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| <h3 class="section">12.11 Block Input/Output</h3> |
| |
| <p>This section describes how to do input and output operations on blocks |
| of data. You can use these functions to read and write binary data, as |
| well as to read and write text in fixed-size blocks instead of by |
| characters or lines. |
| <a name="index-binary-I_002fO-to-a-stream-1006"></a><a name="index-block-I_002fO-to-a-stream-1007"></a><a name="index-reading-from-a-stream_002c-by-blocks-1008"></a><a name="index-writing-to-a-stream_002c-by-blocks-1009"></a> |
| Binary files are typically used to read and write blocks of data in the |
| same format as is used to represent the data in a running program. In |
| other words, arbitrary blocks of memory—not just character or string |
| objects—can be written to a binary file, and meaningfully read in |
| again by the same program. |
| |
| <p>Storing data in binary form is often considerably more efficient than |
| using the formatted I/O functions. Also, for floating-point numbers, |
| the binary form avoids possible loss of precision in the conversion |
| process. On the other hand, binary files can't be examined or modified |
| easily using many standard file utilities (such as text editors), and |
| are not portable between different implementations of the language, or |
| different kinds of computers. |
| |
| <p>These functions are declared in <samp><span class="file">stdio.h</span></samp>. |
| <a name="index-stdio_002eh-1010"></a> |
| <!-- stdio.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: size_t <b>fread</b> (<var>void *data, size_t size, size_t count, FILE *stream</var>)<var><a name="index-fread-1011"></a></var><br> |
| <blockquote><p>This function reads up to <var>count</var> objects of size <var>size</var> into |
| the array <var>data</var>, from the stream <var>stream</var>. It returns the |
| number of objects actually read, which might be less than <var>count</var> if |
| a read error occurs or the end of the file is reached. This function |
| returns a value of zero (and doesn't read anything) if either <var>size</var> |
| or <var>count</var> is zero. |
| |
| <p>If <code>fread</code> encounters end of file in the middle of an object, it |
| returns the number of complete objects read, and discards the partial |
| object. Therefore, the stream remains at the actual end of the file. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: size_t <b>fread_unlocked</b> (<var>void *data, size_t size, size_t count, FILE *stream</var>)<var><a name="index-fread_005funlocked-1012"></a></var><br> |
| <blockquote><p>The <code>fread_unlocked</code> function is equivalent to the <code>fread</code> |
| function except that it does not implicitly lock the stream. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: size_t <b>fwrite</b> (<var>const void *data, size_t size, size_t count, FILE *stream</var>)<var><a name="index-fwrite-1013"></a></var><br> |
| <blockquote><p>This function writes up to <var>count</var> objects of size <var>size</var> from |
| the array <var>data</var>, to the stream <var>stream</var>. The return value is |
| normally <var>count</var>, if the call succeeds. Any other value indicates |
| some sort of error, such as running out of space. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: size_t <b>fwrite_unlocked</b> (<var>const void *data, size_t size, size_t count, FILE *stream</var>)<var><a name="index-fwrite_005funlocked-1014"></a></var><br> |
| <blockquote><p>The <code>fwrite_unlocked</code> function is equivalent to the <code>fwrite</code> |
| function except that it does not implicitly lock the stream. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
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