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VFORK(3P)                  POSIX Programmer's Manual                 VFORK(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       vfork - create a new process; share virtual memory
SYNOPSIS
       #include <unistd.h>
       pid_t vfork(void);

DESCRIPTION
       The vfork() function shall be equivalent to  fork(),  except  that  the
       behavior is undefined if the process created by vfork() either modifies
       any data other than a variable of type pid_t used to store  the  return
       value  from  vfork(), or returns from the function in which vfork() was
       called, or calls any other function before successfully calling _exit()
       or one of the exec family of functions.
RETURN VALUE
       Upon successful completion, vfork() shall return 0 to the child process
       and return the process ID of the child process to the  parent  process.
       Otherwise,  -1  shall be returned to the parent, no child process shall
       be created, and errno shall be set to indicate the error.
ERRORS
       The vfork() function shall fail if:
       EAGAIN The system-wide limit on the total  number  of  processes  under
              execution  would be exceeded, or the system-imposed limit on the
              total number of processes under execution by a single user would
              be exceeded.
       ENOMEM There is insufficient swap space for the new process.

       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       Conforming  applications  are recommended not to depend on vfork(), but
       to use fork() instead. The vfork()  function  may  be  withdrawn  in  a
       future version.
       On some implementations, vfork() is equivalent to fork().
       The vfork() function differs from fork() only in that the child process
       can share code and data with the calling process (parent process). This
       speeds cloning activity significantly at a risk to the integrity of the
       parent process if vfork() is misused.
       The use of vfork() for any purpose except as a prelude to an  immediate
       call to a function from the exec family, or to _exit(), is not advised.
       The  vfork() function can be used to create new processes without fully
       copying the address space of the old process. If a  forked  process  is
       simply going to call exec, the data space copied from the parent to the
       child by fork() is not used. This  is  particularly  inefficient  in  a
       paged  environment,  making vfork() particularly useful. Depending upon
       the size of the parent's data space, vfork()  can  give  a  significant
       performance improvement over fork().
       The  vfork()  function  can normally be used just like fork().  It does
       not work, however, to return while running in the child's context  from
       the caller of vfork() since the eventual return from vfork() would then
       return to a no longer existent stack  frame.   Care  should  be  taken,
       also,  to call _exit() rather than exit() if exec cannot be used, since
       exit() flushes and closes standard I/O channels, thereby  damaging  the
       parent  process' standard I/O data structures. (Even with fork(), it is
       wrong to call exit(), since buffered data would then be flushed twice.)
       If signal handlers are invoked in the child process after vfork(), they
       must follow the same rules as other code in the child process.
RATIONALE
       None.
FUTURE DIRECTIONS
       This function may be withdrawn in a future version.
SEE ALSO
       exec(),   exit(),  fork(),  wait(),  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                            VFORK(3P)