man/man2/vfork.2

.\" Copyright 2014 Garrett D'Amore <garrett@damore.org>
.\" Copyright (c) 2004, Sun Microsystems, Inc.  All Rights Reserved.
.\" Copyright 1989 AT&T.
.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.Dd Aug 20, 2014
.Dt VFORK 2
.Os
.Sh NAME
.Nm vfork ,
.Nm vforkx
.Nd spawn new process in a virtual memory efficient way
.Sh SYNOPSIS
.In unistd.h
.Ft pid_t
.Fn vfork void
.
.In sys/fork.h
.Ft pid_t
.Fn vforkx "int flags"
.Sh DESCRIPTION
The
.Fn vfork
and
.Fn vforkx
functions create a new process without
fully copying the address space of the old process.
These functions are useful in instances where the purpose of a
.Xr fork 2
operation is to create a new
system context for an
.Xr exec 2
operation.
.Lp
Unlike with the
.Fn fork
function, the child process borrows the parent's
memory and thread of control until a call to
.Fn execve
or an exit
.Pq either abnormally or by a call to Xr _exit 2 .
Any modification
made during this time to any part of memory in the child process is reflected
in the parent process on return from
.Fn vfork
or
.Fn vforkx .
The parent process is suspended while the child is using its resources.
.Lp
In a multithreaded application,
.Fn vfork
and
.Fn vforkx
borrow only the thread of control that called
.Fn vfork
or
.Fn vforkx
in the parent; that is, the child contains only one thread.
The use of
.Fn vfork
or
.Fn vforkx
in multithreaded applications, however, is unsafe due to race
conditions that can cause the child process to become deadlocked and
consequently block both the child and parent process from execution
indefinitely.
.Lp
The
.Fn vfork
and
.Fn vforkx
functions can normally be used the same way as
.Fn fork
and
.Fn forkx ,
respectively.
The calling procedure, however, should not return while running in the child's
context, since the eventual return from
.Fn vfork
or
.Fn vforkx
in the parent would be to
a stack frame that no longer exists.
The
.Fn _exit
function should be used
in favor of
.Xr exit 3C
if unable to perform an
.Fn execve
operation, since
.Fn exit
will invoke all functions registered by
.Xr atexit 3C
and will flush and close standard I/O channels, thereby corrupting the parent
process's standard I/O data structures.
Care must be taken in the child process not to modify any global or local data
that affects the behavior of the parent process on return from
.Fn vfork
or
.Fn vforkx ,
unless such an effect
is intentional.
.Lp
Unlike
.Fn fork
and
.Fn forkx ,
fork handlers are not run when
.Fn vfork
and
.Fn vforkx
are called.
.Lp
The
.Fn vfork
and
.Fn vforkx
functions are deprecated.
Their sole legitimate use as a prelude to an immediate call to a function from
the
.Xr exec 2
family can be achieved safely by
.Xr posix_spawn 3C
or
.Xr posix_spawnp 3C .
.Ss "Fork Extensions"
The
.Fn vforkx
function accepts a
.Fa flags
argument consisting of a
bitwise inclusive-OR of zero or more of the following flags, which are defined
in the header
.In sys/fork.h :
.Lp
.Bl -item -compact -offset indent
.It
.Dv FORK_NOSIGCHLD
.It
.Dv FORK_WAITPID
.El
.Lp
See
.Xr fork 2
for descriptions of these flags.
If the
.Fa flags
argument is 0,
.Fn vforkx
is identical to
.Fn vfork .
.Sh RETURN VALUES
Upon successful completion,
.Fn vfork
and
.Fn vforkx
return 0 to
the child process and return the process ID of the child process to the parent
process.
Otherwise, \(mi1 is returned to the parent process, no child process is created,
and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn vfork
and
.Fn vforkx
functions will fail if:
.Bl -tag -width Er
.It Er EAGAIN
The system-imposed limit on the total number of processes under execution
(either system-quality or by a single user) would be exceeded.
This limit is determined when the system is generated.
.
.It Er ENOMEM
There is insufficient swap space for the new process.
.El
.Lp
The
.Fn vforkx
function will fail if:
.Bl -tag -width Er
.It Er EINVAL
The
.Va flags
argument is invalid.
.El
.Sh INTERFACE STABILITY
The
.Fn vfork
function is
.Sy Obsolete Standard .
.Lp
The
.Fn vforkx
function is
.Sy Obsolete Uncommitted .
.Sh MT-LEVEL
.Sy Unsafe .
.Sh SEE ALSO
.Xr exec 2 ,
.Xr exit 2 ,
.Xr fork 2 ,
.Xr ioctl 2 ,
.Xr atexit 3C ,
.Xr exit 3C ,
.Xr posix_spawn 3C ,
.Xr posix_spawnp 3C ,
.Xr wait 3C ,
.Xr signal.h 3HEAD ,
.Xr standards 5
.Sh NOTES
To avoid a possible deadlock situation, processes that are children in the
middle of a
.Fn vfork
or
.Fn vforkx
are never sent
.Dv SIGTTOU
or
.Dv SIGTTIN
signals; rather, output or ioctls are allowed and input attempts
result in an
.Dv EOF
indication.
.Lp
To forestall parent memory corruption due to race conditions with signal
handling,
.Fn vfork
and
.Fn vforkx
treat signal handlers in the child
process in the same manner as the
.Xr exec 2
functions: signals set to be
caught by the parent process are set to the default action
.Pq Dv SIG_DFL
in the child process
.Pq see Xr signal.h 3HEAD .
Any attempt to set a signal
handler in the child before
.Fn execve
to anything other than
.Dv SIG_DFL
or
.Dv SIG_IGN
is disallowed and results in setting the handler to
.Dv SIG_DFL .
.Lp
On some systems, the implementation of
.Fn vfork
and
.Fn vforkx
cause
the parent to inherit register values from the child.
This can create problems for certain optimizing compilers if
.In unistd.h
is not included in the source calling
.Fn vfork
or if
.In sys/fork.h
is not included in the
source calling
.Fn vforkx .
.Sh STANDARDS
The
.Fn vfork
function is available in the following compilation environments.
See
.Xr standards 5 .
.Lp
.Bl -bullet -compact
.It
.St -xpg4.2
.It
.St -susv2
.It
.St -susv3
.El
.Lp
It was marked obsolete in
.St -susv3
and removed from
.St -p1003.1-2008 .
.Lp
The
.Fn vforkx
function is a local extension and not available in any strictly
standards-compliant compilation environment.