21–30 of 419 posts
It has been a hot minute since I've touched C, so I'm failing to grok the issue here. Sscanf is reading the data variable for a float-formatted string into a float variable. How is that also getting the size? What is different about strtof? It looks from the docs that it does something similar, just without using the formatting string.
Earlier quoted context omitted.
Looped algorithms generally outperform recursive algorithms.
does that hold for languages/compilers with tail call optimisation?
It is a good opportunity to mention that you should not use strtod/strtol either if you can help it, since they are impacted by locale. Exactly what to use instead is a bit of a tough nut to crack; you could extract musl’s floatscan code, or implement the Clinger algorithm yourself. Or, of course, use programming languages that have a more reasonable option in the standard library...
Pro tip: the classic recursive approach to implementing the Fibonacci sequence exhibits eye-wateringly poor performance.
Linked deep in the Twitter replies [1], there's an open glibc issue about this, dating back to 2014:
https://sourceware.org/bugzilla/show_bug.cgi?id=17577
C doesn't have any requirements on the complexity of sscanf, so it might not be a bug per se, but it's certainly... a pitfall.
[1] https://twitter.com/impraxical/status/1367194430835425283
I don't think anyone sensible would claim they would never have made this mistake. It's not even surprising that it made it to production. What is surprising is that it lead to a massive and easily-diagnosable slowdown that no one bothered to fix for several years.
I don't think anyone sensible would claim they would never have made this mistake. It's not even surprising that it made it to production. What is surprising is that it lead to a massive and easily-diagnosable slowdown that no one bothered to fix for several years.
The moral of the story, as far as I'm concerned: do NOT parse strings in C! Use a library, prefferably in a higher-level language. C string handling is a mess of viciously surprising APIs, juggling those particular footguns is almost certainly not your least bad option.