1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)fork.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD: src/lib/libc/sys/fork.2,v 1.9.2.6 2002/07/30 19:04:25 silby Exp $ 30.\" $DragonFly: src/lib/libc/sys/fork.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 31.\" 32.Dd June 4, 1993 33.Dt FORK 2 34.Os 35.Sh NAME 36.Nm fork 37.Nd create a new process 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In sys/types.h 42.In unistd.h 43.Ft pid_t 44.Fn fork void 45.Sh DESCRIPTION 46.Fn Fork 47causes creation of a new process. 48The new process (child process) is an exact copy of the 49calling process (parent process) except for the following: 50.Bl -bullet -offset indent 51.It 52The child process has a unique process ID. 53.It 54The child process has a different parent 55process ID (i.e., the process ID of the parent process). 56.It 57The child process has its own copy of the parent's descriptors. 58These descriptors reference the same underlying objects, so that, 59for instance, file pointers in file objects are shared between 60the child and the parent, so that an 61.Xr lseek 2 62on a descriptor in the child process can affect a subsequent 63.Xr read 2 64or 65.Xr write 2 66by the parent. 67This descriptor copying is also used by the shell to 68establish standard input and output for newly created processes 69as well as to set up pipes. 70.It 71The child process' resource utilizations 72are set to 0; see 73.Xr setrlimit 2 . 74.It 75All interval timers are cleared; see 76.Xr setitimer 2 . 77.El 78.Sh RETURN VALUES 79Upon successful completion, 80.Fn fork 81returns a value 82of 0 to the child process and returns the process ID of the child 83process to the parent process. Otherwise, a value of -1 is returned 84to the parent process, no child process is created, and the global 85variable 86.Va errno 87is set to indicate the error. 88.Sh MULTI-THREADING CONSIDERATIONS 89.Fn fork 90can create severe issues for multi-threaded programs due to the fact that 91the memory state of the child process will record the asynchronous state 92of the threads that are running in the parent. 93.Fn Fork 94will only be synchronous for the specific thread making the call. 95In particular, 96locks used internally by 97.Xr pthread 3 98and 99.Xr rtld 1 100can be caught in a bad state. 101To deal with these issues, the pthreads library goes to great lengths 102to synchronize internal locks when 103a 104.Fn fork 105call is issued. 106The threaded program itself as well as third party libraries used by the 107program might or might not properly handle these issues when it comes to 108their own internal state. 109.Pp 110If at all possible, programs should use 111.Xr vfork 2 112instead of 113.Fn fork 114when forking for the purposes of issuing an exec of some sort. 115Attempting to fork a threaded program without issuing an exec is not 116recommended. 117Attempting to bypass pthreads and implement threading manually is also 118not recommended as it is doubtful that homegrown implementations could 119properly deal with rtld races. 120.Sh ERRORS 121.Fn Fork 122will fail and no child process will be created if: 123.Bl -tag -width Er 124.It Bq Er EAGAIN 125The system-imposed limit on the total 126number of processes under execution would be exceeded. 127The limit is given by the 128.Xr sysctl 3 129MIB variable 130.Dv KERN_MAXPROC . 131(The limit is actually ten less than this 132except for the super user). 133.It Bq Er EAGAIN 134The user is not the super user, and 135the system-imposed limit 136on the total number of 137processes under execution by a single user would be exceeded. 138The limit is given by the 139.Xr sysctl 3 140MIB variable 141.Dv KERN_MAXPROCPERUID . 142.It Bq Er EAGAIN 143The user is not the super user, and 144the soft resource limit corresponding to the resource parameter 145.Dv RLIMIT_NPROC 146would be exceeded (see 147.Xr getrlimit 2 ) . 148.It Bq Er ENOMEM 149There is insufficient swap space for the new process. 150.El 151.Sh SEE ALSO 152.Xr execve 2 , 153.Xr rfork 2 , 154.Xr setitimer 2 , 155.Xr setrlimit 2 , 156.Xr vfork 2 , 157.Xr wait 2 158.Sh HISTORY 159A 160.Fn fork 161function call appeared in 162.At v6 . 163