(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."
This page is part of release 3.14 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/.
