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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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POSIX_S...ADDCLOSE(3P) POSIX Programmer's Manual POSIX_S...ADDCLOSE(3P)
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.
posix_spawn_file_actions_addclose,
posix_spawn_file_actions_addopen — add close or open action to
spawn file actions object (ADVANCED REALTIME)
#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);
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.
POSIX.1‐2008 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(3p)).
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.
Upon successful completion, these functions shall return zero;
otherwise, an error number shall be returned to indicate the
error.
The posix_spawn_file_actions_addopen() function shall fail if:
EBADF The value specified by fildes is negative or greater than
or equal to {OPEN_MAX}.
The posix_spawn_file_actions_addclose() function shall fail if:
EBADF The value specified by fildes is negative.
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.
None.
These functions are part of the Spawn option and need not be
provided on all implementations.
Implementations may use file descriptors that must be inherited
into child processes for the child process to remain conforming,
such as for message catalog or tracing purposes. Therefore, an
application that calls posix_spawn_file_actions_addclose() with
an arbitrary integer risks non-conforming behavior, and this
function can only portably be used to close file descriptor
values that the application has obtained through explicit
actions, or for the three file descriptors corresponding to the
standard file streams. In order to avoid a race condition of
leaking an unintended file descriptor into a child process, an
application should consider opening all file descriptors with the
FD_CLOEXEC bit set unless the file descriptor is intended to be
inherited across exec.
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
pathnames 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.
The posix_spawn_file_actions_addclose() function is not required
to check whether the file descriptor is less than {OPEN_MAX}
because on some implementations {OPEN_MAX} reflects the
RLIMIT_NOFILE soft limit and therefore calling setrlimit() to
reduce this limit can result in an {OPEN_MAX} value less than or
equal to an already open file descriptor. Applications need to
be able to close such file descriptors on spawn. On
implementations where {OPEN_MAX} does not change, it is
recommended that posix_spawn_file_actions_addclose() should
return [EBADF] if fildes is greater than or equal to {OPEN_MAX}.
None.
close(3p), dup(3p), open(3p), posix_spawn(3p),
posix_spawn_file_actions_adddup2(3p),
posix_spawn_file_actions_destroy(3p)
The Base Definitions volume of POSIX.1‐2017, spawn.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 POSIX_S...ADDCLOSE(3P)
Pages that refer to this page: spawn.h(0p), posix_spawn(3p), posix_spawn_file_actions_adddup2(3p), posix_spawn_file_actions_addopen(3p), posix_spawn_file_actions_destroy(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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