c - 对使用 O_DIRECT 打开的文件进行“写入”后的用户缓冲区

Question

我正在使用该O_DIRECT标志直接从用户缓冲区写入磁盘。但据我了解，Linux 不保证在这个调用之后，数据被写入。它只是使用 DMA 或其他任何东西直接从用户缓冲区写入物理设备......因此，我不明白在调用“写入”函数后是否可以写入用户缓冲区。

我确信示例代码将有助于理解我的问题：

char *user_buff = malloc(...); /* assume it is aligned as needed */
fd = open(..., O_DIRECT);
write(fd, ...)
memset(user_buff, 0, ...)

最后一行（memset）合法吗？写入用户缓冲区是否有效，DMA 可能使用该缓冲区将数据传输到设备？

Answer 1

It is legal. There's no "loopback" taking place here - consider what you should do if you used a dynamically mallocated array. Could you free it after write()? Couldn't you? - Well, the answer is that the write() function (and syscall) doesn't modify or access the user buffer after it has returned - if immediate write couldn't be performed, a copy of the data will be made. You don't have to worry about implementation details (that's the purpose of having a C standard library, after all...)

Answer 2

将O_SYNC标志与 结合使用O_DIRECT。然后就可以确定write()返回时数据已经写入了。