VFORK(2) System Calls Manual VFORK(2)NAME
vfork - spawn new process in a virtual memory efficient way
pid = vfork()
Vfork can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in
a paged environment. It is useful when the purpose of fork(2) would have been to create a new system context for an execve. Vfork differs
from fork in that the child borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to
exit(2) or abnormally.) The parent process is suspended while the child is using its resources.
Vfork returns 0 in the child's context and (later) the pid of the child in the parent's context.
Vfork can normally be used just like fork. It does not work, however, to return while running in the childs context from the procedure
that called vfork since the eventual return from vfork would then return to a no longer existent stack frame. Be careful, also, to call
_exit rather than exit if you can't execve, since exit will flush and close standard I/O channels, and thereby mess up the parent processes
standard I/O data structures. (Even with fork it is wrong to call exit since buffered data would then be flushed twice.)
SEE ALSO fork(2), execve(2), sigvec(2), wait(2),
Same as for fork.
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.
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.
4th Berkeley Distribution June 30, 1985 VFORK(2)
Check Out this Related Man Page
VFORK(2) Linux Programmer's Manual VFORK(2)NAME
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 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 parent is suspended until the child terminates (either normally, by calling _exit(2), or abnor-
mally, 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)).
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 know-
ing which variables were held in a register.
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
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 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.
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 3.25 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 2009-06-21 VFORK(2)