Re: FD_SETSIZE on Linux?

Thom Brown <thom(at)linux(dot)com> writes:
> I find this in pgbench.c:

> #ifdef FD_SETSIZE
> #define MAXCLIENTS (FD_SETSIZE - 10)
> #else
> #define MAXCLIENTS 1024
> #endif

FD_SETSIZE is supposed to be defined, according to the POSIX spec:

The <sys/select.h> header shall define the following symbolic constant,
which shall have a value suitable for use in #if preprocessing directives:

FD_SETSIZE
Maximum number of file descriptors in an fd_set structure.

It looks like Linux sets it to 1024. On RHEL6, at least, I find this:

$ grep -r FD_SETSIZE /usr/include
/usr/include/linux/posix_types.h:#undef __FD_SETSIZE
/usr/include/linux/posix_types.h:#define __FD_SETSIZE 1024
...
/usr/include/sys/select.h:#define FD_SETSIZE __FD_SETSIZE
...

> #ifdef WIN32
> #define FD_SETSIZE 1024 /* set before winsock2.h is included */
> #endif /* ! WIN32 */

Windows probably hasn't got sys/select.h at all, so it may not provide
this symbol.

Interestingly, it looks like POSIX also requires <sys/time.h> to define
FD_SETSIZE. I wonder whether Windows has that header? It'd definitely
be better to get this symbol from the system than assume 1024 will work.

regards, tom lane