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VFORK(2) Linux Programmer's Manual VFORK(2)
vfork - create a child process and block parent
#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
(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.
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)).
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.
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.
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.)
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
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.
Details of the signal handling are obscure and differ between sys-
tems. The BSD man page states: "To avoid a possible deadlock situa-
tion, 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."
clone(2), execve(2), fork(2), unshare(2), wait(2)
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-08-05 VFORK(2)
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