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| <h4 class="subsection">12.20.3 Controlling Which Kind of Buffering</h4> |
| |
| <p>After opening a stream (but before any other operations have been |
| performed on it), you can explicitly specify what kind of buffering you |
| want it to have using the <code>setvbuf</code> function. |
| <a name="index-buffering_002c-controlling-1141"></a> |
| The facilities listed in this section are declared in the header |
| file <samp><span class="file">stdio.h</span></samp>. |
| <a name="index-stdio_002eh-1142"></a> |
| <!-- stdio.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: int <b>setvbuf</b> (<var>FILE *stream, char *buf, int mode, size_t size</var>)<var><a name="index-setvbuf-1143"></a></var><br> |
| <blockquote><p>This function is used to specify that the stream <var>stream</var> should |
| have the buffering mode <var>mode</var>, which can be either <code>_IOFBF</code> |
| (for full buffering), <code>_IOLBF</code> (for line buffering), or |
| <code>_IONBF</code> (for unbuffered input/output). |
| |
| <p>If you specify a null pointer as the <var>buf</var> argument, then <code>setvbuf</code> |
| allocates a buffer itself using <code>malloc</code>. This buffer will be freed |
| when you close the stream. |
| |
| <p>Otherwise, <var>buf</var> should be a character array that can hold at least |
| <var>size</var> characters. You should not free the space for this array as |
| long as the stream remains open and this array remains its buffer. You |
| should usually either allocate it statically, or <code>malloc</code> |
| (see <a href="Unconstrained-Allocation.html#Unconstrained-Allocation">Unconstrained Allocation</a>) the buffer. Using an automatic array |
| is not a good idea unless you close the file before exiting the block |
| that declares the array. |
| |
| <p>While the array remains a stream buffer, the stream I/O functions will |
| use the buffer for their internal purposes. You shouldn't try to access |
| the values in the array directly while the stream is using it for |
| buffering. |
| |
| <p>The <code>setvbuf</code> function returns zero on success, or a nonzero value |
| if the value of <var>mode</var> is not valid or if the request could not |
| be honored. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>_IOFBF</b><var><a name="index-g_t_005fIOFBF-1144"></a></var><br> |
| <blockquote><p>The value of this macro is an integer constant expression that can be |
| used as the <var>mode</var> argument to the <code>setvbuf</code> function to |
| specify that the stream should be fully buffered. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>_IOLBF</b><var><a name="index-g_t_005fIOLBF-1145"></a></var><br> |
| <blockquote><p>The value of this macro is an integer constant expression that can be |
| used as the <var>mode</var> argument to the <code>setvbuf</code> function to |
| specify that the stream should be line buffered. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>_IONBF</b><var><a name="index-g_t_005fIONBF-1146"></a></var><br> |
| <blockquote><p>The value of this macro is an integer constant expression that can be |
| used as the <var>mode</var> argument to the <code>setvbuf</code> function to |
| specify that the stream should be unbuffered. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>BUFSIZ</b><var><a name="index-BUFSIZ-1147"></a></var><br> |
| <blockquote><p>The value of this macro is an integer constant expression that is good |
| to use for the <var>size</var> argument to <code>setvbuf</code>. This value is |
| guaranteed to be at least <code>256</code>. |
| |
| <p>The value of <code>BUFSIZ</code> is chosen on each system so as to make stream |
| I/O efficient. So it is a good idea to use <code>BUFSIZ</code> as the size |
| for the buffer when you call <code>setvbuf</code>. |
| |
| <p>Actually, you can get an even better value to use for the buffer size |
| by means of the <code>fstat</code> system call: it is found in the |
| <code>st_blksize</code> field of the file attributes. See <a href="Attribute-Meanings.html#Attribute-Meanings">Attribute Meanings</a>. |
| |
| <p>Sometimes people also use <code>BUFSIZ</code> as the allocation size of |
| buffers used for related purposes, such as strings used to receive a |
| line of input with <code>fgets</code> (see <a href="Character-Input.html#Character-Input">Character Input</a>). There is no |
| particular reason to use <code>BUFSIZ</code> for this instead of any other |
| integer, except that it might lead to doing I/O in chunks of an |
| efficient size. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: void <b>setbuf</b> (<var>FILE *stream, char *buf</var>)<var><a name="index-setbuf-1148"></a></var><br> |
| <blockquote><p>If <var>buf</var> is a null pointer, the effect of this function is |
| equivalent to calling <code>setvbuf</code> with a <var>mode</var> argument of |
| <code>_IONBF</code>. Otherwise, it is equivalent to calling <code>setvbuf</code> |
| with <var>buf</var>, and a <var>mode</var> of <code>_IOFBF</code> and a <var>size</var> |
| argument of <code>BUFSIZ</code>. |
| |
| <p>The <code>setbuf</code> function is provided for compatibility with old code; |
| use <code>setvbuf</code> in all new programs. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>setbuffer</b> (<var>FILE *stream, char *buf, size_t size</var>)<var><a name="index-setbuffer-1149"></a></var><br> |
| <blockquote><p>If <var>buf</var> is a null pointer, this function makes <var>stream</var> unbuffered. |
| Otherwise, it makes <var>stream</var> fully buffered using <var>buf</var> as the |
| buffer. The <var>size</var> argument specifies the length of <var>buf</var>. |
| |
| <p>This function is provided for compatibility with old BSD code. Use |
| <code>setvbuf</code> instead. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>setlinebuf</b> (<var>FILE *stream</var>)<var><a name="index-setlinebuf-1150"></a></var><br> |
| <blockquote><p>This function makes <var>stream</var> be line buffered, and allocates the |
| buffer for you. |
| |
| <p>This function is provided for compatibility with old BSD code. Use |
| <code>setvbuf</code> instead. |
| </p></blockquote></div> |
| |
| <p>It is possible to query whether a given stream is line buffered or not |
| using a non-standard function introduced in Solaris and available in the |
| GNU C library. |
| |
| <!-- stdio_ext.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>__flbf</b> (<var>FILE *stream</var>)<var><a name="index-g_t_005f_005fflbf-1151"></a></var><br> |
| <blockquote><p>The <code>__flbf</code> function will return a nonzero value in case the |
| stream <var>stream</var> is line buffered. Otherwise the return value is |
| zero. |
| |
| <p>This function is declared in the <samp><span class="file">stdio_ext.h</span></samp> header. |
| </p></blockquote></div> |
| |
| <p>Two more extensions allow to determine the size of the buffer and how |
| much of it is used. These functions were also introduced in Solaris. |
| |
| <!-- stdio_ext.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: size_t <b>__fbufsize</b> (<var>FILE *stream</var>)<var><a name="index-g_t_005f_005ffbufsize-1152"></a></var><br> |
| <blockquote><p>The <code>__fbufsize</code> function return the size of the buffer in the |
| stream <var>stream</var>. This value can be used to optimize the use of the |
| stream. |
| |
| <p>This function is declared in the <samp><span class="file">stdio_ext.h</span></samp> header. |
| </p></blockquote></div> |
| |
| <!-- stdio_ext.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: size_t <b>__fpending</b> (<var>FILE *stream</var>)<var> The </var><code>__fpending</code><var><a name="index-g_t_005f_005ffpending-1153"></a></var><br> |
| <blockquote><p>function returns the number of bytes currently in the output buffer. |
| For wide-oriented stream the measuring unit is wide characters. This |
| function should not be used on buffers in read mode or opened read-only. |
| |
| <p>This function is declared in the <samp><span class="file">stdio_ext.h</span></samp> header. |
| </p></blockquote></div> |
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