CODEWITHSUNDEEP
clinuxfileblock-device
user1868481
user1868481

Reputation: 31

User buffer after doing 'write' to file opened with O_DIRECT

I'm using the O_DIRECT flag to write to the disk directly from the user buffer. But as far as I understand, Linux doesn't guarantee that after this call, the data is written. It just writes directly from the user buffer to the physical device using DMA or anything else... Therefore, I don't understand if I can write to the user buffer after the call to 'write' function.

I'm sure that example code will help to understand my question:

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

Is the last line (memset) legal? Is writing to the user buffer valid that is maybe used by DMA to transfer data to the device?

Upvotes: 3

Views: 610

Answers (2)

Barmar
Barmar

Reputation: 782166

Use the O_SYNC flag in conjunction with O_DIRECT. Then you can be sure that the data has been written when write() returns.

Upvotes: 1

user529758
user529758

Reputation:

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...)

Upvotes: 2

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