How to force a file descriptor to buffer my output

Lots of people want to switch off buffering on their file descriptors. I want the reverse: I deliberately want to configure a file descriptor to buffer, say, 1K of data before writing to disk.

The reason is that I'm writing a unit test for a "flush" function of a C++ class. To test that it's working I want to write some data, check the size of the file on disk, then flush, then check that the size has grown. But in practice, by the time I do the first file size check the data has already been written.

Note that I'm working with a raw file descriptor here, not a stream or anything.

This is on linux if that matters.

Solution

How to force a file descriptor to buffer my output

If you're using the POSIX write() (or a variant of it), you can't.

The write() call must behave thus:

After a write() to a regular file has successfully returned:

Any successful read() from each byte position in the file that was modified by that write shall return the data specified by the write() for that position until such byte positions are again modified.

Any subsequent successful write() to the same byte position in the file shall overwrite that file data.

Those requirements mean the data written is visible to any other process on the system, and to be consistent, if the data written causes the file size to grow, the file size reported by the kernel must reflect the data written.

I want to write some data, check the size of the file on disk, then flush, then check that the size has grown.

That fundamentally doesn't work with write(). The file size will grow as the data written - write() does not buffer data.

If you want it to do that, you'll have to implement your own filesystem - one that isn't POSIX compliant.