The
vfork system call creates a new process that does not have a new virtual address space, but rather shares address space with the parent, thus avoiding potentially expensive copy-on-write operations normally associated with creating a new process. 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 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.
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(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 can't
execve(2), since
exit(3) will flush and close standard I/O channels, and thereby mess up the standard I/O data structures in the parent process. (Even with
fork(2) it is wrong to call
exit(3) since buffered data would then be flushed twice.)