Question

I’m trying to find the maximum number of threads per process on a UNIX machine and wrote the code below to use sysconf:

#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

int main() {
errno = 0;
long maxThreads = sysconf(_SC_THREAD_THREADS_MAX);
if (maxThreads == -1 && errno == 0)
{
    printf("the variable corresponding to _SC_THREAD_THREADS_MAX "
            "is associated with functionality that is not "
            "supported by the system\n");
    exit(1);
}
if (maxThreads == -1)
{
    printf("errno: %d\n", errno);
    exit(1);
}

printf ("max num threads per process: %ld\n", maxThreads);

exit(0);
}

Unfortunately the sysconf() returns -1 without changing the errno! Does anyone know how to get around this problem and eventually what is the maximum number of Pthreads per process?
Thanks

P.S. I tried it on Solaris and Linux and got the same result. However HPUX did return 8000!

Answer 1

According to my manual for sysconf on my Mac OSX 10.7.X:

If the call to sysconf() is not successful, -1 is returned and errno is set appropriately. Otherwise, if the variable is associated with functionality that is not supported, -1 is returned and errno is not modified. Otherwise, the current variable value is returned.

Under linux it is different:

If name is invalid, -1 is returned, and errno is set to EINVAL. Otherwise, the value returned is the value of the system resource and errno is not changed. In the case of options, a positive value is returned if a queried option is available, and -1 if it is not. In the case of limits, -1 means that there is no definite limit.

So it looks like that _SC_THREAD_THREADS_MAX is not a valid sysconfig variable on that architecture or maybe there is no limit. Or maybe there is another definition for -1 on other architectures. You’ll have to check the manual on each architecture you are trying to get to work.

If _SC_THREAD_THREADS_MAX is not valid then you might want to try processes instead of threads. Maybe there is a max processes setting which also means max threads.