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| <h4 class="subsection">11.1.2 File Position</h4> |
| |
| <p>One of the attributes of an open file is its <dfn>file position</dfn> that |
| keeps track of where in the file the next character is to be read or |
| written. In the GNU system, and all POSIX.1 systems, the file position |
| is simply an integer representing the number of bytes from the beginning |
| of the file. |
| |
| <p>The file position is normally set to the beginning of the file when it |
| is opened, and each time a character is read or written, the file |
| position is incremented. In other words, access to the file is normally |
| <dfn>sequential</dfn>. |
| <a name="index-file-position-899"></a><a name="index-sequential_002daccess-files-900"></a> |
| Ordinary files permit read or write operations at any position within |
| the file. Some other kinds of files may also permit this. Files which |
| do permit this are sometimes referred to as <dfn>random-access</dfn> files. |
| You can change the file position using the <code>fseek</code> function on a |
| stream (see <a href="File-Positioning.html#File-Positioning">File Positioning</a>) or the <code>lseek</code> function on a file |
| descriptor (see <a href="I_002fO-Primitives.html#I_002fO-Primitives">I/O Primitives</a>). If you try to change the file |
| position on a file that doesn't support random access, you get the |
| <code>ESPIPE</code> error. |
| <a name="index-random_002daccess-files-901"></a> |
| Streams and descriptors that are opened for <dfn>append access</dfn> are |
| treated specially for output: output to such files is <em>always</em> |
| appended sequentially to the <em>end</em> of the file, regardless of the |
| file position. However, the file position is still used to control where in |
| the file reading is done. |
| <a name="index-append_002daccess-files-902"></a> |
| If you think about it, you'll realize that several programs can read a |
| given file at the same time. In order for each program to be able to |
| read the file at its own pace, each program must have its own file |
| pointer, which is not affected by anything the other programs do. |
| |
| <p>In fact, each opening of a file creates a separate file position. |
| Thus, if you open a file twice even in the same program, you get two |
| streams or descriptors with independent file positions. |
| |
| <p>By contrast, if you open a descriptor and then duplicate it to get |
| another descriptor, these two descriptors share the same file position: |
| changing the file position of one descriptor will affect the other. |
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