How to correctly fork and finish process to avoid EAGAIN error

Question

I need to fork many short life processes. (But I don't want to wait until it is finished. Just let it run.). After some time, I receive error when executing fork (errno == 11). It looks like the forked processes still exists (at least they are visible in htop).

For instance, please consider the following example. I want to fork a process and then exit it shortly after.

#include <cstdio>
#include <cstdlib>
#include <unistd.h>
#include <errno.h>
#include <signal.h>

void RunProc(){   
    pid_t pid = fork();
    if(pid == 0)
    {
        puts("fork with success");
        //no needed to reproduce issue. Used in my code to run some commands.
        //execvp()             
        exit(0);
    }
    else if (pid == -1) {
        printf("Could not start process\n");
        printf("Errno: %d\n", errno);
    }
    else {
    }
}
 
int main()
{
    while(true){
        RunProc();
    }

    int res = 0;
    int i = 5;
    res = scanf("%d", &i);
    res = printf("End of program %d\n", i);
    return res;
}

At the beginning it runs correctly. After several minutes I receive only errors:

Could not start process Errno: 11

And I'm not able to start new fork anymore. I'm running it on Ubuntu 22.04

Answer 1

You need to call one of the wait functions in the parent process, to reap the child-process when it exits.

You could use e.g. waitpid with the WNOHANG flag to poll for child-process exit. Or use the SIGCHLD signal to learn when a child process have exited.

There's only a limited number of process-slots available in your system, if you don't reap the child processes, they will fill up all the slots and lead to the error you get.

As the fork manual page say about the EAGAIN error:

A system-imposed limit on the number of threads was encountered.