vfork(2)

NAME

vfork − spawn new process in a virtual memory efficient way

SYNTAX

pid = vfork()
int pid;

DESCRIPTION

The 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() would have been to create a new system context for an execve. The vfork system call differs from fork in that the child borrows the parent’s memory and thread of control until a call to execve() or an exit (either by a call to exit() or abnormally.) The parent process is suspended while the child is using its resources.

The vfork system call returns 0 in the child’s context and (later) the pid of the child in the parent’s context.

The vfork system call 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 cannot 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.

RESTRICTIONS

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 ioctls are allowed and input attempts result in an end-of-file indication.

DIAGNOSTICS

Same as for fork.

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