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.\"     @(#)vfork.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd May 22, 2016
.Dt VFORK 2
.Os
.Sh NAME
.Nm vfork
.Nd create a new process without copying the address space
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn vfork void
.Sh DESCRIPTION
.Bf -symbolic
Since this function is hard to use correctly from application software,
it is recommended to use
.Xr posix_spawn 3
or
.Xr fork 2
instead.
.Ef
.Pp
The
.Fn vfork
system call
can be used to create new processes without fully copying the address
space of the old process, which is inefficient in a paged
environment.
It is useful when the purpose of
.Xr fork 2
would have been to create a new system context for an
.Xr execve 2 .
The
.Fn vfork
system call
differs from
.Xr fork 2
in that the child borrows the parent process's address space and the
calling thread's stack
until a call to
.Xr execve 2
or an exit (either by a call to
.Xr _exit 2
or abnormally).
The calling thread is suspended while the child is using its resources.
Other threads continue to run.
.Pp
The
.Fn vfork
system call
returns 0 in the child's context and (later) the pid of the child in
the parent's context.
.Pp
Many problems can occur when replacing
.Xr fork 2
with
.Fn vfork .
For example, it does not work to return while running in the child's context
from the procedure that called
.Fn vfork
since the eventual return from
.Fn vfork
would then return to a no longer existent stack frame.
Also, changing process state which is partially implemented in user space
such as signal handlers with
.Xr libthr 3
will corrupt the parent's state.
.Pp
Be careful, also, to call
.Xr _exit 2
rather than
.Xr exit 3
if you cannot
.Xr execve 2 ,
since
.Xr exit 3
will flush and close standard I/O channels, and thereby mess up the
parent processes standard I/O data structures.
(Even with
.Xr fork 2
it is wrong to call
.Xr exit 3
since buffered data would then be flushed twice.)
.Pp
Unlike
.Xr fork 2 ,
.Fn vfork
does not run
.Xr pthread_atfork 3
handlers.
.Sh RETURN VALUES
Same as for
.Xr fork 2 .
.Sh ERRORS
The
.Fn vfork
system call will fail for any of the reasons described
in the
.Xr fork
man page.
In addition, it will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EINVAL
A system call other than
.Xr _exit()
or
.Xr execve()
(or libc functions that make no system calls other than those)
is called following calling a
.Fn vfork
call.
.El
.Sh SEE ALSO
.Xr _exit 2 ,
.Xr execve 2 ,
.Xr fork 2 ,
.Xr sigaction 2 ,
.Xr wait 2 ,
.Xr exit 3 ,
.Xr posix_spawn 3
.Sh HISTORY
The
.Fn vfork
system call appeared in
.Bx 3 .
.Sh BUGS
To avoid a possible deadlock situation,
processes that are children in the middle
of a
.Fn vfork
are never sent
.Dv SIGTTOU
or
.Dv SIGTTIN
signals; rather,
output or
.Xr ioctl 2
calls
are allowed
and input attempts result in an end-of-file indication.
.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 vfork
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.