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 (con-
       sult 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       withdrawn in a future version.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 exis-
       tent 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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     VFORK(3P)