Does fwrite block until data has been written to disk?

Question

Does the fwrite() function return after the data to be written to disk has been handed over to the operating system or does it return only after the data is actually physically written to the disk?

For my case, I'm hoping that it's the first case since I don't want to wait until all the data is physically written to the disk. I'm hoping that another OS thread transfers it in the background.

I'm curious about behavior on Windows 10 in this particular case.

https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/fwrite

Probably not, and it might depend upon your file system (and its settings). Read about the [page cache](https://en.wikipedia.org/wiki/Page_cache) — Basile Starynkevitch, Jun 23 '18 at 18:14

Richard Chambers · Accepted Answer · 2018-06-23T18:33:45.227

There are several places where there is buffering of data in order to improve efficiency when using fwrite(): buffering within the C++ Runtime and buffering in the operating system file system interface and buffering within the actual disk hardware.

The default for these are to delay the actual physical writing of data to disk until there is an actual request to flush buffers or if appropriate indicators are turned on to perform physical writes as the write requests are made.

If you want to change the behavior of fwrite() take a look at the setbuf() function setbuf redirection as well as setbuff() Linux man page and here is the Microsoft documentation on setbuf().

And if you look at the documentation for the underlying Windows CreateFile() function you will see there are a number of flags which include flags as to whether buffering of data should be done or not.

FILE_FLAG_NO_BUFFERING 0x20000000

The file or device is being opened with no system caching for data reads and writes. This flag does not affect hard disk caching or memory mapped files.

There are strict requirements for successfully working with files opened with CreateFile using the FILE_FLAG_NO_BUFFERING flag, for details see File Buffering.

And see the Microsoft documentation topic File Buffering.

In a simple example, the application would open a file for write access with the FILE_FLAG_NO_BUFFERING flag and then perform a call to the WriteFile function using a data buffer defined within the application. This local buffer is, in these circumstances, effectively the only file buffer that exists for this operation. Because of physical disk layout, file system storage layout, and system-level file pointer position tracking, this write operation will fail unless the locally-defined data buffers meet certain alignment criteria, discussed in the following section.

Take a look at this discussion about settings at the OS level for what looks to be Linux https://superuser.com/questions/479379/how-long-can-file-system-writes-be-cached-with-ext4

Thanks Richard for your helpful answer. I figured that the operating system would do write buffering to improve performance, but I was not aware of options to control buffering at the OS level. — PentiumPro200, Jun 23 '18 at 19:13

score 0 · Answer 2 · answered Jun 23 '18 at 19:29

Does the fwrite(fp,...) function return after the data to be written to disk has been handed over to the operating system or does it return only after the data is actually physically written to the disk?

No. In fact, it does not even (necessarily) wait until data has been handed over to the OS -- fwrite may just put the data in its internal buffer and return immediately without actually writing anything.

To force data to the OS, you need to use fflush(fp) on the FILE pointer, but that still does not necessarily write data to the disk, though it will generally queue it for writing. But it does not wait for those queued writes to finish.

So to guarentee the data is written to disk, you need to do an OS level call to wait until the queued writes complete. On POSIX systems (such as Linux), that is fsync(fileno(fp)). You'll need to study the Windows documentation to figure out how to do the equivalent on Windows.

Does fwrite block until data has been written to disk?

2 Answers2