man-pages/man2/vfork.2

.\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999
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.\" 1999-11-10: Merged text taken from the page contributed by
.\" Reed H. Petty (rhp@draper.net)
.\"
.TH VFORK 2 1999-11-01 "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);
.SH DESCRIPTION
.SS "Standard Description"
(From SUSv2 / POSIX draft.)
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 will be 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 makes a call to
.BR execve (2)
or
.BR _exit (2).
The child shares all memory with its parent, including the stack, until
.BR execve (2)
is issued by the child.
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.
.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, that 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 are
held in a register.
.SH "CONFORMING TO"
4.3BSD, POSIX.1-2001.
.\" FIXME . Mar 07: in the draft of the next POSIX revision, the spec for
.\" vfork() has been removed.  See if this remains so in final version.
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 a call of
.BR execve (2)
or
.BR _exit (2)
and cannot rely on any specific behavior wth 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)