path: root/man2/vfork.2

.\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999
.\"
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.\" manual provided the copyright notice and this permission notice are
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.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
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.\" 1999-11-10: Merged text taken from the page contributed by
.\" Reed H. Petty (rhp@draper.net)
.\"
.TH VFORK 2 2009-06-21 "Linux" "Linux Programmer's Manual"
.SH NAME
vfork \- create a child process and block parent
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
.B pid_t vfork(void);
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR vfork ():
.ad l
.RS 4
.PD 0
.TP 4
Since glibc 2.12:
.nf
_BSD_SOURCE ||
    (_XOPEN_SOURCE\ >=\ 500 ||
        _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
    !(_POSIX_C_SOURCE\ >=\ 200809L || _XOPEN_SOURCE\ >=\ 700)
.TP 4
.fi
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
.PD
.RE
.ad b
.SH DESCRIPTION
.SS "Standard Description"
(From POSIX.1)
The
.BR vfork ()
function has the same effect as
.BR fork (2),
except that the behavior is undefined if the process created by
.BR vfork ()
either modifies any data other than a variable of type
.I pid_t
used to store the return value from
.BR vfork (),
or returns from the function in which
.BR vfork ()
was called, or calls any other function before successfully calling
.BR _exit (2)
or one of the
.BR exec (3)
family of functions.
.SS "Linux Description"
.BR vfork (),
just like
.BR fork (2),
creates a child process of the calling process.
For details and return value and errors, see
.BR fork (2).
.PP
.BR vfork ()
is a special case of
.BR clone (2).
It is used to create new processes without copying the page tables of
the parent process.
It may be useful in performance-sensitive applications
where a child is created which then immediately issues an
.BR execve (2).
.PP
.BR vfork ()
differs from
.BR fork (2)
in that the parent is suspended until the child terminates
(either normally,
by calling
.BR _exit (2),
or abnormally, after delivery of a fatal signal),
or it makes a call to
.BR execve (2).
Until that point, the child shares all memory with its parent,
including the stack.
The child must not return from the current function or call
.BR exit (3),
but may call
.BR _exit (2).
.PP
Signal handlers are inherited, but not shared.
Signals to the parent
arrive after the child releases the parent's memory
(i.e., after the child terminates
or calls
.BR execve (2)).
.SS "Historic Description"
Under Linux,
.BR fork (2)
is implemented using copy-on-write pages, so the only penalty incurred by
.BR fork (2)
is the time and memory required to duplicate the parent's page tables,
and to create a unique task structure for the child.
However, in the bad old days a
.BR fork (2)
would require making a complete copy of the caller's data space,
often needlessly, since usually immediately afterwards an
.BR exec (3)
is done.
Thus, for greater efficiency, BSD introduced the
.BR vfork ()
system call, which did not fully copy the address space of
the parent process, but borrowed the parent's memory and thread
of control until a call to
.BR execve (2)
or an exit occurred.
The parent process was suspended while the
child was using its resources.
The use of
.BR vfork ()
was tricky: for example, not modifying data
in the parent process depended on knowing which variables were
held in a register.
.SH "CONFORMING TO"
4.3BSD, POSIX.1-2001.
POSIX.1-2008 removes the specification of
.BR vfork ().
The requirements put on
.BR vfork ()
by the standards are weaker than those put on
.BR fork (2),
so an implementation where the two are synonymous is compliant.
In particular, the programmer cannot rely on the parent
remaining blocked until the child either terminates or calls
.BR execve (2),
and cannot rely on any specific behavior with respect to shared memory.
.\" In AIXv3.1 vfork is equivalent to fork.
.SH NOTES
.SS Linux Notes
Fork handlers established using
.BR pthread_atfork (3)
are not called when a multithreaded program employing
the NPTL threading library calls
.BR vfork ().
Fork handlers are called in this case in a program using the
LinuxThreads threading library.
(See
.BR pthreads (7)
for a description of Linux threading libraries.)
.SS History
The
.BR vfork ()
system call appeared in 3.0BSD.
.\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
.\" present, but definitely on its way out'.
In 4.4BSD it was made synonymous to
.BR fork (2)
but NetBSD introduced it again,
cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
In Linux, it has been equivalent to
.BR fork (2)
until 2.2.0-pre6 or so.
Since 2.2.0-pre9 (on i386, somewhat later on
other architectures) it is an independent system call.
Support was added in glibc 2.0.112.
.SH BUGS
It is rather unfortunate that Linux revived this specter from the past.
The BSD man page states:
"This system call will be eliminated when proper system sharing mechanisms
are implemented.
Users should not depend on the memory sharing semantics of
.BR vfork ()
as it will, in that case, be made synonymous to
.BR fork (2).\c
"

Details of the signal handling are obscure and differ between systems.
The BSD man page states:
"To avoid a possible deadlock situation, processes that are children
in the middle of a
.BR vfork ()
are never sent
.B SIGTTOU
or
.B SIGTTIN
signals; rather, output or
.IR ioctl s
are allowed and input attempts result in an end-of-file indication."
.\"
.\" As far as I can tell, the following is not true in 2.6.19:
.\" Currently (Linux 2.3.25),
.\" .BR strace (1)
.\" cannot follow
.\" .BR vfork ()
.\" and requires a kernel patch.
.SH "SEE ALSO"
.BR clone (2),
.BR execve (2),
.BR fork (2),
.BR unshare (2),
.BR wait (2)