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VFORK(2)		  FreeBSD System Calls Manual		      VFORK(2)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     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(2).  Vfork() differs
     from fork(2) 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(2).  It does not work, how-
     ever, 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(2) rather than exit(3) if you can't execve(2), since exit(3) will
     flush and close standard I/O channels, and thereby mess up the parent
     processes standard I/O data structures.  (Even with fork(2) it is wrong
     to call exit(3) since buffered data would then be flushed twice.)

SEE ALSO
     execve(2), fork(2), rfork(2), sigvec(2), wait(2), _exit(2), exit(3)

RETURN VALUES
     Same as for fork(2).

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(2) as it will, in that case, be made synonymous to fork(2).

     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 2.9BSD.

FreeBSD 4.9			 June 4, 1993			   FreeBSD 4.9

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