[apple/xnu.git] / bsd / man / man2 / fork.2

.\"	$NetBSD: fork.2,v 1.6 1995/02/27 12:32:36 cgd Exp $
.\"
.\" 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 following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"	@(#)fork.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt FORK 2
.Os BSD 4
.Sh NAME
.Nm fork
.Nd create a new process
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft pid_t
.Fo fork
.Fa void
.Fc
.Sh DESCRIPTION
.Fn 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:
.Bl -bullet -offset indent
.It
The child process has a unique process ID.
.It
The child process has a different parent
process ID (i.e., the process ID of the parent process).
.It
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
.Xr lseek 2
on a descriptor in the child process can affect a subsequent
.Xr read
or
.Xr write
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.
.It
The child processes resource utilizations
are set to 0; see
.Xr setrlimit 2 .
.El
.Sh 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
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn fork
will fail and no child process will be created if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EAGAIN
The system-imposed limit
on the total number of processes under execution
would be exceeded.
This limit is configuration-dependent.
.\" ==========
.It Bq Er EAGAIN
The system-imposed limit
.Dv MAXUPRC
.Pq Aq Pa sys/param.h
on the total number of
processes under execution by a single user would be exceeded.
.\" ==========
.It Bq Er ENOMEM
There is insufficient swap space for the new process.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <unistd.h>
.Pp
The include file
.In sys/types.h
is necessary.
.Sh SEE ALSO
.Xr execve 2 ,
.Xr sigaction 2 ,
.Xr wait 2 ,
.Xr compat 5
.Sh HISTORY
A
.Fn fork
function call appeared in
.At v6 .
.Sh CAVEATS
There are limits to what you can do in the child process.
To be totally safe you should restrict yourself to only
executing async-signal safe operations until such time
as one of the exec functions is called.  All APIs, including
global data symbols, in any framework or library should be 
assumed to be unsafe after a 
.Fn fork
unless explicitly documented to be safe or async-signal 
safe.  If you need to use these frameworks in the child 
process, you must exec.  In this situation it is reasonable 
to exec yourself.
Commit	Line	Data
9bccf70c A	1	.\" $NetBSD: fork.2,v 1.6 1995/02/27 12:32:36 cgd Exp $
	2	.\"
	3	.\" Copyright (c) 1980, 1991, 1993
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" Redistribution and use in source and binary forms, with or without
	7	.\" modification, are permitted provided that the following conditions
	8	.\" are met:
	9	.\" 1. Redistributions of source code must retain the above copyright
	10	.\" notice, this list of conditions and the following disclaimer.
	11	.\" 2. Redistributions in binary form must reproduce the above copyright
	12	.\" notice, this list of conditions and the following disclaimer in the
	13	.\" documentation and/or other materials provided with the distribution.
	14	.\" 3. All advertising materials mentioning features or use of this software
	15	.\" must display the following acknowledgement:
	16	.\" This product includes software developed by the University of
	17	.\" California, Berkeley and its contributors.
	18	.\" 4. Neither the name of the University nor the names of its contributors
	19	.\" may be used to endorse or promote products derived from this software
	20	.\" without specific prior written permission.
	21	.\"
	22	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	23	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	24	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	25	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	26	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	27	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	28	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	29	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	30	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	31	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	32	.\" SUCH DAMAGE.
	33	.\"
	34	.\" @(#)fork.2 8.1 (Berkeley) 6/4/93
	35	.\"
	36	.Dd June 4, 1993
	37	.Dt FORK 2
	38	.Os BSD 4
	39	.Sh NAME
	40	.Nm fork
	41	.Nd create a new process
	42	.Sh SYNOPSIS
9bccf70c A	43	.Fd #include <unistd.h>
9bccf70c A	44	.Ft pid_t
2d21ac55 A	45	.Fo fork
	46	.Fa void
	47	.Fc
9bccf70c	48	.Sh DESCRIPTION
39037602	49	.Fn fork
9bccf70c A	50	causes creation of a new process.
	51	The new process (child process) is an exact copy of the
	52	calling process (parent process) except for the following:
	53	.Bl -bullet -offset indent
	54	.It
	55	The child process has a unique process ID.
	56	.It
	57	The child process has a different parent
	58	process ID (i.e., the process ID of the parent process).
	59	.It
	60	The child process has its own copy of the parent's descriptors.
	61	These descriptors reference the same underlying objects, so that,
	62	for instance, file pointers in file objects are shared between
	63	the child and the parent, so that an
	64	.Xr lseek 2
	65	on a descriptor in the child process can affect a subsequent
	66	.Xr read
	67	or
	68	.Xr write
	69	by the parent.
	70	This descriptor copying is also used by the shell to
	71	establish standard input and output for newly created processes
	72	as well as to set up pipes.
	73	.It
	74	The child processes resource utilizations
	75	are set to 0; see
	76	.Xr setrlimit 2 .
	77	.El
	78	.Sh RETURN VALUES
	79	Upon successful completion,
	80	.Fn fork
2d21ac55 A	81	returns a value of 0 to the child process
	82	and returns the process ID of the child process to the parent process.
	83	Otherwise, a value of -1 is returned to the parent process,
	84	no child process is created, and the global variable
9bccf70c A	85	.Va errno
	86	is set to indicate the error.
	87	.Sh ERRORS
39037602	88	.Fn fork
9bccf70c A	89	will fail and no child process will be created if:
9bccf70c A	90	.Bl -tag -width Er
2d21ac55	91	.\" ==========
9bccf70c	92	.It Bq Er EAGAIN
2d21ac55 A	93	The system-imposed limit
	94	on the total number of processes under execution
	95	would be exceeded.
9bccf70c	96	This limit is configuration-dependent.
2d21ac55	97	.\" ==========
9bccf70c A	98	.It Bq Er EAGAIN
	99	The system-imposed limit
	100	.Dv MAXUPRC
	101	.Pq Aq Pa sys/param.h
	102	on the total number of
	103	processes under execution by a single user would be exceeded.
2d21ac55	104	.\" ==========
9bccf70c A	105	.It Bq Er ENOMEM
	106	There is insufficient swap space for the new process.
	107	.El
2d21ac55 A	108	.Sh LEGACY SYNOPSIS
	109	.Fd #include <sys/types.h>
	110	.Fd #include <unistd.h>
	111	.Pp
	112	The include file
	113	.In sys/types.h
	114	is necessary.
9bccf70c A	115	.Sh SEE ALSO
9bccf70c A	116	.Xr execve 2 ,
2d21ac55 A	117	.Xr sigaction 2 ,
	118	.Xr wait 2 ,
	119	.Xr compat 5
9bccf70c A	120	.Sh HISTORY
9bccf70c A	121	A
55e303ae	122	.Fn fork
9bccf70c A	123	function call appeared in
9bccf70c A	124	.At v6 .
2d21ac55 A	125	.Sh CAVEATS
	126	There are limits to what you can do in the child process.
	127	To be totally safe you should restrict yourself to only
	128	executing async-signal safe operations until such time
	129	as one of the exec functions is called. All APIs, including
	130	global data symbols, in any framework or library should be
	131	assumed to be unsafe after a
	132	.Fn fork
	133	unless explicitly documented to be safe or async-signal
	134	safe. If you need to use these frameworks in the child
	135	process, you must exec. In this situation it is reasonable
	136	to exec yourself.