Creating multiple process using fork() in C

In this section we will see how to use the fork() system call to create child processes in C. The fork() function creates a new process by duplicating the calling process, allowing both parent and child processes to execute different tasks.

When fork() is called, it returns different values to distinguish between processes: a positive value (child's PID) to the parent process, 0 to the child process, and -1 if the fork failed.

Syntax

#include <unistd.h>
pid_t fork(void);

Note: This program requires a Unix-like operating system (Linux, macOS) to compile and run. The fork() system call is not available on Windows.

Return Values

• Greater than 0: Returns the child's process ID (PID) to the parent process
• 0: Returned to the child process
• -1: Fork failed (error occurred)

Example: Basic Fork Implementation

This example demonstrates how to create a child process and execute different code in parent and child processes −

#include <stdio.h>
#include <unistd.h>

int main() {
    int n = fork(); // Create child process
    
    if (n > 0) { 
        // Parent process
        printf("Parent process (PID: %d)<br>", getpid());
        printf("Child PID: %d<br>", n);
    } else if (n == 0) { 
        // Child process
        printf("Child process (PID: %d)<br>", getpid());
        printf("Parent PID: %d<br>", getppid());
    } else {
        // Fork failed
        printf("Fork failed!<br>");
        return 1;
    }
    
    return 0;
}

Parent process (PID: 1234)
Child PID: 1235
Child process (PID: 1235)
Parent PID: 1234

Example: Creating Multiple Processes

This example shows how to create multiple child processes using fork() in a loop −

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

int main() {
    int i;
    pid_t pid;
    
    printf("Main process PID: %d<br>", getpid());
    
    for (i = 0; i < 3; i++) {
        pid = fork();
        
        if (pid == 0) {
            // Child process
            printf("Child %d: PID = %d, Parent = %d<br>", 
                   i+1, getpid(), getppid());
            return 0; // Child exits
        } else if (pid < 0) {
            printf("Fork failed for child %d<br>", i+1);
            return 1;
        }
    }
    
    // Parent waits for all children
    for (i = 0; i < 3; i++) {
        wait(NULL);
    }
    
    printf("All children completed. Parent exiting.<br>");
    return 0;
}

Main process PID: 1234
Child 1: PID = 1235, Parent = 1234
Child 2: PID = 1236, Parent = 1234
Child 3: PID = 1237, Parent = 1234
All children completed. Parent exiting.

Key Points

• The fork() system call creates an exact copy of the calling process
• Both processes continue execution from the point where fork() was called
• Use wait() in the parent process to synchronize with child processes
• Always check the return value of fork() for error handling

Conclusion

The fork() system call is essential for creating multiple processes in Unix-like systems. It allows parallel execution and is the foundation for process management in operating systems.