VFORK(2) BSD System Calls 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 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 con-
trol 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
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 struc-
tures. (Even with fork it is wrong to call exit since buffered data would then be flushed
fork(2), execve(2), sigaction(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 4th Berkeley Distribution