How do I set the command line arguments in a C program so that it's visible when users type "ps aux"?

Question

When you type "ps aux" the ps command shows command arguments that the program was run with. Some programs change this as a way of indicating status. I've tried changing argv[] fields and it doesn't seem to work. Is there a standard way to set the command line arguments so that they appear when the user types ps?

That is, this doesn't work:

int main(int argc,char **argv)
{
    argv[0] = "Hi Mom!";
    sleep(100);
}

09:40 imac3:~$ ./x &
[2] 96087
09:40 imac3:~$ ps uxp 96087 
USER      PID  %CPU %MEM      VSZ    RSS   TT  STAT STARTED      TIME COMMAND
yv32      96087   0.0  0.0  2426560    324 s001  S     9:40AM   0:00.00 ./x
09:40 imac3:~$ cat x.c

Kermanshah

I think ythis is some kind of thing called "bash tab extension". You need to define a tab extension handler for your program and register it in some file, but don't ask me how to do this. – Mediterranean

Not possible? ( steve.org.uk/Reference/Unix/faq_2.html#SEC22 ) You might also like ( netsplit.com/2007/01/10/hiding-arguments-from-ps ) – Hughs

Answer 1

You had the right idea, but you don't change the pointers in argv[n], you must change the string pointed to by argv[0] itself:

#include <string.h>
#include <unistd.h>

int main(int argc,char **argv)
{
    size_t maxlen = strlen(argv[0]);

    memset(argv[0], 0, maxlen);
    strncat(argv[0], "Hi Mom!", maxlen);
    pause();

    return 0;
}

(Note that whether or not this actually changes the command name shown by ps is system-dependent).

Casady

@R..: No it's not. Combined with strlen, @Casady is making a very careful observation that we might not be guaranteed to have more space than the original argv[0] to store our program name. Also, the memset isn't necessary, strncat will always null terminate. Very clean answer, nice use of pause(). – Metabolic

Look again. strncat is being used to concatenate to a zero-length string, i.e. as a cheap strlcpy instead of for actual concatenation. The memset is unnecessary, but you'd at least need argv[0][0]=0; to replace it for the desired functionality. This nonobviousness is why I said it's strange. – Irrepressible

@Kermanshah Not really broken, just misused, misunderstood and their rationale long forgotten... – Styx

@Kermanshah - The UB happens only if you misuse the not null-terminated char arrays. But if that is what you need, then all is well. – Styx

@vy32: ...and strncat() always null-terminates its result, anyway. – Casady

@caf, you are right, strncat() and strncpy() differ in that manner. These inconsistencies are quite annoying. I'll remove my comment. – Kermanshah