CE 155 - fork Example (Home)

This program is a simple demonstration of the fork() function which is used to spawn new child processes. In this program, the main process forks a single child process. After a successful fork(), two processes are now running in parallel, with the new child being a copy of the parent. The fork() call returns the child's pid (process ID) to the parent process, and returns 0 to the child process. This is how the parent gets to know the child's pid, and is also used to separate the program code of the parent and of the child.

Before the child process exits, it asks for an 8-bit number from the user which it uses as its return code. The parent waits for the child to exit. It retrieves the child's exit code, and displays it. The parent then exits shortly thereafter.

The C function and UNIX system calls are declared in the included header files at the start of the code.

fork-ex.c (download)

/* Includes */
#include <unistd.h>     /* Symbolic Constants */
#include <sys/types.h>  /* Primitive System Data Types */ 
#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <sys/wait.h>   /* Wait for Process Termination */
#include <stdlib.h>     /* General Utilities */
int main()
    pid_t childpid; /* variable to store the child's pid */
    int retval;     /* child process: user-provided return code */
    int status;     /* parent process: child's exit status */

    /* only 1 int variable is needed because each process would have its
       own instance of the variable
       here, 2 int variables are used for clarity */
    /* now create new process */
    childpid = fork();
    if (childpid >= 0) /* fork succeeded */
        if (childpid == 0) /* fork() returns 0 to the child process */
            printf("CHILD: I am the child process!\n");
            printf("CHILD: Here's my PID: %d\n", getpid());
            printf("CHILD: My parent's PID is: %d\n", getppid());
            printf("CHILD: The value of my copy of childpid is: %d\n", childpid);
            printf("CHILD: Sleeping for 1 second...\n");
            sleep(1); /* sleep for 1 second */
            printf("CHILD: Enter an exit value (0 to 255): ");
            scanf(" %d", &retval);
            printf("CHILD: Goodbye!\n");    
            exit(retval); /* child exits with user-provided return code */
        else /* fork() returns new pid to the parent process */
            printf("PARENT: I am the parent process!\n");
            printf("PARENT: Here's my PID: %d\n", getpid());
            printf("PARENT: The value of my copy of childpid is %d\n", childpid);
            printf("PARENT: I will now wait for my child to exit.\n");
            wait(&status); /* wait for child to exit, and store its status */
            printf("PARENT: Child's exit code is: %d\n", WEXITSTATUS(status));
            printf("PARENT: Goodbye!\n");             
            exit(0);  /* parent exits */       
    else /* fork returns -1 on failure */
        perror("fork"); /* display error message */