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FORK(2) System Calls Manual FORK(2)

forkcreate a new process

library “libc”

#include <unistd.h>


fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for the following:

In general, the child process should call _exit(2) rather than exit(3). Otherwise, any stdio buffers that exist both in the parent and child will be flushed twice. Similarly, _exit(2) should be used to prevent atexit(3) routines from being called twice (once in the parent and once in the child).

In case of a threaded program, only the thread calling fork() is still running in the child processes.

Child processes of a threaded program have additional restrictions, a child must only call functions that are async-signal-safe. Very few functions are asynchronously safe and applications should make sure they call exec(3) as soon as possible.

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. 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.

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

The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration-dependent.
The limit RLIMIT_NPROC on the total number of processes under execution by this user id would be exceeded.
There is insufficient swap space for the new process.

execve(2), setrlimit(2), vfork(2), wait(2), pthread_atfork(3)

The fork() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”).

A fork() system call appeared in Version 6 AT&T UNIX.

June 10, 2004 NetBSD-7.0.1