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<title>fs/block_dev.c</title>
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<a name='TOP'><h2><a href='../mains.html'>root</a>/<a href='../files/98.html'>fs</a>/block_dev.c</h2>
<i><font color='green'>/* [<][>]<a href='#L23'>[^]</a><a href='#L70'>[v]</a>[top]<a href='#BOTTOM'>[bottom]</a><a href='../mains.html'>[index]</a><a href='../help.html'>[help]</a> */</font></i>
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<h2>DEFINITIONS</h2>
This source file includes following definitions.
<ol>
<li><a href='#L23' title='Defined at 23.'>block_write</a>
<li><a href='#L70' title='Defined at 70.'>block_read</a>
</ol>
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<pre>
<a name='L1'><i><font color='green'>/*</font></i>
<a name='L2'><i><font color='green'>* linux/fs/block_dev.c</font></i>
<a name='L3'><i><font color='green'>*</font></i>
<a name='L4'><i><font color='green'>* (C) 1991 Linus Torvalds</font></i>
<a name='L5'><i><font color='green'>*/</font></i>
<a name='L6'>
<a name='L7'><font color='darkred'>#include</font> <<a href='28.html'>errno.h</a>> <i><font color='green'>// 错误号头文件。包含系统中各种出错号。(Linus 从minix 中引进的)。</font></i>
<a name='L8'>
<a name='L9'><font color='darkred'>#include</font> <<a href='36.html'>linux/sched.h</a>> <i><font color='green'>// 调度程序头文件，定义了任务结构task_struct、初始任务0 的数据，</font></i>
<a name='L10'><i><font color='green'>// 还有一些有关描述符参数设置和获取的嵌入式汇编函数宏语句。</font></i>
<a name='L11'><font color='darkred'>#include</font> <<a href='34.html'>linux/kernel.h</a>> <i><font color='green'>// 内核头文件。含有一些内核常用函数的原形定义。</font></i>
<a name='L12'><font color='darkred'>#include</font> <<a href='24.html'>asm/segment.h</a>> <i><font color='green'>// 段操作头文件。定义了有关段寄存器操作的嵌入式汇编函数。</font></i>
<a name='L13'><font color='darkred'>#include</font> <<a href='25.html'>asm/system.h</a>> <i><font color='green'>// 系统头文件。定义了设置或修改描述符/中断门等的嵌入式汇编宏。</font></i>
<a name='L14'>
<a name='L15'><i><font color='green'>//// 数据块写函数 - 向指定设备从给定偏移处写入指定长度字节数据。</font></i>
<a name='L16'><i><font color='green'>// 参数：dev - 设备号；pos - 设备文件中偏移量指针；buf - 用户地址空间中缓冲区地址；</font></i>
<a name='L17'><i><font color='green'>// count - 要传送的字节数。</font></i>
<a name='L18'><i><font color='green'>// 对于内核来说，写操作是向高速缓冲区中写入数据，什么时候数据最终写入设备是由高速缓冲管理</font></i>
<a name='L19'><i><font color='green'>// 程序决定并处理的。另外，因为设备是以块为单位进行读写的，因此对于写开始位置不处于块起始</font></i>
<a name='L20'><i><font color='green'>// 处时，需要先将开始字节所在的整个块读出，然后将需要写的数据从写开始处填写满该块，再将完</font></i>
<a name='L21'><i><font color='green'>// 整的一块数据写盘（即交由高速缓冲程序去处理）。</font></i>
<a name='L22'><b>int</b>
<a name='L23'><a href='../R/388.html' title='Multiple refered from 2 places.'>block_write</a> (<b>int</b> dev, <b>long</b> *pos, <b>char</b> *buf, <b>int</b> count)
<a name='L24'><font color='red'>{</font>
<a name='L25'><i><font color='green'>// 由pos 地址换算成开始读写块的块序号block。并求出需读第1 字节在该块中的偏移位置offset。</font></i>
<a name='L26'> <b>int</b> block = *<a href='../D/915.html' title='Multiple defined in 9 places.'>pos</a> >> <a href='../S/31.html#L66' title='Defined at 66 in include/linux/fs.h.'>BLOCK_SIZE_BITS</a>;
<a name='L27'> <b>int</b> offset = *<a href='../D/915.html' title='Multiple defined in 9 places.'>pos</a> & (<a href='../D/23.html' title='Multiple defined in 2 places.'>BLOCK_SIZE</a> - 1);
<a name='L28'> <b>int</b> chars;
<a name='L29'> <b>int</b> written = 0;
<a name='L30'> <b>struct</b> buffer_head *bh;
<a name='L31'> <b>register</b> <b>char</b> *p;
<a name='L32'>
<a name='L33'><i><font color='green'>// 针对要写入的字节数count，循环执行以下操作，直到全部写入。</font></i>
<a name='L34'> <b>while</b> (<a href='../D/738.html' title='Multiple defined in 17 places.'>count</a> > 0)
<a name='L35'> <font color='red'>{</font>
<a name='L36'><i><font color='green'>// 计算在该块中可写入的字节数。如果需要写入的字节数填不满一块，则只需写count 字节。</font></i>
<a name='L37'> chars = <a href='../D/23.html' title='Multiple defined in 2 places.'>BLOCK_SIZE</a> - offset;
<a name='L38'> <b>if</b> (chars > <a href='../D/738.html' title='Multiple defined in 17 places.'>count</a>)
<a name='L39'> chars = <a href='../D/738.html' title='Multiple defined in 17 places.'>count</a>;
<a name='L40'><i><font color='green'>// 如果正好要写1 块数据，则直接申请1 块高速缓冲块，否则需要读入将被修改的数据块，并预读</font></i>
<a name='L41'><i><font color='green'>// 下两块数据，然后将块号递增1。</font></i>
<a name='L42'> <b>if</b> (chars == <a href='../D/23.html' title='Multiple defined in 2 places.'>BLOCK_SIZE</a>)
<a name='L43'> bh = <a href='../S/6.html#L301' title='Defined at 301 in fs/buffer.c.'>getblk</a> (<a href='../S/7.html#L136' title='Defined at 136 in fs/char_dev.c.'>dev</a>, block);
<a name='L44'> <b>else</b>
<a name='L45'> bh = <a href='../S/6.html#L478' title='Defined at 478 in fs/buffer.c.'>breada</a> (<a href='../S/7.html#L136' title='Defined at 136 in fs/char_dev.c.'>dev</a>, block, block + 1, block + 2, -1);
<a name='L46'> block++;
<a name='L47'><i><font color='green'>// 如果缓冲块操作失败，则返回已写字节数，如果没有写入任何字节，则返回出错号（负数）。</font></i>
<a name='L48'> <b>if</b> (!bh)
<a name='L49'> <b>return</b> written ? written : -<a href='../D/92.html' title='Multiple defined in 5 places.'>EIO</a>;
<a name='L50'><i><font color='green'>// p 指向读出数据块中开始写的位置。若最后写入的数据不足一块，则需从块开始填写（修改）所需</font></i>
<a name='L51'><i><font color='green'>// 的字节，因此这里需置offset 为零。</font></i>
<a name='L52'> p = offset + bh->b_data;
<a name='L53'> offset = 0;
<a name='L54'><i><font color='green'>// 将文件中偏移指针前移已写字节数。累加已写字节数chars。传送计数值减去此次已传送字节数。</font></i>
<a name='L55'> *<a href='../D/915.html' title='Multiple defined in 9 places.'>pos</a> += chars;
<a name='L56'> written += chars;
<a name='L57'> <a href='../D/738.html' title='Multiple defined in 17 places.'>count</a> -= chars;
<a name='L58'><i><font color='green'>// 从用户缓冲区复制chars 字节到p 指向的高速缓冲区中开始写入的位置。</font></i>
<a name='L59'> <b>while</b> (chars-- > 0)
<a name='L60'> *(p++) = <a href='../D/821.html' title='Multiple defined in 2 places.'>get_fs_byte</a> (<a href='../D/716.html' title='Multiple defined in 16 places.'>buf</a>++);
<a name='L61'><i><font color='green'>// 置该缓冲区块已修改标志，并释放该缓冲区（也即该缓冲区引用计数递减1）。</font></i>
<a name='L62'> bh->b_dirt = 1;
<a name='L63'> <a href='../S/6.html#L377' title='Defined at 377 in fs/buffer.c.'>brelse</a> (bh);
<a name='L64'> <font color='red'>}</font>
<a name='L65'> <b>return</b> written; <i><font color='green'>// 返回已写入的字节数，正常退出。</font></i>
<a name='L66'><font color='red'>}</font>
<a name='L67'>
<a name='L68'><i><font color='green'>//// 数据块读函数 - 从指定设备和位置读入指定字节数的数据到高速缓冲中。</font></i>
<a name='L69'><b>int</b>
<a name='L70'><a href='../R/387.html' title='Multiple refered from 2 places.'>block_read</a> (<b>int</b> dev, <b>unsigned</b> <b>long</b> *pos, <b>char</b> *buf, <b>int</b> count)
<a name='L71'><font color='red'>{</font>
<a name='L72'><i><font color='green'>// 由pos 地址换算成开始读写块的块序号block。并求出需读第1 字节在该块中的偏移位置offset。</font></i>
<a name='L73'> <b>int</b> block = *<a href='../D/915.html' title='Multiple defined in 9 places.'>pos</a> >> <a href='../S/31.html#L66' title='Defined at 66 in include/linux/fs.h.'>BLOCK_SIZE_BITS</a>;
<a name='L74'> <b>int</b> offset = *<a href='../D/915.html' title='Multiple defined in 9 places.'>pos</a> & (<a href='../D/23.html' title='Multiple defined in 2 places.'>BLOCK_SIZE</a> - 1);
<a name='L75'> <b>int</b> chars;
<a name='L76'> <b>int</b> read = 0;
<a name='L77'> <b>struct</b> buffer_head *bh;
<a name='L78'> <b>register</b> <b>char</b> *p;
<a name='L79'>
<a name='L80'><i><font color='green'>// 针对要读入的字节数count，循环执行以下操作，直到全部读入。</font></i>
<a name='L81'> <b>while</b> (<a href='../D/738.html' title='Multiple defined in 17 places.'>count</a> > 0)
<a name='L82'> <font color='red'>{</font>
<a name='L83'><i><font color='green'>// 计算在该块中需读入的字节数。如果需要读入的字节数不满一块，则只需读count 字节。</font></i>
<a name='L84'> chars = <a href='../D/23.html' title='Multiple defined in 2 places.'>BLOCK_SIZE</a> - offset;
<a name='L85'> <b>if</b> (chars > <a href='../D/738.html' title='Multiple defined in 17 places.'>count</a>)
<a name='L86'> chars = <a href='../D/738.html' title='Multiple defined in 17 places.'>count</a>;
<a name='L87'><i><font color='green'>// 读入需要的数据块，并预读下两块数据，如果读操作出错，则返回已读字节数，如果没有读入任何</font></i>
<a name='L88'><i><font color='green'>// 字节，则返回出错号。然后将块号递增1。</font></i>
<a name='L89'> <b>if</b> (!(bh = <a href='../S/6.html#L478' title='Defined at 478 in fs/buffer.c.'>breada</a> (<a href='../S/7.html#L136' title='Defined at 136 in fs/char_dev.c.'>dev</a>, block, block + 1, block + 2, -1)))
<a name='L90'> <b>return</b> read ? read : -<a href='../D/92.html' title='Multiple defined in 5 places.'>EIO</a>;
<a name='L91'> block++;
<a name='L92'><i><font color='green'>// p 指向从设备读出数据块中需要读取的开始位置。若最后需要读取的数据不足一块，则需从块开始</font></i>
<a name='L93'><i><font color='green'>// 读取所需的字节，因此这里需将offset 置零。</font></i>
<a name='L94'> p = offset + bh->b_data;
<a name='L95'> offset = 0;
<a name='L96'><i><font color='green'>// 将文件中偏移指针前移已读出字节数chars。累加已读字节数。传送计数值减去此次已传送字节数。</font></i>
<a name='L97'> *<a href='../D/915.html' title='Multiple defined in 9 places.'>pos</a> += chars;
<a name='L98'> read += chars;
<a name='L99'> <a href='../D/738.html' title='Multiple defined in 17 places.'>count</a> -= chars;
<a name='L100'><i><font color='green'>// 从高速缓冲区中p 指向的开始位置复制chars 字节数据到用户缓冲区，并释放该高速缓冲区。</font></i>
<a name='L101'> <b>while</b> (chars-- > 0)
<a name='L102'> <a href='../S/24.html#L44' title='Defined at 44 in include/asm/segment.h.'>put_fs_byte</a> (*(p++), <a href='../D/716.html' title='Multiple defined in 16 places.'>buf</a>++);
<a name='L103'> <a href='../S/6.html#L377' title='Defined at 377 in fs/buffer.c.'>brelse</a> (bh);
<a name='L104'> <font color='red'>}</font>
<a name='L105'> <b>return</b> read; <i><font color='green'>// 返回已读取的字节数，正常退出。</font></i>
<a name='L106'><font color='red'>}</font>
</pre>
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