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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.\" $FreeBSD$ 30.\" 31.Dd April 20, 2021 32.Dt FORK 2 33.Os 34.Sh NAME 35.Nm fork 36.Nd create a new process 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In unistd.h 41.Ft pid_t 42.Fn fork void 43.Sh DESCRIPTION 44The 45.Fn fork 46system call causes creation of a new process. 47The new process (child process) is an exact copy of the 48calling process (parent process) except for the following: 49.Bl -bullet -offset indent 50.It 51The child process has a unique process ID. 52.It 53The child process has a different parent 54process ID (i.e., the process ID of the parent process). 55.It 56The child process has its own copy of the parent's descriptors, 57except for descriptors returned by 58.Xr kqueue 2 , 59which are not inherited from the parent process. 60These descriptors reference the same underlying objects, so that, 61for instance, file pointers in file objects are shared between 62the child and the parent, so that an 63.Xr lseek 2 64on a descriptor in the child process can affect a subsequent 65.Xr read 2 66or 67.Xr write 2 68by the parent. 69This descriptor copying is also used by the shell to 70establish standard input and output for newly created processes 71as well as to set up pipes. 72.It 73The child process' resource utilizations 74are set to 0; see 75.Xr setrlimit 2 . 76.It 77All interval timers are cleared; see 78.Xr setitimer 2 . 79.It 80The child process has only one thread, 81corresponding to the calling thread in the parent process. 82If the process has more than one thread, 83locks and other resources held by the other threads are not released 84and therefore only async-signal-safe functions 85(see 86.Xr sigaction 2 ) 87are guaranteed to work in the child process until a call to 88.Xr execve 2 89or a similar function. 90.El 91.Sh RETURN VALUES 92Upon successful completion, 93.Fn fork 94returns a value 95of 0 to the child process and returns the process ID of the child 96process to the parent process. 97Otherwise, a value of -1 is returned 98to the parent process, no child process is created, and the global 99variable 100.Va errno 101is set to indicate the error. 102.Sh EXAMPLES 103The following example shows a common pattern of how 104.Fn fork 105is used in practice. 106.Bd -literal -offset indent 107#include <err.h> 108#include <stdio.h> 109#include <stdlib.h> 110#include <unistd.h> 111 112int 113main(void) 114{ 115 pid_t pid; 116 117 switch (pid = fork()) { 118 case -1: 119 err(1, "Failed to fork"); 120 case 0: 121 printf("Hello from child process!\en"); 122 exit(0); 123 default: 124 break; 125 } 126 127 printf("Hello from parent process (child's PID: %d)!\en", pid); 128 129 return (0); 130} 131.Ed 132.Pp 133The output of such a program is along the lines of: 134.Bd -literal -offset indent 135Hello from parent process (child's PID: 27804)! 136Hello from child process! 137.Ed 138.Sh ERRORS 139The 140.Fn fork 141system call will fail and no child process will be created if: 142.Bl -tag -width Er 143.It Bq Er EAGAIN 144The system-imposed limit on the total 145number of processes under execution would be exceeded. 146The limit is given by the 147.Xr sysctl 3 148MIB variable 149.Dv KERN_MAXPROC . 150(The limit is actually ten less than this 151except for the super user). 152.It Bq Er EAGAIN 153The user is not the super user, and 154the system-imposed limit 155on the total number of 156processes under execution by a single user would be exceeded. 157The limit is given by the 158.Xr sysctl 3 159MIB variable 160.Dv KERN_MAXPROCPERUID . 161.It Bq Er EAGAIN 162The user is not the super user, and 163the soft resource limit corresponding to the 164.Fa resource 165argument 166.Dv RLIMIT_NPROC 167would be exceeded (see 168.Xr getrlimit 2 ) . 169.It Bq Er ENOMEM 170There is insufficient swap space for the new process. 171.El 172.Sh SEE ALSO 173.Xr execve 2 , 174.Xr rfork 2 , 175.Xr setitimer 2 , 176.Xr setrlimit 2 , 177.Xr sigaction 2 , 178.Xr vfork 2 , 179.Xr wait 2 180.Sh HISTORY 181The 182.Fn fork 183function appeared in 184.At v1 . 185