fork (2)

Leading comments

 Copyright (c) 1980, 1991, 1993
	The Regents of the University of California.  All rights reserved.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 1. Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.
 2. Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the fol...

(The comments found at the beginning of the groff file "man2/fork.2freebsd".)

NAME

fork - create a new process

LIBRARY

Lb libc

SYNOPSIS

In unistd.h Ft pid_t Fn fork void

DESCRIPTION

The Fn fork system call 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:

• The child process has a unique process ID.
• The child process has a different parent process ID (i.e., the process ID of the parent process).
• The child process has its own copy of the parent's descriptors. 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. 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).
• All interval timers are cleared; see setitimer(2).
• The child process has only one thread, corresponding to the calling thread in the parent process. 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.

RETURN VALUES

Upon successful completion, Fn 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.

ERRORS

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

Bq Er 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).

Bq Er 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

Bq Er EAGAIN

The user is not the super user, and the soft resource limit corresponding to the Fa resource argument RLIMIT_NPROC would be exceeded (see getrlimit(2)).

Bq Er ENOMEM

There is insufficient swap space for the new process.

SEE ALSO

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

HISTORY

The Fn fork function appeared in AT&T System v6 .