recycle PID

recycle.c

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#include <stdbool.h>
#include <stdlib.h>
#include <err.h>
#include <sys/types.h>
#include <sys/wait.h>


bool check_process_exists(pid_t process)
{
    return !(kill(process, 0) == -1 && errno == ESRCH);
}

int main(int argc, char **argv)
{
    int newpid;
    int reqpid;
    int status;

    // Check parameters
    if (argc < 3) {
        errx(EXIT_FAILURE, "usage: %s <pid> <comand> [<params>...]", *argv);
    }

    // The process we're supposed to become
    reqpid = atoi(argv[1]);

    // Verify that pid is available
    if (check_process_exists(reqpid)) {
        errx(EXIT_FAILURE, "the requested pid already exists");
    }

    do {
        if ((newpid = fork()) == 0) {
            if (getpid() == reqpid) {
                execvp(argv[2], &argv[2]);
            }
            exit(1);
        }

        if (newpid < 0) {
            err(EXIT_FAILURE, "fork failed");
        }

        if (waitpid(newpid, &status, 0) != newpid) {
            err(EXIT_FAILURE, "unexpected waitpid result");
        }
    } while (newpid != reqpid);

    if (WIFEXITED(status)) {
        return WEXITSTATUS(status);
    }

    return EXIT_SUCCESS;
}