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

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

SYNOPSIS
     #include <unistd.h>

     pidt
     vfork(void)

DESCRIPTION
     Vfork() can be used to create new processes without fully copying the ad-
     dress 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).  Vfork() differs
     from fork in that the child borrows the parent's memory and thread of
     control until a call to execve 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 process ID 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 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
     rather than exit(3) if you can't execve,  since exit will flush and close
     standard I/O channels, and thereby mess up the parent process's standard
     I/O data structures.  (Even with fork it is wrong to call exit since
     buffered data would then be flushed twice.)

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

DIAGNOSTICS
     Same as for fork.

BUGS
     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.

     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.

HISTORY
     The vfork function call appeared in 3.0BSD.

4th Berkeley Distribution       March 26, 1993                               1