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 .