POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE(3P)                POSIX Programmer's Manual

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
       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.  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()).

       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
       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.

EXAMPLES
       None.

APPLICATION USAGE
       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.

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 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}.

FUTURE DIRECTIONS
       None.

SEE ALSO
       close(), dup(), open(), posix_spawn(),
       posix_spawn_file_actions_adddup2(), posix_spawn_file_actions_destroy()

       The Base Definitions volume of POSIX.1-2017, <spawn.h>

COPYRIGHT
       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_SPAWN_FILE_ACTIONS_ADDCLOSE(3P)