string.h on rhES 7

[Tom Tromey]

Hi.  gdb recently updated gnulib to 

GNULIB_COMMIT_SHA1="0cda5beb7962f6567f0c4e377df870fa05c6d681"

This caused a build failure here at AdaCore, where we are using a
relatively recent GCC (11.2) on an rhES 7 host:

  CC       libgnu_a-openat-proc.o
In file included from /usr/include/string.h:633,
                 from ./string.h:41,
                 from ../../../binutils-gdb/gnulib/import/openat-proc.c:30:
./string.h:1105:1: error: expected identifier or '(' before '__extension__'
 1105 | _GL_FUNCDECL_SYS (strndup, char *,
      | ^~~~~~~~~~~~~~~~

We worked around this locally by reverting gnulib commit cbdb5ea63.

Today I looked into it a bit more and found that, after preprocessing,
the line in question looks like:

extern char * (__extension__ (__builtin_constant_p (char const *__s) && 
((size_t)(const void *)((char const *__s) + 1) - (size_t)(const void *)(char 
const *__s) == 1) ? (((const char *) (char const *__s))[0] == '\0' ? (char *) 
calloc ((size_t) 1, (size_t) 1) : ({ size_t __len = strlen (char const *__s) + 
1; size_t __n = (size_t __n); char *__retval; if (__n < __len) __len = __n + 1; 
__retval = (char *) malloc (__len); if (__retval != ((void *)0)) { 
__retval[__len - 1] = '\0'; __retval = (char *) memcpy (__retval, char const 
*__s, __len - 1); } __retval; })) : __strndup (char const *__s, size_t __n))) 
__attribute__ ((__nonnull__ (1))) 


That is, there's an unexpected macro expansion coming from glibc:

# 1314 "/usr/include/bits/string2.h" 3 4
[...]
#define __strndup(s,n) (__extension__ (__builtin_constant_p (s) && 
__string2_1bptr_p (s) ? (((const char *) (s))[0] == '\0' ? (char *) calloc 
((size_t) 1, (size_t) 1) : ({ size_t __len = strlen (s) + 1; size_t __n = (n); 
char *__retval; if (__n < __len) __len = __n + 1; __retval = (char *) malloc 
(__len); if (__retval != NULL) { __retval[__len - 1] = '\0'; __retval = (char 
*) memcpy (__retval, s, __len - 1); } __retval; })) : __strndup (s, n)))
# 1342 "/usr/include/bits/string2.h" 3 4
#define strndup(s,n) __strndup (s, n)


I'm not 100% sure of the best way to fix this in gnulib, as I'm not
familiar with all the ins and outs of working on it, but I can report
that the appended works.

thanks,
Tom

diff --git a/gnulib/import/string.in.h b/gnulib/import/string.in.h
index b6840fa9121..cb344ed0498 100644
--- a/gnulib/import/string.in.h
+++ b/gnulib/import/string.in.h
@@ -594,6 +594,7 @@ _GL_CXXALIAS_SYS (strndup, char *, (char const *__s, size_t 
__n));
 _GL_CXXALIASWARN (strndup);
 #else
 # if __GNUC__ >= 11
+#undef strndup
 /* For -Wmismatched-dealloc: Associate strndup with free or rpl_free.  */
 _GL_FUNCDECL_SYS (strndup, char *,
                   (char const *__s, size_t __n)