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VFORK(2)		    BSD	System Calls Manual		      VFORK(2)

NAME
     vfork -- spawn new	process	in a virtual memory efficient way

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <unistd.h>

     pid_t
     vfork(void);

DESCRIPTION
     The vfork system call creates a new process that does not have a new vir-
     tual address space, but rather shares address space with the parent, thus
     avoiding potentially expensive copy-on-write operations normally associ-
     ated 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.)

RETURN VALUES
     Same as for fork(2).

ERRORS
     Same as for fork(2).

SEE ALSO
     execve(2),	fork(2), sigaction(2), wait(2)

HISTORY
     The vfork() function call appeared	in 3.0BSD.  In 4.4BSD, the semantics
     were changed to only suspend the parent.  The original semantics were
     reintroduced in NetBSD 1.4.

BUGS
     Users should not depend on	the memory sharing semantics of	vfork()	as
     other ways	of speeding up the fork	process	may be developed in the	fu-
     ture.

     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.

BSD				January	3, 1998				   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | BUGS

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