unistd's execl() without passing any arguments

Question

In my code I need to execute /bin/bash, but I wan't to do it without passing any arguments to it. I wrote this:

execl("/bin/bash", NULL);

Then, through some research, I realized that I also need to add type cast:

execl("bin/bash", (char*) NULL);

but GCC is still giving me warnings:

main.c:18:5: warning: null argument where non-null required (argument 2) [-Wnonnull]
     if(execl("/bin/bash", (char*) NULL) == -1) {
     ^
main.c:18:5: warning: not enough variable arguments to fit a sentinel [-Wformat=]

What is the proper way of doing this, am I misunderstanding something, or am I using a completely wrong function call?

Varitype

You've got a couple of substantially identical answers so far. I'll observe that the classic way of getting a login shell was execl("/bin/sh", "-sh", (char *)0), where the - indicated that it was a login shell. Bash (and some other shells) have regularized it so that execl("/bin/bash", "bash", "-l", (char *)0) is a login shell. – Cirone

Answer 1

You are getting the warning because the convention is that the first argument is always the same as the path of the program being run, like this:

execl("/bin/bash", "/bin/bash", (char*) NULL);

This is essentially what happens when you run a program without arguments in the shell.

This argument will go into the executed program's argv[0], which it can use to check how it was run.

Piwowar

The convention is that the first arg is the full path as it was run, not just the filename. You got it right in your example however. – Arista

Answer 2

Per the man page, execl() has the following defintion:

   int execl(const char *path, const char *arg, ...
                   /* (char  *) NULL */);

GCC is telling you the execl() function is expecting a non-NULL argument.

Thorpe

Answer 3

Just had a similar problem. I found out that if I specified the full path to my exe (starting with /) neither execlp nor execv passed any argument. And also that I could not output anything to stderr.

Since I only want to replace my current process with a clone version of itself, I save and use the argv[0] of the parent as the 2 first arguments of execlp/v and everything is fine.

Gastrolith

Answer 4

The first two parameters of execl() should be the full path and then the process name.

At least that's how I interpreted it here:

Comradery

Why don't you use an official documentation link, like this one exec(3)? – Baskerville