VFORK

Section: Linux Programmer's Manual (2)
Updated: 2009-06-21
Index Return to Main Contents

NAME

vfork - create a child process and block parent

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

pid_t vfork(void);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

vfork(): _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 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."

COLOPHON

This page is part of release 3.22 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/.