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@(#)vfork.2 8.1 (Berkeley) 6/4/93
$FreeBSD: releng/10.2/lib/libc/sys/vfork.2 283912 2015-06-02 15:02:28Z jhb $
.Dd April 6, 2015 .Dt VFORK 2 .Os .Sh NAME .Nm vfork .Nd create a new process without copying the address space .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft pid_t .Fn vfork void .Sh DESCRIPTION The .Fn 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 .Xr fork 2 would have been to create a new system context for an .Xr execve 2 . The .Fn vfork system call 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 The .Fn vfork system call returns 0 in the child's context and (later) the pid of the child in the parent's context.
p The .Fn vfork system call can normally be used just like .Xr fork 2 . It does not work, however, to return while running in the child's 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 cannot .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 RETURN VALUES Same as for .Xr fork 2 . .Sh SEE ALSO .Xr execve 2 , .Xr _exit 2 , .Xr fork 2 , .Xr rfork 2 , .Xr sigaction 2 , .Xr wait 2 , .Xr exit 3 .Sh HISTORY The .Fn vfork system call appeared in x 3 . .Sh BUGS 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.