¿Cómo obtener la cantidad de CPU en Linux usando C?

#include <stdio.h>
#include <sys/sysinfo.h>

int main(int argc, char *argv[])
{
    printf("This system has %d processors configured and "
        "%d processors available.\n",
        get_nprocs_conf(), get_nprocs());
    return 0;
}

https://linux.die.net/man/3/get_nprocs

#include <unistd.h>
long number_of_processors = sysconf(_SC_NPROCESSORS_ONLN);