VFORK(2)	    UNIX Programmer's Manual		 VFORK(2)


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

SYNOPSIS
     pid = vfork()
     int pid;

DESCRIPTION
     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 use-
     ful when the purpose of fork(2) would have been to create a
     new system context for an execve.	Vfork differs from fork
     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.

     Vfork returns 0 in the child's context and (later) the pid
     of the child in the parent's context.

     Vfork can normally be used just like fork. 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 rather than exit if
     you can't execve, since exit will flush and close standard
     I/O channels, and thereby mess up the parent processes stan-
     dard I/O data structures.	(Even with fork it is wrong to
     call exit since buffered data would then be flushed twice.)

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

DIAGNOSTICS
     Same as for fork.

BUGS
     This system call will be eliminated when proper system shar-
     ing 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.

     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 ioctls are allowed and
     input attempts result in an end-of-file indication.


Printed 11/26/99	  June 30, 1985                         1


 
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