vfork (2)
Leading comments
Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999 and Copyright 2006, 2012, 2017 Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is di...
NAME
vfork - create a child process and block parentSYNOPSIS
#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:
-
_BSD_SOURCE || (_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) && !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700) - Before glibc 2.12: _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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. 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."