Thread: Exec() Question?

Code:

char* ls_args[2];
ls_args[0] = "../..";
ls_args[1] = 0;

execve("/bin/ls", ls_args, 0);

When executing the above code, it just does an ls on the current directory. Array element [0] is completely ignored.

char* ls_args[3];
ls_args[0] = "the first array value has absolutely no effect whatsoever";
ls_args[1] = "../..";
ls_args[2] = 0;

execve("/bin/ls", ls_args, 0);

This works; it actually does an ls on the directory two levels up. However, notice the contents of array element [0].

What is going on here?

I tried looking it up on the man pages, but I don't understand what it says. Here is an excerpt:

http:// www.opengroup.org/onlinepubs/000095399/functions/exec.html

execl(<shell path>, arg0, file, arg1, ..., (char *)0);

The argument arg0 should point to a filename that is associated with the process being started by one of the exec functions.

... Huh? What file?

Also, the examples on the same page always have the command listed twice, such as:

ret = execle ("/bin/ls", "ls", "-l", (char *)0, env);

Can anyone explain the purpose of the first array element in laymen's terms?

"file is the basename stripped of its path"

Sure but what is the purpose of it? Notice what I put in the example in my first post. It seems like it doesn't matter what you put at all.

Originally Posted by Paul22000

"file is the basename stripped of its path"

Semantically speaking, yes that's what it should be though nothing prevents anybody from putting something else there (sort of like what you did). Only caveat or downside is that the exec() family of functions does not enforce the fact that "file is the basename of the program".

Originally Posted by Paul22000

Sure but what is the purpose of it? Notice what I put in the example in my first post. It seems like it doesn't matter what you put at all.

No purpose except that it's the string that argv[0] points to within the exec()'d file; so for ex. if "oneprog" calls "someprog" as in:

Code:

execl("/full/path/to/someprog", "no effect whatsoever", "firstarg", "secondarg", (char*)0);  /* inside oneprog */

When someprog is executed the arguments passed to its main() function would be:

Code:

argv[0] points to "no effect whatsover"
argv[1] points to "firstarg"
argv[2] points to "secondarg"

Which is semantically incorrect since argv[0] is the name of the program which in this case should be "someprog" and not "no effect whatsoever" as in.

Code:

execl("/full/path/to/someprog", "someprog", "firstarg", "secondarg", (char*)0);