SOURCE      /* <-- the problem */

#include "../mem/mem.h"
#include "../ut.h"       /* pulls in <time.h>: clock_gettime, ctime_r, CLOCK_REALTIME */

The difference between the two libcs is when the feature-test macros are evaluated:

• musl re-checks the feature-test macros in every system header. Its features.h only turns on the permissive default set (_BSD_SOURCE + _XOPEN_SOURCE 700, which is what exposes clock_gettime/ctime_r) when no source macro is defined:

  #if !defined(_POSIX_SOURCE) && !defined(_POSIX_C_SOURCE) \
   && !defined(_XOPEN_SOURCE) && !defined(_GNU_SOURCE) \
   && !defined(_BSD_SOURCE) && !defined(__STRICT_ANSI__)
  #define _BSD_SOURCE 1
  #define _XOPEN_SOURCE 700
  #endif

  When <stdio.h> first includes features.h, _GNU_SOURCE is defined, so that default block is skipped and features.h latches (include guard). The following #undef _GNU_SOURCE then leaves the translation unit with no feature-test macro at all. When ut.h later includes <time.h>, features.h is guard-skipped and never re-derives the defaults, so time.h's guarded prototypes (clock_gettime, ctime_r, CLOCK_REALTIME) are all absent -> implicit declarations -> build error.

• glibc latches internal __USE_POSIX*/__USE_GNU macros at the first header inclusion. The later #undef _GNU_SOURCE does not clear those, so the prototypes stay visible and the build succeeds. That is why this has gone unnoticed.

A one-line reproducer confirms the mechanism on musl:

#define _GNU_SOURCE
#include <stdio.h>
#undef _GNU_SOURCE
#include <time.h>
int main(void){ struct timespec t; clock_gettime(CLOCK_REALTIME,&t); return 0; }
// gcc: implicit declaration of 'clock_gettime'; 'CLOCK_REALTIME' undeclared
// remove the #undef line -> compiles clean

The fix

Drop the #undef and keep _GNU_SOURCE defined for the whole translation unit (with a comment explaining why). This matches how other core sources already enable it unconditionally (io_wait.h, transformations.c), and is a no-op on glibc.

+/* _GNU_SOURCE must stay defined for the rest of this file. On musl libc
+ * the feature-test macros are re-evaluated by every system header, so
+ * undefining it here would hide the clock_gettime()/ctime_r() prototypes
+ * that ut.h and dprint.h (via <time.h>) rely on, breaking the build. */
 #define _GNU_SOURCE
 #include <stdio.h>
 #include <string.h>
-#undef _GNU_SOURCE
 
 #include "../mem/mem.h"
 #include "../ut.h"

Testing

• Alpine 3.24 / musl 1.2.x / gcc 15.2.0: make opensips fails at lib/url.o before this change; builds to completion after it. Full core binary produced, modules build and load, runtime verified.
• Debian / glibc: unchanged (the #undef was already a no-op there).

lib/url.c is the only core file that uses this #define/#undef _GNU_SOURCE bracket, so this one file is the whole fix for the core.

---

Opening as a draft for maintainer review of the approach (per-file fix here vs. defining _GNU_SOURCE tree-wide in the build).