vfork(2) - NetBSD Manual Pages

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VFORK(2)                  NetBSD 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

     The vfork system call creates a new process that does not have a new vir-
     tual address space, but rather shares address space with the parent, thus
     avoiding potentially expensive copy-on-write operations normally associ-
     ated with creating a new process.  It is useful when the purpose of
     fork(2) would have been to create a new system context for an execve(2).
     The vfork system call 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.

     The vfork system call returns 0 in the child's context and (later) the
     pid of the child in the parent's context.

     The vfork system call can normally be used just like fork(2).  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(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 standard I/O data structures in the parent process.  (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).


ERRORS

     Same as for fork(2).


SEE ALSO

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


HISTORY

     The vfork() function call appeared in 3.0BSD.  In 4.4BSD, the semantics
     were changed to only suspend the parent.  The original semantics were
     reintroduced in NetBSD 1.4.


BUGS

     Users should not depend on the memory sharing semantics of vfork() as
     other ways of speeding up the fork process may be developed in the
     future.

     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.

NetBSD 2.1                      January 3, 1998                     NetBSD 2.1