///////////////
// pido
// writes the pid, then execs something.
//
// Usage:
//   pido pidfile /absolute/path/prog progarg1 progarg2 ...

// If the first arg is "-" then we write to STDOUT.
// If the first arg is "--" then we write to STDERR.

// Pido is important for scripts in init.d/ if they
// want to start a pipeline.

// If you use the command:  
//      aa | bb | cc &
// then $! returns the pid of cc.
// Alas, aa and/or bb may be just as important as cc
// (or even more important).
// For example, cc might be a simple logging utility,
// while aa is the daemon of interest.
//
// It is often true BUT NOT PROVABLY TRUE that the pid
// of bb is $! minus 1, and the pid of aa is $! minus 2.

// If you want something provably correct, do this:
//   pido /var/run/aa.pid /absolute/path/aa | bb | cc &
// Then you know the correct pid of aa has been written 
// to the file aa.pid.

// The perfect test for this is:
//	 ./pido - ./pido -       
// which should print the same pid number twice.

// There might be a way to do this with a simple shell script: 
//	echo ... exec ... 
// but if there is, I haven't figured it out.

using namespace std;

#include <unistd.h>
#include <stdlib.h>		/* for exit() */
#include <errno.h>
#include <sys/types.h>		/* for fork(), wait() */
#include <stdio.h>



////////////////////////////////////////
// Here with data coming in on fd 0.
// and control coming in on ft 1.

int main(int argc, char* argv[], char* env[]) {

  if (argc < 2 || argv[1][0] == '-') {
    if (argv[1][1] == '-') {
      fprintf(stderr, "%d\n", getpid());
    } else {
      fprintf(stdout, "%d\n", getpid());
    }
  } else {
    FILE* ouch = fopen (argv[1], "w");
    if (!ouch) {
      fprintf(stderr, "pido: cannot open pidfile '%s': ", argv[1]);
      perror(0);
      exit (1);
    }
    fprintf(ouch, "%d\n", getpid());
    fclose(ouch);
  }
  if (argc > 2) {
     execve(argv[2], argv+2, env);
     fprintf(stderr, "hi-q: failed to exec '%s': ", argv[2]);
     perror(0);
     exit(2);
  }
  return 0;
}