FORK(2)

💖 FORK(2) 💖

okay so bestie, you're reading the FORK(2) page from the System Calls Manual and honestly? this one is a whole moment. it's giving "i made a whole entire human being (a process) out of thin air" energy. let's get into it 🫶

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✨ NAME ✨

fork — create a new process

📚 LIBRARY 📚

Standard C Library (libc, -lc)

🎀 SYNOPSIS 🎀

#include <unistd.h>

pid_t
fork(void);

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🌸 DESCRIPTION 🌸

The fork() system call causes creation of a new process. no cap, it literally clones you. the new process (the child process) is an exact copy of the calling process (the parent process) except for the following, and bestie you NEED to know these exceptions because they are the whole plot:

• The child process has a unique process ID. yes, its own PID, its own little identity 💅
• The child process has a different parent process ID (i.e., the process ID of the parent process). it knows who its mom is, but the number is different.
• The child process has its own copy of the parent's descriptors, except for descriptors returned by kqueue(2), which are not inherited from the parent process. and here's the tea: these descriptors reference the same underlying objects, so that, for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the child process can affect a subsequent read(2) or write(2) by the parent. 😭 shared file pointers, iconic but also chaotic. this descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes.
• The child process' resource utilizations are set to 0; see setrlimit(2). fresh start, clean slate, new me era ✨
• All interval timers are cleared; see setitimer(2). gone. deleted. bye.
• The child process has only one thread, corresponding to the calling thread in the parent process. so if the process has more than one thread, locks and other resources held by the other threads are not released and therefore only async-signal-safe functions (see sigaction(2)) are guaranteed to work in the child process until a call to execve(2) or a similar function. 🚨 this one is important, don't be out here calling non-async-signal-safe stuff in a multithreaded fork child, i am BEGGING you.

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💫 RETURN VALUES 💫

Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. (yes the return value is literally how you tell which one you are. main character test fr fr.)

Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indicate the error. 😭

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🔥 ERRORS 🔥

The fork() system call will fail and no child process will be created if:

• [EAGAIN] — The system-imposed limit on the total number of processes under execution would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC. (The limit is actually ten less than this except for the super user). 🙈 ten. literally ten. the super user gets the extra ten, wild.
• [EAGAIN] — The user is not the super user, and the system-imposed limit on the total number of processes under execution by a single user would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROCPERUID.
• [EAGAIN] — The user is not the super user, and the soft resource limit corresponding to the resource argument RLIMIT_NPROC would be exceeded (see getrlimit(2)).
• [ENOMEM] — There is insufficient swap space for the new process. 💀 out of memory, out of vibes, out of luck.

lowkey note: yes, EAGAIN shows up three separate times. that's not a typo, that's just three different ways to run out of process budget. bestie you're gonna have to read all three.

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🌈 SEE ALSO 🌈

execve(2), rfork(2), setitimer(2), setrlimit(2), sigaction(2), vfork(2), wait(2)

iykyk, the whole process family is here 🧚

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👑 HISTORY 👑

The fork() function appeared in Version 1 AT&T UNIX. yes. Version 1. this function is ancient and it ATE and it's still serving. periodt. 💜

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okay bestie that's the whole page 🥰 this gem came from Debian, dated December 1, 2017, and it's FORK(2) — go forth and multiply your processes responsibly. mwah 🎀🦋⭐