1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
|
.\" $OpenBSD: fork.2,v 1.18 2015/09/10 17:55:21 schwarze Exp $
.\" $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. 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 $Mdocdate: September 10 2015 $
.Dt FORK 2
.Os
.Sh NAME
.Nm fork
.Nd create a new process
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn fork void
.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,
which also does not match any existing process group 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 a single thread.
.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 2
or
.Xr 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.
.It
The child process has no
.Xr fcntl 2 Ns -style
file locks.
.It
The child process' resource utilizations
are set to 0; see
.Xr getrusage 2 .
.It
All interval timers are cleared; see
.Xr setitimer 2 .
.It
The child process' semaphore undo values are set to 0; see
.Xr semop 2 .
.It
The child process' pending signals set is empty.
.It
The child process has no memory locks; see
.Xr mlock 2
and
.Xr mlockall 2 .
.El
.Pp
In general, the child process should call
.Xr _exit 2
rather than
.Xr exit 3 .
Otherwise, any stdio buffers that exist both in the parent and child
will be flushed twice.
Similarly,
.Xr _exit 2
should be used to prevent
.Xr atexit 3
routines from being called twice (once in the parent and once in the child).
.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 [EAGAIN]
.It Bq Er EAGAIN
The system-imposed limits on the total
number of processes or total number of threads
under execution would be exceeded.
These limits are configuration dependent.
.It Bq Er EAGAIN
The limit
.Dv RLIMIT_NPROC
on the total number of processes under execution by the user ID
would be exceeded.
.It Bq Er ENOMEM
There is insufficient swap space for the new process.
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr getrusage 2 ,
.Xr wait 2
.Sh STANDARDS
The
.Fn fork
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn fork
system call first appeared in
.At v1 .
|
