# Documenting sscanf()'s and fscanf's underflow - Thread: https://externals.io/message/131005 - Original archive: https://news-web.php.net/php.internals/131005 - Started by mickmackusa on 2026-05-24T20:38:16+00:00 - Messages: 3 - Reply by email to internals@lists.php.net - More for agents: search with https://externals.io/search.md?q={query} · RFCs at https://externals.io/rfc.md · JSON API at https://externals.io/api · documentation at https://externals.io/llms.txt --- ## #131005 — mickmackusa — 2026-05-24T20:38:16+00:00 URL: https://externals.io/message/131005#131005 Hi all, may I request that the documentation pages for `sscanf()` and `fscanf()` be reviewed? Last week, I tripped over a bug in my code because, despite using and loving `sscanf()` for many years, the function unexpectedly returned -1 for the number of successful captures when I expected 0. I've documented the encounter here: https://stackoverflow.com/q/79942833/2943403 I have been doing a fair amount of Google searching, but either my keywords are not ideal or there really is no mention of this floating around (even the C++ pages that I read don't mention it). Thanks, mickmackusa --- ## #131006 — Jordi Kroon — 2026-05-24T22:55:00+00:00 URL: https://externals.io/message/131005#131006 · In reply to: #131005 > Hi all, may I request that the documentation pages for `sscanf()` and > `fscanf()` be reviewed? > > Last week, I tripped over a bug in my code because, despite using and > loving `sscanf()` for many years, the function unexpectedly returned -1 > for the number of successful captures when I expected 0. > > I've documented the encounter here: https://stackoverflow.com/ > q/79942833/2943403 > > I have been doing a fair amount of Google searching, but either my > keywords are not ideal or there really is no mention of this floating > around (even the C++ pages that I read don't mention it). > > Thanks, > mickmackusa `sscanf()` and `fscanf()` may return -1 when the end of the input is reached before any value has been successfully parsed. That behaviour is currently missing from the documentation. I opened a PR to cover this: https://github.com/php/doc-en/pull/5580 Specific commit: https://github.com/php/doc-en/commit/7af7c98e4f7c983581dc05c58b958bb6c0c1267b Feel free to check and reply in the PR conversation if you think the wording should be adjusted. -- Regards, Jordi Kroon --- ## #131020 — Hans Krentel — 2026-05-26T13:37:53+00:00 URL: https://externals.io/message/131005#131020 · In reply to: #131005 > Hi all, may I request that the documentation pages for `sscanf()` and > `fscanf()` be reviewed? Thank you for the detailed documentation on SO and invoking some initiative here on the list. > I have been doing a fair amount of Google searching, but either my keywords > are not ideal or there really is no mention of this floating around (even > the C++ pages that I read don't mention it). C++ might be a red herring, sscanf is more ANSI C and for PHP the original code from TCL8.3.0: "The main site for Tcl/Tk source distributions is SourceForge" [1], that version's sources and manual: * tcl8.3.0.tar.gz -- 2001-07-30 2.5 MB * tcl8.3.0.html.tar.gz -- 2001-07-30 582.1 kB For sscanf ANSI C I have found The Unix Heritage Society an invaluable resource to learn about ANSI draft interpretations as they have archives of Usenet comp.std.c in indexable hypertext documents as well as gzipped tarballs: I found it helpful in context of PHP to understand the communication in the user's network (Usenet/USENIX network) with the Unix-to-Unix Copy Protocol (UUCP) and also the nomenclature of C and other computer lanugages standardization processes. To give an example for questions like why the PHP implementation of `fscanf()` might be reading one line at a time as you briefly mentioned off-list IIRC, I found a message 'Hints for using "scanf"' [2] that helped me to reflect such kind of thoughts. Let me illustrate with an excerpt of the exemplary message written by Martin Weitzel in January 1990: > If you want to analyze interactive (or unpredictable) input, > my advice is to read a complete line with "fgets" (avoid "gets"!!) > or read up to some other significant token with "gets/fgets". > After that, use "sscanf" on the buffer you just read. In our sources we can find the following lines: buf = php_stream_get_line((php_stream *) what, NULL, 0, &len); if (buf == NULL) { RETURN_FALSE; } result = php_sscanf_internal(buf, format, argc, args, 0, return_value); This is PHP's way of `fgets()` followed by `sscanf()` on the buffer, as illustrated by M. Weitzel back in 1990. As you requested, I illustrative now "reviewed" the manual page for the `fscanf()` function [3] in this regard: > Each call to `fscanf()` reads one line from the file. Find out more about networked computing not only in comp.infosystems.www.authoring.cgi (check June 8 '95) but also in the comp.* hierarchy when you want to research in the archives. best -- hakre p.s. I took some time yesterday to document an example listing of how to make use of the "underflow behavior", e.g. for testing in GH-19088 [4] in case of interest. [1]: https://www.tcl-lang.org/software/tcltk/download.html [2]: https://www.tuhs.org/Usenet/comp.std.c/1990-January/000290.html [3]: https://web.archive.org/web/20170812235246/http://php.net/fscanf [4]: https://github.com/php/php-src/issues/19088#issuecomment-4534153067