fork() in C

In this section we will see what is the fork system call in C. This fork system call is used to create a new process. This newly created process is known as child process. The current process which is creating another child process is called the parent process.

A child process uses the same program counter, CPU register, same files that are used by the parent process.

Syntax

#include <sys/types.h>
#include <unistd.h>

pid_t fork(void);

Return Value

The fork() does not take any parameter, it returns integer values. It may return three types of integer values −

• Negative Number: It returns negative number when child process creation is failed
• Zero Value: It returns Zero for the newly created child process
• Positive Value: The positive value is returned to the parent process.

Example: Basic Fork Usage

Here is a simple example to demonstrate how fork() creates a child process −

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

int main() {
    fork(); // make a child process of same type
    printf("Fork testing code
");
    return 0;
}

The output of the above code is −

Fork testing code
Fork testing code

Example: Identifying Parent and Child Process

To differentiate between parent and child processes, we can check the return value of fork() −

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

int main() {
    pid_t process_id = fork();
    
    if (process_id == 0) {
        printf("This is child process. PID: %d
", getpid());
    } else if (process_id > 0) {
        printf("This is parent process. PID: %d, Child PID: %d
", 
               getpid(), process_id);
    } else {
        printf("Fork failed
");
    }
    
    return 0;
}

The output of the above code is −

This is parent process. PID: 1234, Child PID: 1235
This is child process. PID: 1235

Note: The fork() system call is Unix/Linux specific and may not work on Windows systems. Use a Unix/Linux environment or compiler that supports POSIX functions.

Key Points

• Both parent and child processes execute the code after fork() independently
• The child process gets a copy of the parent's memory space
• Process IDs (PID) are unique for each process

Conclusion

The fork() system call is essential for creating new processes in Unix/Linux systems. Understanding its return values helps distinguish between parent and child processes for proper program flow control.