build: add portable getprogname() support

Add a compatibility layer for getprogname() to allow utilities to
retrieve the short program name uniformly, removing the need for
manual argv[0] parsing.

The logic prioritizes the GNU extension variable
'program_invocation_short_name', falling back to getprogname.

If neither is available, the build fails with an #error.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>

configure.ac

@@ -182,6 +182,13 @@ elif test -f /etc/mtab; then
    mtab_type=file
 fi
 
+# Check program_invocation_short_name and getprogname()
+AC_CHECK_DECLS([program_invocation_short_name], [], [],
+               [[#include <errno.h>]])
+
+AS_IF([test "x$ac_cv_have_decl_program_invocation_short_name" != "xyes"],
+      [AC_CHECK_FUNCS([getprogname])])
+
 # Check for conditional libraries and headers.
 AS_IF([test "${with_blkid}" = "yes"], [
     NILFS_UTILS_PKG_CHECK([BLKID], [blkid], [

include/compat.h

@@ -203,4 +203,14 @@
 #endif  /* HAVE_SYS_SYSMACROS_H */
 
 
+#ifndef HAVE_GETPROGNAME
+#if defined(HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME) && \
+    HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME
+#include <errno.h>
+#define getprogname() ((const char *)program_invocation_short_name)
+#else
+#error "No way to retrieve the program name"
+#endif
+#endif  /* HAVE_GETPROGNAME */
+
 #endif /* __COMPAT_H__ */