Run N concurrent processes in C - c

Question

I am trying to run N concurrent processes in a C program. I've built a simple example that takes commands as arguments, creates a fork for each one, and executes it.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int i;
for(i = 1; i < argc; i++)
{
pid_t pid = fork();
if(pid < 0)
{
fprintf(stderr, "forking error\n");
exit(1);
}
else if(pid > 0)
{
int status;
waitpid(pid, &status, 0);
printf("Command %s has completed successfully by PID=%d\n", argv[i], pid);
}
else
{
char cmd[1024];
sprintf(cmd, "%s", argv[i], i);
system(cmd);
_exit(1);
}
}
printf("Finished\n");
return 0;
}
This seems to run the processes correctly, but not concurrently. Any ideas as to what am I doing wrong?
EDIT: I've edited based on suggestions, but this also does not seem to work.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int i;
pid_t *pids = malloc( sizeof(pid_t) * (argc) );
int *statuses = malloc( sizeof(int) * (argc) );
for(i = 1; i < argc; i++)
{
pid_t pid = fork();
if(pid < 0)
{
fprintf(stderr, "forking error\n");
exit(1);
}
else if(pid > 0)
{
//int status;
//waitpid(pid, &status, 0);
//printf("Command %s has completed successfully by PID=%d\n", argv[i], pid);
pids[i] = pid;
}
else
{
char cmd[1024];
sprintf(cmd, "%s > out.%d", argv[i], i);
system(cmd);
_exit(1);
}
}
int needtowait = 0;
do
{
needtowait = 0;
for(i = 1; i < argc; i++)
{
if(pids[i] > 0)
{
if(waitpid(pids[i], &statuses[i], 0) != 0)
{
pids[i] = 0;
char *successstr = "successfully";
if(statuses[i])
{
successstr = "unsuccessfully";
}
printf("Command %s has completed %s by PID=%d\n", argv[i], successstr, pids[i]);
}
}
else
{
needtowait = 1;
}
sleep(0);
}
} while(needtowait);
printf("Finished!\n");
return 0;
}

Answer 1

The reason you are not running these processes concurrently is in this line:
waitpid(pid, &status, 0);
The main process that forks out the child process waits for the child process to exit before continuing with the loop, and starting the next process.
Since you want to run your processes concurrently, you can do this: allocate an array of pid_t for process IDs, and fill it in inside the loop. Once you are out of the loop, you can wait for the individual processes to complete by executing waitpid calls in a loop.
pid_t *pids = malloc(argc * sizeof(pid_t));
for (int i = 0 ; i < argc ; i++) { // Start i at 0, not at 1
pid_t pid = fork();
if (pid < 0) {
...
} else if (pid > 0) {
pids[i] = pid;
} else {
char cmd[1024];
sprintf(cmd, "%s", argv[i+1], i+1);
system(cmd);
_exit(1);
}
}
for (int i = 0 ; i < argc ; i++) {
int status;
waitpid(pids[i], &status, 0);
printf("Command %s has completed successfully by PID=%d\n", argv[i+1], pids[i]);
}

Answer 2

Sure. Your parent process is waiting for the child process to finish executing before forking again. You're just running cmd sequentially N times.