On Jan 26 18:35, Steve Beck via Cygwin wrote:
> 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):
>
> [...]
> if (fork() == 0)
> {
> extern char **__argv;
>
> strcpy(__argv[0], "bar");
Don't do that. It's much too dangerous.
> __argv[0] = "bar";
That's ok, but see below.
> printf("Retry in Child '%s' (argv[0]) with pid %d setting
> __argv[0] = 'bar': ps ...\n", argv[0], getpid());
> system("ps");
Make sure to install the procps-ng package, and then call
system("procps -f");
> Here is the output when running foo.exe:
> [...]
> 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?
Actually, that's not your fault. ps(1) from the cygwin base package
does not support this kind of faking the process name, because it does
not even get the command line of a process. The information given to
ps(1) is pretty minimal and doesn't allow for stuff like that. The
process name is always the real executable path the process has been
started through.
However, there's the ps(1) from the procps-ng package, which is called
procps.exe so as not to collide with the basic Cygwin ps. This ps
fetches process information from /proc, which *is* updated when you
change __argv[0], because the info is fetched in realtime from the
process itself.
So with `procps -f', the output looks like this:
Retry in Child './foo' (argv[0]) with pid 570 setting __argv[0] = 'bar': ps ...
UID PID PPID C STIME TTY TIME CMD
corinna 569 496 15 10:49 pty0 00:00:00 ./foo
corinna 570 569 19 10:49 pty0 00:00:00 bar
corinna 571 570 99 10:49 pty0 00:00:00 procps -f
corinna 496 495 0 10:18 pty0 00:00:00 -tcsh
Bottom line:
Cygwin does not support this officially. Changing the global __argv[0]
is a bad hack. It works, but is non-portable. In fact, there is no
portable way to do this. SOme systems have prctl, some have
setproctitle, and some have nothing like this at all, stock Windows for
example.
It's also too late to add something along these lines to Cygwin 3.5,
which is due this week.
What we can do is to support this in the next major version of Cygwin,
in which case I'd prefer to implement this via setproctitle(3), given
this API exists on BSD and Linux. Whether that implies changing
Cygwin's very simple ps(1) as well, I can't say yet.
HTH,
Corinna
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