example/posix/processes-fork.c

Shows how to fork and wait for a child process

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

int main(int argc, const char* argv[])
{
    srand(getpid());
    fprintf(stdout, "Main process: forking\n");

    // try to fork the process
    pid_t fork_result = fork();

    // if that failed
    if(fork_result < 0)
    {
        // complain and exit
        fprintf(
            stdout,
            "Main process: forking failed with '%s'\n",
            strerror(fork_result)
        );
        return fork_result;
    }

    // if we are here and fork_result is 0 then we are
    // in the child process
    if(fork_result == 0)
    {
        // do some "work"
        fprintf(stderr, "Child process %d: started\n", getpid());
        sleep(2);
        int child_result = rand() % 256;
        fprintf(
            stderr,
            "Child process %d: quitting with exit code %d\n",
            getpid(),
            child_result
        );
        return child_result;
    }
    // otherwise this is executing in the parent process
    else
    {
        fprintf(stdout, "Main process: successfully forked\n");

        // randomly either send a signal to the process
        // or let it finish normally
        if(rand() % 2 == 0)
        {
            sleep(1);
            fprintf(stdout, "Main process: signaling the child\n");
            if(rand() % 2 == 0)
                kill(fork_result, SIGTERM);
            else
                kill(fork_result, SIGINT);
        }

        fprintf(stdout, "Main process: waiting for child\n");

        // wait for the child process to finish
        // get the exit status and examine it
        int status = 0;
        wait(&status);

        // if the process exitted normally
        if(WIFEXITED(status))
        {
            fprintf(
                stdout,
                "Main process: child %d exitted with code %d\n",
                fork_result,
                WEXITSTATUS(status)
            );
        }

        // if the process was terminated by a signal
        if(WIFSIGNALED(status))
        {
            fprintf(
                stdout,
                "Main process: child %d terminated by signal %d\n",
                fork_result,
                WTERMSIG(status)
            );
        }
    }
    fprintf(stdout, "Main process: quitting\n");
    return 0;
}