Printing the PID of a program immediately before it runs

Question

I need to know (for monitoring purposes) the PID of a program immediately before I start it. There could be multiple of the same program launched at the same time, so monitoring ps or top isn't really an option. It dawned on me as I was exploring various bash-related options that I could make use of C's exec functions to try and pull this off. With that in mind, I whipped up this little piece of code:

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

int main(int argc, char **argv)
{
    char **argv_copy;
    int i;

    if (argc >= 2)
    {
        argv_copy = malloc(argc * sizeof *argv_copy);

        for (i = 1; i < argc; i++)
        {
            argv_copy[i-1] = argv[i];
        }
        argv_copy[argc] = (char *) NULL;

        printf("%lu\n", (long unsigned) getpid());

        execvp(argv[1], argv_copy);
    }
    return 0;
}

It works as expected - it immediately prints out the PID of the process and then loads the actual process I want to run. What I'm not convinced of is the security or robustness of this little hack. I don't need it to be 100% bullet-proof security-wise, but if you can drive an SUV through it I'd like to know.

Could I get some advice on these two areas specifically?

Answer 1

Bug

This line:

    argv_copy[argc] = (char *) NULL;

should be:

    argv_copy[argc-1] = NULL;

You are removing one argument so you need to terminate the array at the right place.

Copy unneeded

Instead of:

    execvp(argv[1], argv_copy);

you could do:

    execvp(argv[1], &argv[1]);

and avoid making a copy.

Answer 2

It fails silently when you pass it gibberish. Not quite a bug, but still not the best.

$ ./a.out nemo
8620
$ echo $?
0

$ nemo
bash: nemo: command not found
$ echo $?
127

An easy fix would be to print an error message and return 127 after the execvp.

Answer 3

Note that you can also do this trivially with a shell script:

#!/bin/sh

echo "PID: $$"
exec "$@"

That's a lot simpler and less bug-prone than doing it in C.

Answer 4

Can't comment, sorry.

I might be totally wrong here but the man page for getpid() states that it returns the PID of the calling process. This would be your application instead of the one you want to execute.

So instead of printing the PID of a program immediately before it runs, you print the PID of a program immediatly before another program runs.

Answer 5

pid_t is certainly some integer type.

Looking forward, little reason to assume an unsigned long is sufficient range to encompass a pid_t.

So why be stingy? Use a very wide type when trying to print types of unknown width.

//printf("%lu\n", (long unsigned) getpid());
printf("%jd\n", (intmax_t) getpid());
// or 
printf("%lld\n", (long long) getpid());

If absolutely certain, use the unsigned counter parts.

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For completeness, code should free() memory it allocates.