NAME
vfork - spawn new process in a virtual memory efficient way

SYNOPSIS
pid_t vfork()

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.