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NAME
posix_spawn_file_actions_addclose, posix_spawn_file_actions_addopen - add close or open
action to spawn file actions object (ADVANCED REALTIME)
SYNOPSIS
#include <spawn.h>
int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *
file_actions, int fildes);
int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *
restrict file_actions, int fildes,
const char *restrict path, int oflag, mode_t mode);
DESCRIPTION
These functions shall add or delete a close or open action to a spawn file actions object.
A spawn file actions object is of type posix_spawn_file_actions_t (defined in <spawn.h>)
and is used to specify a series of actions to be performed by a posix_spawn() or
posix_spawnp() operation in order to arrive at the set of open file descriptors for the
child process given the set of open file descriptors of the parent. IEEE Std 1003.1-2001
does not define comparison or assignment operators for the type
posix_spawn_file_actions_t.
A spawn file actions object, when passed to posix_spawn() or posix_spawnp(), shall specify
how the set of open file descriptors in the calling process is transformed into a set of
potentially open file descriptors for the spawned process. This transformation shall be as
if the specified sequence of actions was performed exactly once, in the context of the
spawned process (prior to execution of the new process image), in the order in which the
actions were added to the object; additionally, when the new process image is executed,
any file descriptor (from this new set) which has its FD_CLOEXEC flag set shall be closed
(see posix_spawn() ).
The posix_spawn_file_actions_addclose() function shall add a close action to the object
referenced by file_actions that shall cause the file descriptor fildes to be closed (as if
close( fildes) had been called) when a new process is spawned using this file actions
object.
The posix_spawn_file_actions_addopen() function shall add an open action to the object
referenced by file_actions that shall cause the file named by path to be opened (as if
open( path, oflag, mode) had been called, and the returned file descriptor, if not fildes,
had been changed to fildes) when a new process is spawned using this file actions object.
If fildes was already an open file descriptor, it shall be closed before the new file is
opened.
The string described by path shall be copied by the posix_spawn_file_actions_addopen()
function.
RETURN VALUE
Upon successful completion, these functions shall return zero; otherwise, an error number
shall be returned to indicate the error.
ERRORS
These functions shall fail if:
EBADF The value specified by fildes is negative or greater than or equal to {OPEN_MAX}.
These functions may fail if:
EINVAL The value specified by file_actions is invalid.
ENOMEM Insufficient memory exists to add to the spawn file actions object.
It shall not be considered an error for the fildes argument passed to these functions to
specify a file descriptor for which the specified operation could not be performed at the
time of the call. Any such error will be detected when the associated file actions object
is later used during a posix_spawn() or posix_spawnp() operation.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
These functions are part of the Spawn option and need not be provided on all
implementations.
RATIONALE
A spawn file actions object may be initialized to contain an ordered sequence of close(),
dup2(), and open() operations to be used by posix_spawn() or posix_spawnp() to arrive at
the set of open file descriptors inherited by the spawned process from the set of open
file descriptors in the parent at the time of the posix_spawn() or posix_spawnp() call.
It had been suggested that the close() and dup2() operations alone are sufficient to
rearrange file descriptors, and that files which need to be opened for use by the spawned
process can be handled either by having the calling process open them before the
posix_spawn() or posix_spawnp() call (and close them after), or by passing filenames to
the spawned process (in argv) so that it may open them itself. The standard developers
recommend that applications use one of these two methods when practical, since detailed
error status on a failed open operation is always available to the application this way.
However, the standard developers feel that allowing a spawn file actions object to specify
open operations is still appropriate because:
1. It is consistent with equivalent POSIX.5 (Ada) functionality.
2. It supports the I/O redirection paradigm commonly employed by POSIX programs designed
to be invoked from a shell. When such a program is the child process, it may not be
designed to open files on its own.
3. It allows file opens that might otherwise fail or violate file ownership/access rights
if executed by the parent process.
Regarding 2. above, note that the spawn open file action provides to posix_spawn() and
posix_spawnp() the same capability that the shell redirection operators provide to
system(), only without the intervening execution of a shell; for example:
system ("myprog <file1 3<file2");
Regarding 3. above, note that if the calling process needs to open one or more files for
access by the spawned process, but has insufficient spare file descriptors, then the open
action is necessary to allow the open() to occur in the context of the child process after
other file descriptors have been closed (that must remain open in the parent).
Additionally, if a parent is executed from a file having a "set-user-id" mode bit set and
the POSIX_SPAWN_RESETIDS flag is set in the spawn attributes, a file created within the
parent process will (possibly incorrectly) have the parent's effective user ID as its
owner, whereas a file created via an open() action during posix_spawn() or posix_spawnp()
will have the parent's real ID as its owner; and an open by the parent process may
successfully open a file to which the real user should not have access or fail to open a
file to which the real user should have access.
File Descriptor Mapping
The standard developers had originally proposed using an array which specified the mapping
of child file descriptors back to those of the parent. It was pointed out by the ballot
group that it is not possible to reshuffle file descriptors arbitrarily in a library
implementation of posix_spawn() or posix_spawnp() without provision for one or more spare
file descriptor entries (which simply may not be available). Such an array requires that
an implementation develop a complex strategy to achieve the desired mapping without
inadvertently closing the wrong file descriptor at the wrong time.
It was noted by a member of the Ada Language Bindings working group that the approved Ada
Language Start_Process family of POSIX process primitives use a caller-specified set of
file actions to alter the normal fork()/ exec semantics for inheritance of file
descriptors in a very flexible way, yet no such problems exist because the burden of
determining how to achieve the final file descriptor mapping is completely on the
application. Furthermore, although the file actions interface appears frightening at first
glance, it is actually quite simple to implement in either a library or the kernel.
FUTURE DIRECTIONS
None.
SEE ALSO
close() , dup() , open() , posix_spawn() , posix_spawn_file_actions_adddup2() ,
posix_spawn_file_actions_destroy() , posix_spawnp() , the Base Definitions volume of
IEEE Std 1003.1-2001, <spawn.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 .