The fork system call is one of the most fundamental operations in Unix and Unix-like operating systems. It was introduced with the first version of Unix in the early 1970s by Ken Thompson and Dennis Ritchie at Bell Laboratories.
Overview
The primary function of the fork system call is to create a new process, which is a copy of the calling process. Here's how it works:
- The parent process (the process that calls
fork()) results in two identical processes:
- The parent process continues to execute from where
fork() was called.
- The child process, which is an exact duplicate of the parent at the time of the call, also starts executing from the same point.
- The
fork() system call returns different values to the parent and child processes:
- In the parent process,
fork() returns the process ID (PID) of the child process.
- In the child process, it returns zero.
- If an error occurs,
fork() returns -1 in the parent process, and no child process is created.
- Both processes share the same file descriptors, but they have separate memory spaces, which means changes in one do not affect the other after the fork.
Historical Context
The fork system call was designed to facilitate process creation in a simple and efficient manner:
- It allows for the creation of processes without needing to load a new program into memory, as the child process starts with a copy of the parent's memory.
- It was pivotal for implementing the shell, where background processes could be easily spawned to run concurrently with the shell.
Usage and Implications
The fork system call has several implications:
- Resource Sharing: Initially, the parent and child share the same file descriptors, which means they can read or write to the same file or socket without additional setup.
- Memory: While the memory is copied, modern implementations use a technique known as Copy-on-Write to avoid unnecessary copying until modifications are made.
- Concurrency: It allows for concurrent execution of programs, facilitating parallelism in computing.
- System Load: Excessive forking can lead to high system load if not managed properly, leading to issues like fork bombs.
Modern Implementations
Over time, several optimizations and variations of the fork system call have been developed:
- Vfork - A variation that does not copy the parent's memory space, intended for quick child process creation before an immediate exec.
- posix_spawn - A newer API that combines
fork and exec in a single call, reducing the overhead associated with forking.
External Resources
Related Topics