Re: When is it OK to pass NULLs to the 2nd and 3rd args of execve()?

[<tomas@tuxteam.de>]

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On Fri, Apr 09, 2021 at 11:53:39AM -0500, Peng Yu via Libc-help wrote:
> Hi,
> 
> I am wondering when (all possible legitimate cases) it is OK to pass
> NULLs to execve's 2nd and 3rd args.
> 
> #include <unistd.h>
> #include <stdio.h>
> 
> int main(int argc, char *argv[]) {
> 	if(execve(argv[1], NULL, NULL) == -1) {
> 		perror("execvp()");
> 		return 1;
> 	}
> 	return 0;
> }
> 
> 
> I tried the above program. It works when it is called with `/bin/sh`.
> My guess is that when the program being exec'ed don't use its args and
> envs, then then 2nd and 3rd args of execve() can be NULLs. Could
> anybody correct me if I am wrong?

I wouldn't count on that. It seems to depend on the operating
system. I quote the Linux Programmer's Manual execve(2) man page
from my box:

  On Linux, argv and envp can be specified as NULL.  In both
  cases, this has the same effect as specifying the argument
  as a pointer to a list containing a single null pointer.
  **Do not take  advantage of this nonstandard and nonportable
  misfeature!**  On many other UNIX systems, specifying argv
  as NULL will result in an error (EFAULT).  Some other UNIX
  systems treat the envp==NULL case the same as Linux.

So... I wouldn't rely on it :-)

Cheers
 - t

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