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GNU/Linux |
Debian 6.0.5(Squeeze) |
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vfork(2) |
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vfork − create a child process and block parent
#include
<sys/types.h>
#include <unistd.h>
pid_t vfork(void);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vfork():
Since glibc 2.12:
_BSD_SOURCE ||
(_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
!(_POSIX_C_SOURCE >= 200809L ||
_XOPEN_SOURCE >= 700)
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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 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 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 shared memory.
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.
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."
clone(2), execve(2), fork(2), unshare(2), wait(2)
This page is part of release 3.27 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/.
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vfork(2) | |