freebsd-dev/lib/libc/sys/fork.2

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.\"	@(#)fork.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd December 1, 2017
.Dt FORK 2
.Os
.Sh NAME
.Nm fork
.Nd create a new process
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn fork void
.Sh 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:
.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,
except for descriptors returned by
.Xr kqueue 2 ,
which are not inherited from the parent process.
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' resource utilizations
are set to 0; see
.Xr setrlimit 2 .
.It
All interval timers are cleared; see
.Xr setitimer 2 .
.It
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
.Xr sigaction 2 )
are guaranteed to work in the child process until a call to
.Xr execve 2
or a similar function.
.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
The
.Fn fork
system call 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.
The limit is given by the
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROC .
(The limit is actually ten less than this
except for the super user).
.It 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
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROCPERUID .
.It Bq Er EAGAIN
The user is not the super user, and
the soft resource limit corresponding to the
.Fa resource
argument
.Dv RLIMIT_NPROC
would be exceeded (see
.Xr getrlimit 2 ) .
.It Bq Er ENOMEM
There is insufficient swap space for the new process.
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr rfork 2 ,
.Xr setitimer 2 ,
.Xr setrlimit 2 ,
.Xr sigaction 2 ,
.Xr vfork 2 ,
.Xr wait 2
.Sh HISTORY
The
.Fn fork
function appeared in
.At v1 .