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@(#)vfork.2 8.1 (Berkeley) 6/4/93
$FreeBSD: head/lib/libc/sys/vfork.2 84306 2001-10-01 16:09:29Z ru $
.Dd June 4, 1993 .Dt VFORK 2 .Os .Sh NAME .Nm vfork .Nd spawn new process in a virtual memory efficient way .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn vfork void .Sh DESCRIPTION .Fn 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 .Xr fork 2 would have been to create a new system context for an .Xr execve 2 . .Fn Vfork differs from .Xr fork 2 in that the child borrows the parent's memory and thread of control until a call to .Xr execve 2 or an exit (either by a call to .Xr _exit 2 or abnormally). The parent process is suspended while the child is using its resources.
p .Fn Vfork returns 0 in the child's context and (later) the pid of the child in the parent's context.
p .Fn Vfork can normally be used just like .Xr fork 2 . It does not work, however, to return while running in the childs context from the procedure that called .Fn vfork since the eventual return from .Fn vfork would then return to a no longer existent stack frame. Be careful, also, to call .Xr _exit 2 rather than .Xr exit 3 if you can't .Xr execve 2 , since .Xr exit 3 will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (Even with .Xr fork 2 it is wrong to call .Xr exit 3 since buffered data would then be flushed twice.) .Sh SEE ALSO .Xr execve 2 , .Xr fork 2 , .Xr rfork 2 , .Xr sigvec 2 , .Xr wait 2 , .Xr _exit 2 , .Xr exit 3 .Sh RETURN VALUES Same as for .Xr fork 2 . .Sh BUGS This system call will be eliminated when proper system sharing mechanisms are implemented. Users should not depend on the memory sharing semantics of .Xr vfork 2 as it will, in that case, be made synonymous to .Xr fork 2 .
p To avoid a possible deadlock situation, processes that are children in the middle of a .Fn vfork are never sent .Dv SIGTTOU or .Dv SIGTTIN signals; rather, output or .Xr ioctl 2 calls are allowed and input attempts result in an end-of-file indication. .Sh HISTORY The .Fn vfork function call appeared in x 2.9 .