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VFORK(2)                 DragonFly 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>

     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(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,
     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(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.)


RETURN VALUES

     Same as for fork(2).


SEE ALSO

     execve(2), fork(2), rfork(2), sigaction(2), wait(2), _exit(2), exit(3)


HISTORY

     The vfork() function call appeared in 2.9BSD.


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(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.

DragonFly 5.5                  November 16, 2012                 DragonFly 5.5

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