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3669. std::filesystem operations should be observable behaviour

Section: 4.1.2 [intro.abstract] Status: New Submitter: Jens Maurer Opened: 2022-02-03 Last modified: 2026-07-01

Priority: 3

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Discussion:

4.1.2 [intro.abstract] p6.2 should be amended to say that filesystem operations such as mkdir are observable behaviour. Any resolution would need CWG review.

[2022-03-04; Reflector poll]

Set priority to 3 after reflector poll.

[Brno 2026-06-12; LWG discussion]

Cross reference is now p8.2. Add a bullet with the words from 31.12.13.1 [fs.op.funcs.general].

[2026-06-30; Tim adds wording]

Proposed resolution:

This wording is relative to N5050.

1. Modify 4.1.2 [intro.abstract]p8 as indicated:

   -8- The following specify the observable behavior of the program:

   1. (8.1) — Accesses through volatile glvalues are evaluated strictly according to the rules of the abstract machine.

   2. (8.2) — Data is delivered to the host environment to be written into files (See also: ISO/IEC 9899:2024, 7.23.3).

      [Note 4: Delivering such data is followed by an observable checkpoint (31.13.1 [cstdio.syn]). Not all host environments provide access to file contents before program termination. — end note]

   3. (8.?) — Files in external storage are modified (31.12.13.1 [fs.op.funcs.general]).

   4. (8.3) —The input and output dynamics of interactive devices shall take place in such a fashion that prompting output is actually delivered before a program waits for input. What constitutes an interactive device is implementation-defined.