Portable equivalent of OPEN_MAX

nftw wants a parameter for number of file handles to use, and doesn't seem to have a way to say 'as many as possible'. Specifying 255 seems to work on Linux, but fails on BSD. Apparently OPEN_MAX is the recommended solution on BSD, but I can't use this as it doesn't work on Linux.

Is there a portable equivalent of OPEN_MAX that will work on both Linux and BSD?

Alternatively, is there a portable number, some number large enough to not slow things down, that is portable for practical purposes (ideally specified in POSIX, or at least that will work on every Unix-like system with significant market share)?

Solution

Advanced Programming in the Unix Environment, 2nd Ed gives us the following code which should work everywhere; though it is pretty clever, I think it is a little unfortunate it doesn't also check the rlimits of the process, since the rlimits can further constrain how many open files a process may use. That aside, here's the code from The Master:

#ifdef  OPEN_MAX
static long openmax = OPEN_MAX;
#else
static long openmax = 0;
#endif

/*
 * If OPEN_MAX is indeterminate, we're not
 * guaranteed that this is adequate.
 */
#define OPEN_MAX_GUESS  256

long
open_max(void)
{
    if (openmax == 0) {     /* first time through */
        errno = 0;
        if ((openmax = sysconf(_SC_OPEN_MAX)) < 0) {
            if (errno == 0)
                openmax = OPEN_MAX_GUESS;   /* it's indeterminate */
            else
                err_sys("sysconf error for _SC_OPEN_MAX");
        }
    }

    return(openmax);
}

(err_sys() is provided in the apue.h header with the sources -- should be easy to code a replacement for your routine.)