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

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

SYNOPSIS
     #include <unistd.h>

     pid_t
     vfork(void);

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 useful 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 standard
     I/O data structures.  (Even with fork it is wrong to call exit since
     buffered data would then be flushed twice.)

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

ERRORS
     The vfork() system call will fail for any of the reasons described in the
     fork man page.  In addition, it will fail if:

     [EINVAL]           A system call other than _exit() or execve() (or libc
                        functions that make no system calls other than those)
                        is called following calling a vfork() call.

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        June 4, 1993        4th Berkeley Distribution