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NAME
vfork - create a child process and block parent
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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 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.
STANDARDS
4.3BSD; POSIX.1-2001 (but marked OBSOLETE). 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
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(2)." 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.
Caveats
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.
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
History
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.
BUGS
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), _exit(2), fork(2), unshare(2), wait(2)