spawnwithpid.c

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>

int forkstatus;
int exitstatus = 0;
void catch_sigterm () {
  if (forkstatus != 0)
    kill(0, SIGTERM);
  _exit(exitstatus);
}

int main (int argc, char *const argv[]) {
  if (argc < 3) {
    printf("usage: spawnwithpid <pid> <cmd>\n");
    return 1;
  }

  int wantpid = atoi(argv[1]);
  if (!(0 < wantpid && wantpid < 32768)) { // ignoring pid_max
    printf("invalid pid %s.\n", argv[1]);
    return 2;
  }

  // note: if we get lucky on the first try, we actually won't succeed
  // too lazy to fix this
  int procstatus = kill(wantpid, 0);
  if (!(procstatus == -1 && errno == ESRCH)) {
    printf("pid already taken or other error. (%s)\n", argv[1]);
    return 3;
  }

  signal(SIGTERM, catch_sigterm);

  // spawn children until pid is hit, then exit
  // children: wait until parent exits
  do {
    forkstatus = fork();
  } while (forkstatus > 0 && forkstatus != wantpid);

  if (forkstatus == -1) {
    // common problems
    if (errno == EAGAIN) {
      // observation: linux seems to stop spawning short of full exhaustion,
      // making this fail every second call during debugging
      printf("could not fork anymore, reason: out of pids (EAGAIN). Retrying usually helps.\n");
      exitstatus = 5;
      kill(0, SIGTERM);
    } else {
      printf("could not fork for unexpected reason %d\n", errno);
      exitstatus = 4;
      kill(0, SIGTERM);
    }
  } else if (forkstatus == 0) {
    // children
    if (wantpid == getpid()) {
      // success
      printf("got pid %d!\n", wantpid);
      setsid();
      kill(getppid(), SIGTERM);
      execvp(argv[2], &argv[2]);
    } else {
      // dud
      pause();
    }
  } else if (forkstatus == wantpid) {
    // parent
    pause();
  }

  return 99;
}