posix_spawn_file_actions_addclose(3p) — Linux manual page



PROLOG         top

       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         top

       posix_spawn_file_actions_addopen — add close or open action to
       spawn file actions object (ADVANCED REALTIME)

SYNOPSIS         top

       #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         top

       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

       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

       The string described by path shall be copied by the
       posix_spawn_file_actions_addopen() function.

RETURN VALUE         top

       Upon successful completion, these functions shall return zero;
       otherwise, an error number shall be returned to indicate the

ERRORS         top

       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         top



       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         top

       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

       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}.



SEE ALSO         top

       close(3p), dup(3p), open(3p), posix_spawn(3p),

       The Base Definitions volume of POSIX.1‐2017, spawn.h(0p)

COPYRIGHT         top

       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)