确定一个进程是子进程还是父进程（不使用fork的返回值）

Question

How to write a programm to show whether it's a child or parent process after the fork() is used?

I need to find other way then checking the return value of fork() or getpid() function.

I know eg that process resource utilizations ( getrusage(2) ) and CPU time counters ( times(2) ) are reset to zero in the child. How i can use for example that knowledge (or other child-parents differences) to determine my problem?

Thank you for your help!

Answer 1

• Setup a signal handler for SIGCHLD in one of the processes and end the other one. If the SIGCHLD handler gets called you know the child died. If not the parent died.

• Another (less distructive) way it to call wait() . If it returns ECHLD no child is around, thus the calling process is the child. If it blocks, at least one child is around, thus the calling process is the parent.

• Or call waitpid() with PID of the other process. The same logic on the result as for wait() applies.

Answer 2

You could exploit the fact that the parent will be a process group leader (and thus its pid will be the same as its process group id) while the child will be a member of that process group and thus its pid will be different from the process group id:

#include <stdio.h>
#include <unistd.h>

main(int argc, char **argv) {
    char *self = NULL;

    fork(); /* looking at return value would be cheating */
    int pid = getpid();
    int pgid = getpgid(0);

    if (pid == pgid)
        self = "Parent";
    else
        self = "Child";

    printf("%s pid = %d pgid = %d\n", self, pid, pgid);
}

Running yields:

$ ./pgid
Parent pid = 29400 pgid = 29400
Child pid = 29401 pgid = 29400

This does of course still use getpid() but not in the same way you describe above.

Answer 3

On Linux, you might also use proc(5) . The process of pid 1234 is described by the directory /proc/1234/ and you could read sequentially and parse the /proc/1234/status pseudo-file to get all that information.

To understand a bit more, read the proc(5) man page, then type

 cat /proc/self/status
 cat /proc/$$/status
 cat /proc/self/maps
 cat /proc/$$/maps

in a terminal, and try to understand the output. You might parse it programmatically (eg with FILE*f = fopen("/proc/1234/status","r"); then do some fgets or fscanf on f ; then fclose(f); )