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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | VERSIONS | STANDARDS | HISTORY | CAVEATS | BUGS | SEE ALSO | COLOPHON |
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vfork(2) System Calls Manual vfork(2)
vfork - create a child process and block parent
Standard C library (libc, -lc)
#include <unistd.h>
pid_t vfork(void);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
vfork():
Since glibc 2.12:
(_XOPEN_SOURCE >= 500) && ! (_POSIX_C_SOURCE >= 200809L)
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
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 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) (which would
have the effect of calling exit handlers established by the
parent process and flushing the parent's stdio(3) buffers), 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)).
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
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.
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.” 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.)
• On systems where memory is constrained, vfork() avoids the
need to temporarily commit memory (see the description of
/proc/sys/vm/overcommit_memory in proc(5)) in order to execute
a new program. (This can be especially beneficial where a
large parent process wishes to execute a small helper program
in a child process.) By contrast, using fork(2) in this
scenario requires either committing an amount of memory equal
to the size of the parent process (if strict overcommitting is
in force) or overcommitting memory with the risk that a
process is terminated by the out-of-memory (OOM) killer.
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.)
A call to vfork() is equivalent to calling clone(2) with flags
specified as:
CLONE_VM | CLONE_VFORK | SIGCHLD
None.
4.3BSD; POSIX.1-2001 (but marked OBSOLETE). POSIX.1-2008 removes
the specification of vfork().
The vfork() system call appeared in 3.0BSD. In 4.4BSD it was
made synonymous to fork(2) but NetBSD introduced it again; see
⟨http://www.netbsd.org/Documentation/kernel/vfork.html⟩. In
Linux, it has been equivalent to fork(2) until Linux 2.2.0-pre6
or so. Since Linux 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.
The child process should take care not to modify the memory in
unintended ways, since such changes will be seen by the parent
process once the child terminates or executes another program.
In this regard, signal handlers can be especially problematic: if
a signal handler that is invoked in the child of vfork() changes
memory, those changes may result in an inconsistent process state
from the perspective of the parent process (e.g., memory changes
would be visible in the parent, but changes to the state of open
file descriptors would not be visible).
When vfork() is called in a multithreaded process, only the
calling thread is suspended until the child terminates or
executes a new program. This means that the child is sharing an
address space with other running code. This can be dangerous if
another thread in the parent process changes credentials (using
setuid(2) or similar), since there are now two processes with
different privilege levels running in the same address space. As
an example of the dangers, suppose that a multithreaded program
running as root creates a child using vfork(). After the
vfork(), a thread in the parent process drops the process to an
unprivileged user in order to run some untrusted code (e.g.,
perhaps via plug-in opened with dlopen(3)). In this case,
attacks are possible where the parent process uses mmap(2) to map
in code that will be executed by the privileged child process.
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."
clone(2), execve(2), _exit(2), fork(2), unshare(2), wait(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 vfork(2)
Pages that refer to this page: strace(1), clone(2), fork(2), getpid(2), ptrace(2), setns(2), syscalls(2), unshare(2), posix_spawn(3), persistent-keyring(7), pid_namespaces(7), session-keyring(7), user-keyring(7), user-session-keyring(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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