Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

The InOut function
The InOut function is called by the Read, Readln, Write, Writeln, Eof, Eoln, SeekEof, SeekEoln, and CloseFile
standard routines whenever input or output from the device is required.
When Mode is fmInput, the InOut function reads up to BufSize characters into BufPtr^, and returns the number of
characters read in BufEnd. In addition, it stores zero in BufPos. If the InOut function returns zero in BufEnd as a
result of an input request, Eof becomes True for the file.
When Mode is fmOutput, the InOut function writes BufPos characters from BufPtr^, and returns zero in BufPos.