Are there any platforms where using structure copy on an fd_set (for select() or pselect()) causes problems?

Question

The select() and pselect() system calls modify their arguments (the \'fd_set *\' arguments), so the input value tells the system which file descriptors to check

Answer 1

I've done a little research on MacOS X, Linux, AIX, Solaris and HP-UX, and there are some interesting results. I used the following program:

#if __STDC_VERSION__ >= 199901L
#define _XOPEN_SOURCE 600
#else
#define _XOPEN_SOURCE 500
#endif /* __STDC_VERSION__ */

#ifdef SET_FD_SETSIZE
#define FD_SETSIZE SET_FD_SETSIZE
#endif

#ifdef USE_SYS_TIME_H
#include 
#else
#include 
#endif /* USE_SYS_TIME_H */

#include 

int main(void)
{
    printf("FD_SETSIZE = %d; sizeof(fd_set) = %d\n", (int)FD_SETSIZE, (int)sizeof(fd_set));
    return 0;
}

It was compiled twice on each platform:

cc -o select select.c
cc -o select -DSET_FD_SETSIZE=16384

(And on one platform, HP-UX 11.11, I had to add -DUSE_SYS_TIME_H to get things to compile at all.) I separately did a visual check on FD_COPY - only MacOS X seemed to include it, and that had to be activated by ensuring that _POSIX_C_SOURCE was not defined or by defining _DARWIN_C_SOURCE.

AIX 5.3

• Default FD_SETSIZE is 65536
• The FD_SETSIZE parameter can be resized
• No FD_COPY

HP-UX 11.11

• No header - use instead
• Default FD_SETSIZE is 2048
• The FD_SETSIZE parameter can be resized
• No FD_COPY

HP-UX 11.23

• Has
• Default FD_SETSIZE is 2048
• The FD_SETSIZE parameter can be resized
• No FD_COPY

Linux (kernel 2.6.9, glibc 2.3.4)

• Default FD_SETSIZE is 1024
• The FD_SETSIZE parameter cannot be resized
• No FD_COPY

MacOS X 10.6.2

• Default FD_SETSIZE is 1024
• The FD_SETSIZE parameter can be resized
• FD_COPY is defined if strict POSIX compliance is not requested or if _DARWIN_C_SOURCE is specified

Solaris 10 (SPARC)

• Default FD_SETSIZE is 1024 for 32-bit, 65536 for 64-bit
• The FD_SETSIZE parameter can be resized
• No FD_COPY

Clearly, a trivial modification to the program allows automatic checking of FD_COPY:

#ifdef FD_COPY
    printf("FD_COPY is a macro\n");
#endif

What is not necessarily trivial is finding out how to ensure that it is available; you end up doing the manual scan and working out how to trigger it.

On all these machines, it looks like an fd_set can be copied by a structure copy without running into risk of undefined behaviour.