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 correspond-
       ing 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 The vfork() function may  be  withdrawn  in  a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 Technol-
       ogy -- 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)