VFORK(2) BSD Programmer's Manual VFORK(2)
vfork - spawn new process in a virtual memory efficient way
Vfork() can be used to create new processes without fully copying the ad-
dress 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 ab-
normally.) The parent process is suspended while the child is using its
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.)
fork(2), execve(2), sigvec(2), wait(2),
Same as for fork.
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 ioctl(2) calls are allowed and input attempts result in
an end-of-file indication.
The vfork function call appeared in 3.0BSD.
4th Berkeley Distribution June 4, 1993 1