R-exts provides some header preamble for using alloca() and suggests that it
"suffices for known R platforms", with the caveat that it "should be included
before standard C headers such as stdlib.h". But attempting to compile a
minimal program using Xcode 15.3 with macOS 14.4 SDK, including the preamble
followed by stdlib.h, results in an error:
Source:
#include <Rconfig.h> // for HAVE_ALLOCA_H
#ifdef __GNUC__
// this covers gcc, clang, icc
# undef alloca
# define alloca(x) __builtin_alloca((x))
#elif defined(HAVE_ALLOCA_H)
// needed for native compilers on Solaris and AIX
# include <alloca.h>
#endif
#include <stdlib.h>
int main()
{
return 0;
}
Output:
$ clang -I"`R RHOME`/include" test.c -o test.o
In file included from test.c:11:
In file included from
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/stdlib.h:68:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/alloca.h:32:7:
error: conflicting types for '__builtin_alloca'
void *alloca(size_t); /* built-in for gcc */
^
test.c:5:20: note: expanded from macro 'alloca'
# define alloca(x) __builtin_alloca((x))
^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/alloca.h:32:7:
note: '__builtin_alloca' is a builtin with type 'void *(unsigned long)'
test.c:5:20: note: expanded from macro 'alloca'
# define alloca(x) __builtin_alloca((x))
^
1 error generated.
where it seems that the macro alloca() defined at line 5 is expanded in
alloca.h, producing a nonsense declaration there. I conclude that the currently
documented advice is not quite portable here ...
I know that I can just prefix identifier as below:
#include <Rconfig.h> // for HAVE_ALLOCA_H
#ifdef __GNUC__
// this covers gcc, clang, icc
# define prefixed_alloca(x) __builtin_alloca((x))
#else
# ifdef HAVE_ALLOCA_H
// needed for native compilers on Solaris and AIX
# include <alloca.h>
# endif
# define prefixed_alloca(x) alloca((x))
#endif
but if there is another way to remain portable, then perhaps it could be
documented.
Mikael
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