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海皮飞 edited this page May 28, 2013
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read系统调用的核心实现定义在vfs中。
PUBLIC int read_write(rw_flag)
int rw_flag; /* READING or WRITING */
{
...
// 这里会区分三种设备，对于不同的设备用不同的函数读写。
// 字符设备
if (char_spec) { /* Character special files. */
dev_t dev;
int suspend_reopen;
suspend_reopen = (f->filp_state != FS_NORMAL);
dev = (dev_t) vp->v_sdev;
r = dev_io(op, dev, who_e, m_in.buffer, position, m_in.nbytes, oflags,
suspend_reopen);
if (r >= 0) {
cum_io = r;
position = add64ul(position, r);
r = OK;
}
// 块设备
} else if (block_spec) { /* Block special files. */
r = req_breadwrite(vp->v_bfs_e, who_e, vp->v_sdev, position,
m_in.nbytes, m_in.buffer, rw_flag, &res_pos, &res_cum_io);
if (r == OK) {
position = res_pos;
cum_io += res_cum_io;
}
// 普通文件
} else { /* Regular files */
if (rw_flag == WRITING && block_spec == 0) {
/* Check for O_APPEND flag. */
if (oflags & O_APPEND) position = cvul64(vp->v_size);
}
/* Issue request */
r = req_readwrite(vp->v_fs_e, vp->v_inode_nr, position, rw_flag, who_e,
m_in.buffer, m_in.nbytes, &new_pos, &cum_io_incr);
if (r >= 0) {
if (ex64hi(new_pos))
panic("read_write: bad new pos");
position = new_pos;
cum_io += cum_io_incr;
}
}
/* On write, update file size and access time. */
// 写的时候，会更新文件大小和更新时间。
if (rw_flag == WRITING) {
if (regular || mode_word == I_DIRECTORY) {
if (cmp64ul(position, vp->v_size) > 0) {
if (ex64hi(position) != 0) {
panic("read_write: file size too big ");
}
vp->v_size = ex64lo(position);
}
}
}
f->filp_pos = position;
if (r == OK) return(cum_io);
return(r);
}