Make stdarg.h compatible with FreeBSD

[arichardson]

Summary

On FreeBSD it is currently not possible to use the clang builtin
headers to build the base system. The build will fail with the
following error if I don't delete the std* headers from the clang
install directory:

/sources/freebsd-mips/include/stdio.h:72:19: error:
redefinition of typedef 'va_list' is a C11 feature
[-Werror,-Wtypedef-redefinition]
typedef __va_list       va_list;
                        ^
/home/alr48/cheri/output/sdk/lib/clang/7.0.0/include/stdarg.h:30:27:
note: previous definition is here
typedef __builtin_va_list va_list;

/sources/freebsd-mips/sys/sys/_stdarg.h:46:11: error: 'va_start' macro redefined
[-Werror,-Wmacro-redefined]
  #define       va_start(ap, last)      __builtin_va_start((ap), (last))
                ^
/home/alr48/cheri/output/sdk/lib/clang/7.0.0/include/stdarg.h:35:9:
note: previous definition is here
  #define va_start(ap, param) __builtin_va_start(ap, param)

FreeBSD includes definitions of va_list, va_copy, etc in the
<machine/stdarg.h> header and some files end up including both that
header and <stdarg.h>.

Only defining the va_* macros if they are not yet defined fixes the
macro redefiniton issue and probably also makes sense for other
operating systems. However, for the va_list typedef I can't think
of a better solution than to also check for _VA_LIST_DECLARED and
define it.

[dim]

I'm fine with either this approach, or with the approach in stdint.h, e.g. using __has_include_next(), and in that case first including the system-provided header before this one.

[krytarowski]

This does not fix the potential problem on NetBSD. We define __VA_LIST_DECLARED (with extra _ on the beginning).

The proper solution looks to go for:

#if __STDC_HOSTED__ && __has_include_next(<stdarg.h>)

[bsdjhb]

FWIW, I ended up fixing FreeBSD to only use <machine/stdarg.h> in freestanding environments and always use <stdarg.h> in userland which makes this patch no longer necessary. (Only one place needed to be fixed.)

[arichardson]

Hopefully no longer required.

lib/Headers/stdarg.h

  * THE SOFTWARE.
  *
  *===-----------------------------------------------------------------------===
  */
 
 #ifndef __STDARG_H
 #define __STDARG_H
 
-#ifndef _VA_LIST
+#if !defined(_VA_LIST) && !defined(_VA_LIST_DECLARED)
 typedef __builtin_va_list va_list;
 #define _VA_LIST
+#define _VA_LIST_DECLARED /* FreeBSD */
 #endif
+#ifndef va_start
 #define va_start(ap, param) __builtin_va_start(ap, param)
+#endif
+#ifndef va_end
 #define va_end(ap)          __builtin_va_end(ap)
+#endif
+#ifndef va_arg
 #define va_arg(ap, type)    __builtin_va_arg(ap, type)
+#endif
 
 /* GCC always defines __va_copy, but does not define va_copy unless in c99 mode
  * or -ansi is not specified, since it was not part of C90.
  */
+#ifndef __va_copy
 #define __va_copy(d,s) __builtin_va_copy(d,s)
+#endif
 
 #if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L || !defined(__STRICT_ANSI__)
+#ifndef va_copy
 #define va_copy(dest, src)  __builtin_va_copy(dest, src)
 #endif
+#endif
 
 #ifndef __GNUC_VA_LIST
 #define __GNUC_VA_LIST 1
 typedef __builtin_va_list __gnuc_va_list;
 #endif
 
 #endif /* __STDARG_H */