I think you meant _IONBF instead of _IOFBF -- otherwise it's at odds with the comment you add. But what is the justification for that addition? I don't see us doing that anywhere else.
No.
_IOFBF
Full buffering: On output, data is written once the buffer is full (or flushed). On Input, the buffer is filled when an input operation is requested and the buffer is empty.
_IONBF No buffering: No buffer is used. Each I/O operation is written as soon as possible. In this case, the buffer and size parameters are ignored.
_IONBF ignores buffer and size.
Without setvbuf, fread uses an internal buffer, default 4096 bytes (OS dependent).
If fread uses an internal buffer, then it needs a copy to the buffer provided by the function.
setvbuf, does the same as read function low level, copies directly to the final buffer.
