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 2.2.0" "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 "STANDARD DESCRIPTION"
(From XPG4 / SUSv2 / POSIX draft.)
The
.BR vfork ()
function has the same effect as
.BR fork (),
except that the behaviour is undefined if the process created by
.BR vfork ()
either modifies any data other than a variable of type 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 ()
or one of the
.BR exec ()
family of functions.
.SH ERRORS
.TP
.B EAGAIN
Too many processes; try again.
.TP
.B ENOMEM
There is insufficient swap space for the new process.
.SH "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 () .
.PP
.BR vfork ()
differs from 
.BR fork ()
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 ()
is issued by the child.  The child must not return from the
current function or call
.BR exit (),
but may call
.BR _exit ().
.PP
Signal handlers are inherited, but not shared.  Signals to the parent
arrive after the child releases the parent.
.SH "HISTORIC DESCRIPTION"
Under Linux,
.BR fork ()
is implemented using copy-on-write pages, so the only penalty incurred by
.BR fork ()
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 ()
would require making a complete copy of the caller's data space,
often needlessly, since usually immediately afterwards an
.BR exec ()
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 ()
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 BUGS
It is rather unfortunate that Linux revived this spectre from the past.
The BSD manpage 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 ().\c
"

Formally speaking, the standard description given above does not allow
one to use
.BR vfork ()
since a following
.BR exec ()
might fail, and then what happens is undefined.

Details of the signal handling are obscure and differ between systems.
The BSD manpage states:
"To avoid a possible deadlock situation, processes that are children
in the middle of a
.BR vfork ()
are never sent SIGTTOU or SIGTTIN signals; rather, output or
.IR ioctl s
are allowed and input attempts result in an end-of-file indication."

Currently (Linux 2.3.25),
.BR strace (1)
cannot follow
.BR vfork ()
and requires a kernel patch.
.SH 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 ()
but NetBSD introduced it again,
cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
In Linux, it has been equivalent to
.BR fork ()
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 "CONFORMING TO"
The
.BR vfork ()
call may be a bit similar to calls with the same name in other
operating systems. The requirements put on
.BR vfork ()
by the standards are weaker than those put on
.BR fork (),
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 ()
or
.BR _exit ()
and cannot rely on any specific behaviour w.r.t. shared memory.
.\" In AIXv3.1 vfork is equivalent to fork.
.SH "SEE ALSO"
.BR clone (2),
.BR execve (2),
.BR fork (2),
.BR unshare (2),
.BR wait (2)