#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int parallel_max = 4;
int n_jobs = 0;

void wait_for_job(void)
{
	pid_t p;
	p = wait(NULL);
	if (p <= 0) {
		perror("wait failed");
		exit(1);
	}
	--n_jobs;
}

void wait_for_all_jobs(void)
{
	while (n_jobs) {
		wait_for_job();
	}
}

void start_job(char *const argv[])
{
	pid_t p;
	if (n_jobs == parallel_max) {
		wait_for_job();
	}
	p = fork();
	if (p < 0) {
		perror("fork failed");
		exit(1);
	}
	else if (p == 0) {
		execvp(argv[0], argv);
		perror("exec failed");
		exit(1);
	}
	++n_jobs;
}

int main(void)
{
	char *cmd[] = { "bash", "-c", "s=$[$RANDOM % 10 + 1]; echo start $s; sleep $s; echo done", NULL };
	int i;
	for (i=0; i<10; ++i) {
		start_job(cmd);
	}
	wait_for_all_jobs();
	return 0;
}