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///////////////
// 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, "pido: failed to exec '%s': ", argv[2]);
perror(0);
for (int ii = 0; ii < argc; ii++){
fprintf(stderr, "'%s' ", argv[ii]);
}
fprintf(stderr, "\n");
exit(2);
}
return 0;
}
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