Detailed explanation of fork function in Linux

fork() is a very important system call in Linux and other Unix-like systems. It is used to create a new process. This new process is a copy of the current process, called a child process. The child process will obtain a copy of the parent process's code, data, heap, stack, etc., but the two processes will have different process IDs and some other resources, such as open file descriptors.

The following is the basic usage and precautions of the fork() function:

Function prototype

c

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

Return value

If fork() is successfully called in the parent process, the process ID of the newly created child process is returned.

If fork() is successfully called in the child process, 0 is returned.

If the fork() call fails, -1 is returned.

Features

Parent-child process: The process calling fork() is the parent process, and the newly created process is the child process.

Data copy: The data of the parent process (including code, heap, stack, etc.) will be copied to the child process, but the two processes have independent address spaces.

Asynchronicity: After fork() creates a child process, the parent process and the child process execute in an asynchronous manner, and they can run independently of each other.

Process ID: Each process has a unique process ID (PID), and the PID of the child process is different from the PID of the parent process.

Example

c

#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
  
int main() {  
    pid_t pid;  
  
    pid = fork();  
  
    if (pid < 0) {  
        // fork 失败  
        fprintf(stderr, "Fork failed\n");  
        exit(1);  
    } else if (pid == 0) {  
        // 子进程  
        printf("I am the child process, my PID is %d\n", getpid());  
    } else {  
        // 父进程  
        printf("I am the parent process, my PID is %d, my child&#39;s PID is %d\n", getpid(), pid);  
    }  
  
    return 0;  
}

Note

Resource copying: fork() will copy all resources of the parent process, which may cause performance issues, especially in in large programs. Therefore, it is usually recommended to use the exec() series of functions to replace the code of the child process after fork() to avoid unnecessary resource copying.

Race conditions: Because the parent process and the child process execute asynchronously, race conditions may occur. For example, two processes might try to access or modify the same file at the same time, resulting in inconsistent data.

Error handling: After calling fork(), its return value should always be checked to handle possible error conditions.

In general, fork() is a very basic and important system call in the Linux system, used to create new processes. However, due to its complexity and potential performance issues, extreme caution is required when using it.

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