vfork(2)
NAME
vfork - create a child process and block parent
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
pid_t vfork(void);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vfork():
Since glibc 2.12:
(_XOPEN_SOURCE >= 500) && ! (_POSIX_C_SOURCE >= 200809L)
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
DESCRIPTION
Standard description
(From POSIX.1) The vfork() function has the same effect as fork(2),
except that the behavior is undefined if the process created by vfork()
either modifies any data other than a variable of type pid_t used to
store the return value from vfork(), or returns from the function in
which vfork() was called, or calls any other function before
successfully calling _exit(2) or one of the exec(3) family of
functions.
Linux description
vfork(), just like fork(2), creates a child process of the calling
process. For details and return value and errors, see fork(2).
vfork() is a special case of 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 execve(2).
vfork() differs from fork(2) in that the calling thread is suspended
until the child terminates (either normally, by calling _exit(2), or
abnormally, after delivery of a fatal signal), or it makes a call to
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 exit(3), but may call _exit(2).
As with fork(2), the child process created by vfork() inherits copies
of various of the caller's process attributes (e.g., file descriptors,
signal dispositions, and current working directory); the vfork() call
differs only in the treatment of the virtual address space, as
described above.
Signals sent to the parent arrive after the child releases the parent's
memory (i.e., after the child terminates or calls execve(2)).
Historic description
Under Linux, fork(2) is implemented using copy-on-write pages, so the
only penalty incurred by 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 fork(2) would
require making a complete copy of the caller's data space, often
needlessly, since usually immediately afterward an exec(3) is done.
Thus, for greater efficiency, BSD introduced the 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
execve(2) or an exit occurred. The parent process was suspended while
the child was using its resources. The use of vfork() was tricky: for
example, not modifying data in the parent process depended on knowing
which variables were held in a register.
CONFORMING TO
4.3BSD; POSIX.1-2001 (but marked OBSOLETE). POSIX.1-2008 removes the
specification of vfork().
The requirements put on vfork() by the standards are weaker than those
put on 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 execve(2),
and cannot rely on any specific behavior with respect to shared memory.
NOTES
Some consider the semantics of vfork() to be an architectural blemish,
and the 4.2BSD man page stated: "This system call will be eliminated
when proper system sharing mechanisms are implemented. Users should
not depend on the memory sharing semantics of vfork() as it will, in
that case, be made synonymous to fork(2)." However, even though modern
memory management hardware has decreased the performance difference
between fork(2) and vfork(), there are various reasons why Linux and
other systems have retained vfork():
* Some performance-critical applications require the small performance
advantage conferred by vfork().
* vfork() can be implemented on systems that lack a memory-management
unit (MMU), but fork(2) can't be implemented on such systems.
(POSIX.1-2008 removed vfork() from the standard; the POSIX rationale
for the posix_spawn(3) function notes that that function, which
provides functionality equivalent to fork(2)+exec(3), is designed to
be implementable on systems that lack an MMU.)
Linux notes
Fork handlers established using pthread_atfork(3) are not called when a
multithreaded program employing the NPTL threading library calls
vfork(). Fork handlers are called in this case in a program using the
LinuxThreads threading library. (See pthreads(7) for a description of
Linux threading libraries.)
A call to vfork() is equivalent to calling clone(2) with flags
specified as:
CLONE_VM | CLONE_VFORK | SIGCHLD
History
The vfork() system call appeared in 3.0BSD. In 4.4BSD it was made
synonymous to fork(2) but NetBSD introduced it again, cf.
http://www.netbsd.org/Documentation/kernel/vfork.html. In Linux, it
has been equivalent to 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.
BUGS
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 vfork() are never sent
SIGTTOU or SIGTTIN signals; rather, output or ioctls are allowed and
input attempts result in an end-of-file indication."
SEE ALSO
clone(2), execve(2), fork(2), unshare(2), wait(2)
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A
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latest version of this page, can be found at
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