The vfork system call 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(2). The vfork system call differs from fork(2) in that the child borrows the parents 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.
The vfork system call returns 0 in the childs context and (later) the pid of the child in the parents context.
The vfork system call can normally be used just like fork(2). 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(2) rather than exit(3) if you cannot execve(2), since exit(3) will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (Even with fork(2) it is wrong to call exit(3) since buffered data would then be flushed twice.)
Same as for fork(2).
execve(2), _exit(2), fork(2), rfork(2), sigvec(2), wait(2), exit(3)