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VFORK
Section: POSIX Programmer's Manual (3P)
Updated: 2003
Index
Return to Main Contents
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withdrawn in a future version.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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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