243 lines
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243 lines
13 KiB
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<title>posix_spawn_file_actions_addclose</title>
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<script type="text/javascript" language="JavaScript" src="../jscript/codes.js">
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<basefont size="3"> <a name="posix_spawn_file_actions_addclose"></a> <a name="tag_03_428"></a>
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<!-- posix_spawn_file_actions_addclose -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_428_01"></a>NAME</h4>
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<blockquote>posix_spawn_file_actions_addclose, posix_spawn_file_actions_addopen - add close or open action to spawn file actions
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object (<b>ADVANCED REALTIME</b>)</blockquote>
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<h4><a name="tag_03_428_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('SPN')">SPN</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/spawn.h.html">spawn.h</a>><br>
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<br>
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int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *<br>
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</tt> <i>file_actions</i><tt>, int</tt> <i>fildes</i><tt>);<br>
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int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *<br>
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restrict</tt> <i>file_actions</i><tt>, int</tt> <i>fildes</i><tt>,<br>
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const char *restrict</tt> <i>path</i><tt>, int</tt> <i>oflag</i><tt>, mode_t</tt>
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<i>mode</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_428_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>These functions shall add or delete a close or open action to a spawn file actions object.</p>
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<p>A spawn file actions object is of type <b>posix_spawn_file_actions_t</b> (defined in <a href=
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"../basedefs/spawn.h.html"><i><spawn.h></i></a>) and is used to specify a series of actions to be performed by a <a href=
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"../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href="../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a>
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operation in order to arrive at the set of open file descriptors for the child process given the set of open file descriptors of
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the parent. IEEE Std 1003.1-2001 does not define comparison or assignment operators for the type
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<b>posix_spawn_file_actions_t</b>.</p>
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<p>A spawn file actions object, when passed to <a href="../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href=
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"../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a>, shall specify how the set of open file descriptors in the calling
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process is transformed into a set of potentially open file descriptors for the spawned process. This transformation shall be as if
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the specified sequence of actions was performed exactly once, in the context of the spawned process (prior to execution of the new
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process image), in the order in which the actions were added to the object; additionally, when the new process image is executed,
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any file descriptor (from this new set) which has its FD_CLOEXEC flag set shall be closed (see <a href=
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"posix_spawn.html"><i>posix_spawn</i>()</a> ).</p>
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<p>The <i>posix_spawn_file_actions_addclose</i>() function shall add a <i>close</i> action to the object referenced by
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<i>file_actions</i> that shall cause the file descriptor <i>fildes</i> to be closed (as if <i>close</i>( <i>fildes</i>) had been
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called) when a new process is spawned using this file actions object.</p>
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<p>The <i>posix_spawn_file_actions_addopen</i>() function shall add an <i>open</i> action to the object referenced by
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<i>file_actions</i> that shall cause the file named by <i>path</i> to be opened (as if <i>open</i>( <i>path</i>, <i>oflag</i>,
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<i>mode</i>) had been called, and the returned file descriptor, if not <i>fildes</i>, had been changed to <i>fildes</i>) when a new
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process is spawned using this file actions object. If <i>fildes</i> was already an open file descriptor, it shall be closed before
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the new file is opened.</p>
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<p>The string described by <i>path</i> shall be copied by the <i>posix_spawn_file_actions_addopen</i>() function.</p>
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</blockquote>
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<h4><a name="tag_03_428_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, these functions shall return zero; otherwise, an error number shall be returned to indicate the
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error.</p>
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</blockquote>
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<h4><a name="tag_03_428_05"></a>ERRORS</h4>
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<blockquote>
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<p>These functions shall fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The value specified by <i>fildes</i> is negative or greater than or equal to {OPEN_MAX}.</dd>
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</dl>
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<p>These functions may fail if:</p>
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<dl compact>
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<dt>[EINVAL]</dt>
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<dd>The value specified by <i>file_actions</i> is invalid.</dd>
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<dt>[ENOMEM]</dt>
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<dd>Insufficient memory exists to add to the spawn file actions object.</dd>
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</dl>
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<p>It shall not be considered an error for the <i>fildes</i> argument passed to these functions to specify a file descriptor for
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which the specified operation could not be performed at the time of the call. Any such error will be detected when the associated
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file actions object is later used during a <a href="../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href=
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"../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> operation.</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_428_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_428_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>These functions are part of the Spawn option and need not be provided on all implementations.</p>
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</blockquote>
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<h4><a name="tag_03_428_08"></a>RATIONALE</h4>
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<blockquote>
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<p>A spawn file actions object may be initialized to contain an ordered sequence of <a href=
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"../functions/close.html"><i>close</i>()</a>, <a href="../functions/dup2.html"><i>dup2</i>()</a>, and <a href=
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"../functions/open.html"><i>open</i>()</a> operations to be used by <a href=
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"../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href="../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> to
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arrive at the set of open file descriptors inherited by the spawned process from the set of open file descriptors in the parent at
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the time of the <a href="../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href=
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"../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> call. It had been suggested that the <a href=
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"../functions/close.html"><i>close</i>()</a> and <a href="../functions/dup2.html"><i>dup2</i>()</a> operations alone are sufficient
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to rearrange file descriptors, and that files which need to be opened for use by the spawned process can be handled either by
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having the calling process open them before the <a href="../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href=
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"../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> call (and close them after), or by passing filenames to the spawned
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process (in <i>argv</i>) so that it may open them itself. The standard developers recommend that applications use one of these two
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methods when practical, since detailed error status on a failed open operation is always available to the application this way.
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However, the standard developers feel that allowing a spawn file actions object to specify open operations is still appropriate
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because:</p>
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<ol>
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<li>
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<p>It is consistent with equivalent POSIX.5 (Ada) functionality.</p>
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</li>
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<li>
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<p>It supports the I/O redirection paradigm commonly employed by POSIX programs designed to be invoked from a shell. When such a
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program is the child process, it may not be designed to open files on its own.</p>
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</li>
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<li>
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<p>It allows file opens that might otherwise fail or violate file ownership/access rights if executed by the parent process.</p>
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</li>
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</ol>
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<p>Regarding 2. above, note that the spawn open file action provides to <a href=
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"../functions/posix_spawn.html"><i>posix_spawn</i>()</a> and <a href="../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> the
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same capability that the shell redirection operators provide to <a href="../functions/system.html"><i>system</i>()</a>, only
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without the intervening execution of a shell; for example:</p>
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<pre>
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<tt>system ("myprog <file1 3<file2");
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</tt>
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</pre>
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<p>Regarding 3. above, note that if the calling process needs to open one or more files for access by the spawned process, but has
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insufficient spare file descriptors, then the open action is necessary to allow the <a href=
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"../functions/open.html"><i>open</i>()</a> to occur in the context of the child process after other file descriptors have been
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closed (that must remain open in the parent).</p>
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<p>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
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in the spawn attributes, a file created within the parent process will (possibly incorrectly) have the parent's effective user ID
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as its owner, whereas a file created via an <a href="../functions/open.html"><i>open</i>()</a> action during <a href=
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"../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href="../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> will
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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
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should not have access or fail to open a file to which the real user should have access.</p>
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<h5><a name="tag_03_428_08_01"></a>File Descriptor Mapping</h5>
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<p>The standard developers had originally proposed using an array which specified the mapping of child file descriptors back to
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those of the parent. It was pointed out by the ballot group that it is not possible to reshuffle file descriptors arbitrarily in a
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library implementation of <a href="../functions/posix_spawn.html"><i>posix_spawn</i>()</a> or <a href=
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"../functions/posix_spawnp.html"><i>posix_spawnp</i>()</a> without provision for one or more spare file descriptor entries (which
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simply may not be available). Such an array requires that an implementation develop a complex strategy to achieve the desired
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mapping without inadvertently closing the wrong file descriptor at the wrong time.</p>
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<p>It was noted by a member of the Ada Language Bindings working group that the approved Ada Language <i>Start_Process</i> family
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of POSIX process primitives use a caller-specified set of file actions to alter the normal <a href=
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"../functions/fork.html"><i>fork</i>()</a>/ <i><a href="../functions/exec.html">exec</a></i> semantics for inheritance of file
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descriptors in a very flexible way, yet no such problems exist because the burden of determining how to achieve the final file
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descriptor mapping is completely on the application. Furthermore, although the file actions interface appears frightening at first
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glance, it is actually quite simple to implement in either a library or the kernel.</p>
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</blockquote>
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<h4><a name="tag_03_428_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_428_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="close.html"><i>close</i>()</a> , <a href="dup.html"><i>dup</i>()</a> , <a href="open.html"><i>open</i>()</a> , <a href=
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"posix_spawn.html"><i>posix_spawn</i>()</a> , <a href=
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"posix_spawn_file_actions_adddup2.html"><i>posix_spawn_file_actions_adddup2</i>()</a> , <a href=
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"posix_spawn_file_actions_destroy.html"><i>posix_spawn_file_actions_destroy</i>()</a> , <a href=
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"posix_spawnp.html"><i>posix_spawnp</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/spawn.h.html"><i><spawn.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_428_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 6. Derived from IEEE Std 1003.1d-1999.</p>
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<p>IEEE PASC Interpretation 1003.1 #105 is applied, adding a note to the DESCRIPTION that the string pointed to by <i>path</i> is
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copied by the <i>posix_spawn_file_actions_addopen</i>() function.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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