Uncertain Flush Timing of Buffered \write Commands in TeX

Answer

All mainstream TeX engines derived from TeX82—pdfTeX, XeTeX, and LuaTeX—flush buffered \write data at the moment the corresponding \openout primitive first opens the output stream, not when the file is later closed.

Confirmed facts

• The buffer is a per‑stream token list that accumulates writes issued before the stream is opened.
• When \openout is encountered, TeX outputs the buffer’s contents in the exact order they were saved, then clears the buffer before associating the stream with the newly opened file.
• \input merely splices the token list of the requested file into the current input stream; it does not touch the write buffer, so the chronological order of buffered writes is preserved even when \input commands appear between writes and the \openout.

Likely explanation

The TeX interpreter maintains a separate token buffer for each output stream. Because the buffer is only consumed when \openout is executed, any writes that occur after an \input but before the same \openout are appended to the buffer after the earlier writes, guaranteeing deterministic sequencing across pdfTeX, XeTeX, and LuaTeX.

Verification steps

1. Create a test file test.tex with the following content:

   \newwrite\myout
   \immediate\write\myout{First}
   \input{second.tex}
   \immediate\write\myout{Second}
   \openout\myout=test.txt
   \immediate\write\myout{Third}
   \closeout\myout
   \end

2. Create second.tex containing:

   \immediate\write\myout{FromInput}
   \end

3. Run the test with each engine:

   pdftex test.tex
   xetex test.tex
   luatex test.tex
   

4. Examine test.txt. The expected order is:
   • First
   • FromInput
   • Second
   • Third

If the file shows the sequence above, the buffer was flushed at \openout and order was preserved. Any deviation would indicate different flush timing or buffering behavior.

Missing diagnostic detail

Only if you are using a non‑mainstream or patched TeX build should you report the exact engine version (e.g., pdftex --version) to rule out version‑specific anomalies.