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| <h3 class="section">13.4 Descriptors and Streams</h3> |
| |
| <p><a name="index-streams_002c-and-file-descriptors-1226"></a><a name="index-converting-file-descriptor-to-stream-1227"></a><a name="index-extracting-file-descriptor-from-stream-1228"></a> |
| Given an open file descriptor, you can create a stream for it with the |
| <code>fdopen</code> function. You can get the underlying file descriptor for |
| an existing stream with the <code>fileno</code> function. These functions are |
| declared in the header file <samp><span class="file">stdio.h</span></samp>. |
| <a name="index-stdio_002eh-1229"></a> |
| <!-- stdio.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Function: FILE * <b>fdopen</b> (<var>int filedes, const char *opentype</var>)<var><a name="index-fdopen-1230"></a></var><br> |
| <blockquote><p>The <code>fdopen</code> function returns a new stream for the file descriptor |
| <var>filedes</var>. |
| |
| <p>The <var>opentype</var> argument is interpreted in the same way as for the |
| <code>fopen</code> function (see <a href="Opening-Streams.html#Opening-Streams">Opening Streams</a>), except that |
| the ‘<samp><span class="samp">b</span></samp>’ option is not permitted; this is because GNU makes no |
| distinction between text and binary files. Also, <code>"w"</code> and |
| <code>"w+"</code> do not cause truncation of the file; these have an effect only |
| when opening a file, and in this case the file has already been opened. |
| You must make sure that the <var>opentype</var> argument matches the actual |
| mode of the open file descriptor. |
| |
| <p>The return value is the new stream. If the stream cannot be created |
| (for example, if the modes for the file indicated by the file descriptor |
| do not permit the access specified by the <var>opentype</var> argument), a |
| null pointer is returned instead. |
| |
| <p>In some other systems, <code>fdopen</code> may fail to detect that the modes |
| for file descriptor do not permit the access specified by |
| <code>opentype</code>. The GNU C library always checks for this. |
| </p></blockquote></div> |
| |
| <p>For an example showing the use of the <code>fdopen</code> function, |
| see <a href="Creating-a-Pipe.html#Creating-a-Pipe">Creating a Pipe</a>. |
| |
| <!-- stdio.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>fileno</b> (<var>FILE *stream</var>)<var><a name="index-fileno-1231"></a></var><br> |
| <blockquote><p>This function returns the file descriptor associated with the stream |
| <var>stream</var>. If an error is detected (for example, if the <var>stream</var> |
| is not valid) or if <var>stream</var> does not do I/O to a file, |
| <code>fileno</code> returns -1. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>fileno_unlocked</b> (<var>FILE *stream</var>)<var><a name="index-fileno_005funlocked-1232"></a></var><br> |
| <blockquote><p>The <code>fileno_unlocked</code> function is equivalent to the <code>fileno</code> |
| function except that it does not implicitly lock the stream if the state |
| is <code>FSETLOCKING_INTERNAL</code>. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
| |
| <p><a name="index-standard-file-descriptors-1233"></a><a name="index-file-descriptors_002c-standard-1234"></a>There are also symbolic constants defined in <samp><span class="file">unistd.h</span></samp> for the |
| file descriptors belonging to the standard streams <code>stdin</code>, |
| <code>stdout</code>, and <code>stderr</code>; see <a href="Standard-Streams.html#Standard-Streams">Standard Streams</a>. |
| <a name="index-unistd_002eh-1235"></a> |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <dl> |
| <dt><code>STDIN_FILENO</code><dd><a name="index-STDIN_005fFILENO-1236"></a>This macro has value <code>0</code>, which is the file descriptor for |
| standard input. |
| <a name="index-standard-input-file-descriptor-1237"></a> |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>STDOUT_FILENO</code><dd><a name="index-STDOUT_005fFILENO-1238"></a>This macro has value <code>1</code>, which is the file descriptor for |
| standard output. |
| <a name="index-standard-output-file-descriptor-1239"></a> |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <br><dt><code>STDERR_FILENO</code><dd><a name="index-STDERR_005fFILENO-1240"></a>This macro has value <code>2</code>, which is the file descriptor for |
| standard error output. |
| </dl> |
| <a name="index-standard-error-file-descriptor-1241"></a> |
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