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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:
_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. ⟨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
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