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POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE(3P)ammer's_ManualFILE_ACTIONS_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.1‐2008, spawn.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013POSIX_SPAWN_FILE_ACTIONS_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)