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(): _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 will
be created which then immediately issues an execve(2).
vfork() differs from fork(2) in that the parent 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).
Signal handlers are inherited, but not shared. Signals 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 afterwards 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 are held in a register.
CONFORMING TO
4.3BSD, POSIX.1-2001. 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
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.)
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
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,
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 3.24 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/.