WriteFile lpNumberOfBytesWritten less than nNumberOfBytesToWrite

Question

When calling WriteFile can the returned lpNumberOfBytesWritten ever be less than nNumberOfBytesToWrite in synchronous write mode where an error has not occured (return is TRUE)? Writing is done to a proper file (not a socket, pipe, or other special handle). lpOverlapped is not used.

It is not entirely clear from the docs.

Answer 1

The documentation states:

The WriteFile function returns when one of the following conditions occur:

• The number of bytes requested is written.
• A read operation releases buffer space on the read end of the pipe (if the write was blocked). For more information, see the Pipes section.
• An asynchronous handle is being used and the write is occurring asynchronously.
• An error occurs.

Only the first one of these meets your criteria so the answer is that the documentation is clear *lpNumberOfBytesWritten is always equal to nNumberOfBytesToWrite when the function returns when called as you specify.