Comment by astrea
4 years ago
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.
> sscanf() converts the string you pass in to an _IO_FILE* to make the string look like a "file". This is so the same internal _IO_vfscanf() can be used for both a string and a FILE*.
> However, as part of that conversion, done in a _IO_str_init_static_internal() function, it calls __rawmemchr (ptr, '\0'); essentially a strlen() call, on your input string. This conversion is done on every call to sscanf(), and since your input buffer is rather large, it'll spend a fair amount of time calculating the length of the input string.
https://stackoverflow.com/a/23924112
and the proper remedy is to use fmemopen(3), which makes the temporary FILE object explicit and persistent for the whole parse, and needs just one strlen call.
https://news.ycombinator.com/item?id=26343149
Everybody in this thread seems to be missing a particularly large elephant in this particular room, which is that sscanf() supports scientific notation while strtod() and strtof() do not.
Or at least, they didn't originally support it.
Has this been fixed in the 20+ years since I noticed it and started using sscanf() everywhere instead?
Sscanf shouldn't need to get the size. The fact that it does is a flaw in a particular implementation of the c standard library.