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

// If the first arg is missing or is "-" or is ""
//   then we write to STDOUT.
// If the first arg is "--" then we write to STDERR.
// Otherwise the first arg is an ordinary filename.

// 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.


#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[]) {

  FILE* ouch = stdout;
  int isfile = 1;
  if (argc > 1) {
    if (argv[1][0] == '-' || argv[1][0] == 0) {
      isfile = 0;
      if (argv[1][1] == '-') ouch = stderr;
    }
    if (isfile) {
      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());

  if (isfile) 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;
}