VFORK(2) Linux Programmers Manual VFORK(2)
vfork - create a child process and block parent
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vfork(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
(From SUSv2 / POSIX draft.) The vfork() function has the same effect
as fork(2), except that the behavior is undefined if the process cre
ated 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
vfork(), just like fork(2), creates a child process of the calling pro
cess. For details and return value and errors, see fork(2).
vfork() is a special case of clone(2). It is used to create new pro
cesses 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 execve(2).
vfork() differs from fork(2) in that the parent is suspended until the
child makes a call to execve(2) or _exit(2). The child shares all mem
ory with its parent, including the stack, until execve(2) is issued by
the child. The child must not return from the current function or call
exit(3), but may call _exit(2).
Signal handlers are inherited, but not shared. Signals to the parent
arrive after the child releases the parents memory.
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 parents page tables, and to create a unique task struc
ture for the child. However, in the bad old days a fork(2) would
require making a complete copy of the callers data space, often need
lessly, since usually immediately afterwards an exec(3) is done. Thus,
for greater efficiency, BSD introduced the vfork() system call, that
did not fully copy the address space of the parent process, but bor
rowed the parents 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 are held in a register.
4.3BSD, POSIX.1-2001. 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 a call of execve(2) or
_exit(2) and cannot rely on any specific behavior with respect to
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.)
The vfork() system call appeared in 3.0BSD. In 4.4BSD it was made syn
onymous 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.
It is rather unfortunate that Linux revived this specter from the past.
The BSD man page states: "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)."
Details of the signal handling are obscure and differ between systems.
The BSD man page states: "To avoid a possible deadlock situation, pro
cesses 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 3.05 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 2007-07-26 VFORK(2)