Setting process command name in forked process

[Steve Beck]

Thanks so much for the reply, Anton!  Really appreciate it.

I tried what you proposed.  Here is the code trying both ways (overwriting what is referenced by __argv[0] and then reassigning the reference).  I compile this code (foo.c) simply as follows: gcc -o foo foo.c (with no errors or warnings):

#include        <stdio.h>
#include        <sys/types.h>
#include        <stdlib.h>
#include        <string.h>

extern  pid_t   getpid(void), fork(void);
extern  int     sleep(int);

int
main (int argc, char *argv[])
{
        printf("%s with pid %d: Entered.  About to fork child ...\n",
                argv[0], getpid());

        if (fork() == 0)
        {
                extern  char    **__argv;

                strcpy(__argv[0], "bar");

                printf("Child '%s' (argv[0]) with pid %d: ps ...\n",
                        argv[0], getpid());
                system("ps");

                __argv[0] = "bar";

                printf("Retry in Child '%s' (argv[0]) with pid %d setting __argv[0] = 'bar': ps ...\n", argv[0], getpid());
                system("ps");

                exit(0);
        }

        printf("%s parent with pid %d: sleep(5) ...\n", argv[0], getpid());
        sleep(5);

        exit(0);
}

Here is the output when running foo.exe:

foo with pid 497: Entered.  About to fork child ...
foo parent with pid 497: sleep(5) ...
Child 'bar' (argv[0]) with pid 498: ps ...
      PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
      446     445     446     108276  pty0     1207519 10:19:37 /usr/bin/bash
      445       1     445     126652  ?        1207519 10:19:36 /usr/bin/mintty
      499     498     497     125904  pty0     1207519 10:26:41 /usr/bin/ps
      498     497     497     128424  pty0     1207519 10:26:41 /home/sbeck/foo
      497     446     497     123772  pty0     1207519 10:26:41 /home/sbeck/foo
Retry in Child 'bar' (argv[0]) with pid 498 setting __argv[0] = 'bar': ps ...
      PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
      446     445     446     108276  pty0     1207519 10:19:37 /usr/bin/bash
      445       1     445     126652  ?        1207519 10:19:36 /usr/bin/mintty
      498     497     497     128424  pty0     1207519 10:26:41 /home/sbeck/foo
      497     446     497     123772  pty0     1207519 10:26:41 /home/sbeck/foo
      500     498     497     128948  pty0     1207519 10:26:41 /usr/bin/ps

As you can see, in neither case does the ps command seem to accurately reflect the change in __argv[0] (although within the program, the change occurs to argv[0]).

Can you see what I'm doing wrong?

Many thanks,
Steve

________________________________
From: Lavrentiev, Anton (NIH/NLM/NCBI) [C] <lavr@ncbi.nlm.nih.gov>
Sent: Friday, January 26, 2024 7:47 AM
To: Steve Beck <stevebeck99@hotmail.com>; cygwin@cygwin.com <cygwin@cygwin.com>
Subject: RE: Setting process command name in forked process

> (I'm assuming it's too late by the time the /proc entry has been set up).

Actually, it's not.  But beware, the suggested solution is absolutely NOT portable:

These are the externals that /proc is referring to...  (Not argc, argv passed to main().)

    extern char** __argv;
    extern int    __argc;

So you can change everything that it shows by reassigning __argv(and/or __argc) with a totally
different set of arguments, or just modify __argv[0], provided that it fits into the space
occupied by the original argument [0].

HTH,

Anton Lavrentiev
Contractor NIH/NLM/NCBI