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@(#)vfork.2 4.1 (Berkeley) 05/09/85

C 4 vfork - spawn new process in a virtual memory efficient way vfork() 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 exec. Vfork differs from fork in that the child borrows the parent's memory and thread of control until a call to exec (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 which 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 exec, 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.)

Similarly when using the new signal mechanism of sigset (3) mechanism be sure to call sigsys rather than signal (2).

fork(2), exec(2), sigsys(2), wait(2), Same as for fork . This system call may be unnecessary if the system sharing mechanisms allow fork to be implemented more efficiently; users should not depend on the memory sharing semantics of vfork as it could, in that case, be made synonymous to fork.

To avoid a possible deadlock situation, processes which are children in the middle of a vfork are never sent SIGTTOU or SIGTTIN signals; rather, output or ioctl s are allowed and input attempts result in an end-of-file indication.

This call is peculiar to this version of UNIX.