[libc++] Provide c++03 alternative for va_copy if available in xlocale.h

[jasonliu]

Summary

If we are on c++03 mode for some reason, and __va_copy is available, then use it instead of error out on not having va_copy in 03 mode.

[ldionne]

libcxx/include/__support/ibm/xlocale.h

314

This is part of the C standard and should be provided by your implementation. Yes, I know C++03 is technically not built on top of C99 IIRC, but asking your implementation to support it as an extension is the way to go IMO. Using an identifier internal to the specific C library you're using isn't a good solution if there are other alternatives IMO.

[jasonliu]

Use __builtin_va_copy instead.

[jasonliu]

libcxx/include/__support/ibm/xlocale.h

314

It might not be a good idea to expose va_copy in non-C99 or above mode because it could pollute user's namespace.
I used the builtin version here instead. Let me know if that address your concern.

[jasonliu]

Gentle ping.

[jasonliu]

Ping.
@ldionne Any comments?

[ldionne]

libcxx/include/__support/ibm/xlocale.h

314

This seems reasonable to me, but please add a comment like // va_copy may not be provided by the C library in C++03 mode

libcxx/include/__support/ibm/xlocale.h

 static inline
 int vasprintf(char **strp, const char *fmt, va_list ap) {
   const size_t buff_size = 256;
   if ((*strp = (char *)malloc(buff_size)) == NULL) {
     return -1;
   }
 
   va_list ap_copy;
+  // va_copy may not be provided by the C library in C++ 03 mode.
+#if defined(_LIBCPP_CXX03_LANG) && __has_builtin(__builtin_va_copy)
+  __builtin_va_copy(ap_copy, ap);
+#else
   va_copy(ap_copy, ap);
+#endif
   int str_size = vsnprintf(*strp, buff_size, fmt,  ap_copy);
   va_end(ap_copy);
 
   if ((size_t) str_size >= buff_size) {
     if ((*strp = (char *)realloc(*strp, str_size + 1)) == NULL) {
       return -1;
     }
     str_size = vsnprintf(*strp, str_size + 1, fmt,  ap);
   }
   return str_size;
 }
 
 #ifdef __cplusplus
 }
 #endif
 #endif // _LIBCPP_SUPPORT_IBM_XLOCALE_H