Provided by: freebsd-manpages_7.0-2_all
vfork - spawn new process in a virtual memory efficient way
Standard C Library (libc, -lc)
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 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 child’s 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
Same as for fork(2).
execve(2), _exit(2), fork(2), rfork(2), sigvec(2), wait(2), exit(3)
The vfork() system call appeared in 2.9BSD.
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(2).
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.