A simple shell command parallelizer.

parallelize.c

/*  parallelize: reads commands from stdin and executes them in parallel.

    The sole argument is the number of simultaneous processes (optional) to
      run. If omitted, the number of logical CPUs available will be used.
    
    Build: gcc -pthread parallelize.c -o parallelize
    
    Demo: (for i in {1..10}; do echo "echo $i ; sleep 5" ; done ) | ./parallelize
    
    By Marco Arment, released into the public domain with no guarantees.
*/
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/wait.h>

pthread_mutex_t g_input_mutex;

void *thread_main(void *_)
{
    while (1) {
        while (pthread_mutex_lock(&g_input_mutex)) { }
        size_t length = 0;
        char *line = NULL;
        int retval = getline(&line, &length, stdin);
        pthread_mutex_unlock(&g_input_mutex);
        if (retval == -1) break;
        
        pid_t child_pid = fork();
        if (child_pid) {
            waitpid(child_pid, NULL, 0);
        } else {
            system(line);
            exit(0);
        }
        free(line);
    }
    
    pthread_exit(NULL);
}

int main(int argc, const char **argv)
{
    int thread_count = argc < 2 ? sysconf(_SC_NPROCESSORS_ONLN) : atoi(argv[1]);    
    if (thread_count < 1) {
        fprintf(stderr, "Usage: parallelize {thread_count}\n");
        exit(1);
    }

    pthread_mutex_init(&g_input_mutex, NULL);

    pthread_t *threads = malloc(thread_count * sizeof(pthread_t));
    for (int i = 0; i < thread_count; i++) {
        if (pthread_create(&threads[i], NULL, thread_main, NULL)) {
            fprintf(stderr, "parallelize: cannot create worker thread %d\n", i);
            exit(-1);
        }
    }

    pthread_exit(NULL);
}