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vfork(2) [bsd man page]

VFORK(2)							System Calls Manual							  VFORK(2)

NAME
vfork - spawn new process in a virtual memory efficient way SYNOPSIS
pid = vfork() int pid; DESCRIPTION
Vfork can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in a paged environment. It is useful when the purpose of fork(2) would have been to create a new system context for an execve. Vfork differs from fork in that the child borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to exit(2) or abnormally.) The parent process is suspended while the child is using its resources. Vfork returns 0 in the child's context and (later) the pid of the child in the parent's context. Vfork can normally be used just like fork. It does not work, however, to return while running in the childs context from the procedure that called vfork since the eventual return from vfork would then return to a no longer existent stack frame. Be careful, also, to call _exit rather than exit if you can't execve, since exit will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (Even with fork it is wrong to call exit since buffered data would then be flushed twice.) SEE ALSO
fork(2), execve(2), sigvec(2), wait(2), DIAGNOSTICS
Same as for fork. BUGS
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. 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. 4th Berkeley Distribution June 30, 1985 VFORK(2)

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VFORK(2)						     Linux Programmer's Manual							  VFORK(2)

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 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 abnor- mally, 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 know- ing which variables were 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." SEE ALSO
clone(2), execve(2), fork(2), unshare(2), wait(2) COLOPHON
This page is part of release 3.25 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/. Linux 2009-06-21 VFORK(2)

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