在文件成功打开之后,进程将使用内核提供的read和write系统调用,来读取或修改文件的数据。内核中文件读写操作的系统调用实现基本都一样,下面我们看看文件的读取。
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"code" class="cpp">
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SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
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{
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struct file *file;
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ssize_t ret = -EBADF;
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int fput_needed;
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file = fget_light(fd, &fput_needed);
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if (file) {
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loff_t pos = file_pos_read(file);
"code"
class="cpp">"white-space:pre">
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ret = vfs_read(file, buf, count, &pos);
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file_pos_write(file, pos);
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fput_light(file, fput_needed);
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}
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return ret;
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}
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"font-size:18px">读取实现,返回读取字节数
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"code"
class="cpp">ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
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{
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ssize_t ret;
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if (!(file->f_mode & FMODE_READ))
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return -EBADF;
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if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))
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return -EINVAL;
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if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
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return -EFAULT;
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ret = rw_verify_area(READ, file, pos, count);
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if (ret >= 0) {
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count = ret;
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if (file->f_op->read)
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ret = file->f_op->read(file, buf, count, pos);
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else
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ret = do_sync_read(file, buf, count, pos);
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if (ret > 0) {
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fsnotify_access(file->f_path.dentry);
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add_rchar(current, ret);
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}
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inc_syscr(current);
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}
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return ret;
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}
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