Execve

In the previous posts, we focused on fork() and wait(). With those two system calls alone, we created concurrency — fork() created a child process and wait() instructed the kernel to reap the child process, the parent process not proceeding with executing the next line of code until that the child process finished. But how can we take advantage of the fact that fork() creates a child process and perhaps run a separate program all together?

That’s where execve() comes in.

When called within a process, execve system calls overwrites the virtual memory within a process, replacing the stack and variables and so on. Moreover, the process gets overwritten with the program specified in the argument(s) passed when calling execve(). For example, let’s write a simple little program that forks a subprocess which calls ls.

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

int main(int argc, char * argv[])
{
    pid_t pid;
    int child_status;

    printf("Forking a child process from parent pid %d\n", getpid());

    if ( (pid = fork()) == 0){
	// In the child process
	char * args[] = {"ls", "-altrh", NULL};
	char * envp[] = {NULL};
	execve("/bin/ls", args, envp);
	printf("This line should never run!!!\n");
    } else {
	printf("Waiting for %d to finish executing\n", pid);
	waitpid(pid, &child_status, 0);
	printf("Exiting!\n");
	exit(0);
    }

}

Compiling and running this program yields the following output:

$ ./execveing 
Forking a child process from parent pid 28630
Waiting for 28631 to finish executing
total 128
-rw-r--r--   1 mattchung  staff   399B Oct 19 20:32 zombie.c
-rwxr-xr-x   1 mattchung  staff   8.4K Oct 19 20:32 zombie
-rw-r--r--   1 mattchung  staff   252B Oct 19 20:32 forking.c
-rwxr-xr-x   1 mattchung  staff   8.3K Oct 19 20:32 forking
-rw-r--r--   1 mattchung  staff   397B Oct 20 10:01 waiting.c
-rwxr-xr-x   1 mattchung  staff   8.4K Oct 20 10:01 waiting
drwxr-xr-x  17 mattchung  staff   578B Oct 21 13:50 ..
-rw-r--r--   1 mattchung  staff   567B Oct 21 15:29 execveing.c
-rwxr-xr-x   1 mattchung  staff   8.6K Oct 21 15:29 execveing
drwxr-xr-x  10 mattchung  staff   340B Oct 21 15:29 .
Exiting!

Pretty neat! Bear in mind to keep the (above) to a minimum, I omitted checking the exit code when calling execve().