Fork() Process

 The fork() system call is used in Unix/Linux systems to create a new process by duplicating the calling process. Here's a complete and FAANG-level explanation of how it works:

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🔹 What is fork()?

The fork() system call creates a child process that is a copy of the parent process. This is a fundamental system call for multitasking in Unix-based systems.

#include <unistd.h>

pid_t fork(void);

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🔸 Return Values of fork()

Returned Value	Meaning
> 0	Returned to the parent: PID of child.
0	Returned to the child process.
-1	Error: fork() failed (e.g., no memory).

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✅ Example

#include <stdio.h> #include <unistd.h> int main() {     pid_t pid = fork();     if (pid > 0) {         printf("Parent process: child PID = %d\n", pid);     } else if (pid == 0) {         printf("Child process: my PID = %d\n", getpid());     } else {         perror("fork failed");     }     return 0; }

📌 Output (example):

Parent process: child PID = 12345

Child process: my PID = 12345

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🔍 Detailed Explanation

🔸 1. Cloning the Parent

• fork() creates a separate child process with a copy of the parent’s memory, stack, heap, and file descriptors.
  
  

• Both processes continue executing from the point where fork() was called.
  
  

🔸 2. Return Differentiation

• This return value is crucial to distinguish execution flows:
  
  

• Parent uses pid > 0 to know the child’s PID.
  
  

• Child sees pid == 0, knows it's the child.
  
  

• If fork() fails due to resource limits (e.g., max number of processes), it returns -1.
  
  

🔸 3. Process IDs

• Each process gets its own unique PID.
  
  

• The child gets a new PID, and the parent can track it using the returned PID.
  
  

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🔧 Practical Use Cases

Use Case	Behavior
Creating daemons	Detach from terminal with fork + setsid + second fork.
Multi-processing servers	Spawn child workers to handle requests.
Shells/terminals	Spawn each command as a new process via fork() + exec().

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🚫 fork() Failure Cases

• Returns -1 if:
  
  

• Process limit reached (ulimit -u)
  
  

• Not enough memory
  
  

• OS policy blocks it
  
  

Debug:

ulimit -a        # View limits dmesg | tail     # Kernel logs if fork fails

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🔄 Common Fork Pattern

pid_t pid = fork(); if (pid == 0) {     // Child process     execvp(...); // Replace child with a new program } else if (pid > 0) {     wait(NULL);  // Parent waits for child } else {     // Error }

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⚠️ Interview Tip (FAANG)

Interviewers often check if you:

• Know the return semantics of fork()
  
  

• Can distinguish between parent and child
  
  

• Handle errors properly
  
  

• Understand zombie processes when wait() is missing
  
  

• Avoid race conditions when using fork() in multithreaded programs
  
  

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Would you like a fork vs exec vs clone vs vfork comparison table too?