August 8th, 2025
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Under what conditions could a Read­File or Write­File fail to transfer all of the bytes, and how do I detect that?

Raymond Chen

A customer wanted to understand the conditions under which the Read­File and Write­File functions would fail to transfer all of the bytes, and how to detect that this has occurred.

The obvious reason why the Read­File functions would fail to transfer all the bytes is if there aren’t that many bytes to read. For a disk file, this typically happens because you are reading past the end of the file. You can also get this for other types of file handles: For a pipe in nonblocking mode, there may not be enough bytes in the pipe. Or you might have a message pipe, and the message is smaller than the size of your buffer. Or you might be accessing a device, and the device doesn’t have all the bytes available.

Similarly, the obvious reason why the Write­File function would fail to transfer all the bytes is if there isn’t enough room for all the bytes. For a disk file, the disk might be full, or you have reached your disk quota. For a pipe in nonblocking mode, a write may be short if there is not enough buffer space in the pipe to hold all the requested data. In all of these cases, you can detect the short write by checking whether the actual number of bytes written is less than the number of bytes requested.

If the number of bytes actually transferred is nonzero, then the Read­File and Write­File functions will return success, but the actual number of bytes transferred will be less than the number of bytes requested.

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Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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  • Cake Cheese

    Seems `­` in the title is breaking some rss feed parsing of this blog: https://github.com/alexdebril/feed-io/issues/439
    Is it possible to fix it?
    Sorry for disturbing with little off topic issue.

  • Alex Alabuzhev

    > the obvious reason why the Write­File function would fail to transfer all the bytes is if there isn’t enough room for all the bytes. For a disk file, the disk might be full, or you have reached your disk quota.

    Experiments show that for a disk file the (synchronous) WriteFile writes nothing and returns false when there isn’t enough room for all the bytes.

  • Luca Bacci

    Probably it’s up to the driver handling IRP_MJ_WRITE. However drivers should report the number of bytes consumed, not the number of bytes produced

  • Georg Rottensteiner

    Is it also possible for WriteFile to return a value higher than passed?

    I happened on something like this on a Tandem mainframe, where a special file type insists on always having an even size. To my (then) surprise writing one byte would return to have written 2.

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