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Operating Systems: Virtualization, Concurrency & Persistence

Finally, The exec() System Call

Explore the exec() system call to understand how it replaces the running program with a new executable in UNIX process virtualization. Learn its role alongside fork() and how exec() allows running different programs within a process, essential for mastering process creation APIs.

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A final and important piece of the process creation API is the exec() system call.

Running p3.c

This system call is useful when you want to run a program that is different from the calling program. For example, calling fork() in p2.c is only useful if you want to keep running copies of the same in the program. However, often you want to run a different program; exec() does just that (see the code below).

C
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
int main(int argc, char *argv[]){
    printf("hello world (pid:%d)\n", (int) getpid());
    fflush(stdout);
    int rc = fork();
    if (rc < 0) {
        // fork failed; exit
        fprintf(stderr, "fork failed\n");
        exit(1);
    } else if (rc == 0) {
        // child (new process)
        printf("hello, I am child (pid:%d)\n", (int) getpid());
        fflush(stdout);
        char *myargs[3];
        myargs[0] = strdup("wc");   // program: "wc" (word count)
        myargs[1] = strdup("p3.c"); // argument: file to count
        myargs[2] = NULL;           // marks end of array
        execvp(myargs[0], myargs);  // runs word count
        printf("this shouldn't print out");
    } else {
        // parent goes down this path (original process)
        int wc = wait(NULL);
        printf("hello, I am parent of %d (wc:%d) (pid:%d)\n",
	       rc, wc, (int) getpid());
    }
    return 0;
}

In this example, the child ...