C get number of CPU threads

ncpus.c

int ncpus(void) {
    FILE *cmd = popen("grep '^processor' /proc/cpuinfo | wc -l", "r");

    if (cmd == NULL)
        return -1;

    unsigned nprocs;
    size_t n;
    char buff[8];

    if ((n = fread(buff, 1, sizeof(buff)-1, cmd)) <= 0)
        return -1;

    buff[n] = '\0';
    if (sscanf(buff, "%u", &nprocs) != 1)
        return -1;

    pclose(cmd);

    return nprocs;
}