5 ms·

Unspecified, really? cppreference's [C documentation][1] says that it returns zero. The [OpenGroup][2] documentation doesn't specify a return value when the con

Unspecified, really? cppreference's [C documentation][1] says that it returns zero. The [OpenGroup][2] documentation doesn't specify a return value when the conversion can't be performed. This recent [draft][3] of the ISO standard for C says that if the value cannot be represented (does that mean over/underflow, bad parse, both, neither?), then it's undefined behavior. So three references give three different answers. You could always use sscanf instead, which tells you how many values were scanned (e.g. zero or one). [1]: https://en.cppreference.com/w/c/string/byte/atoi.html https://en.cppreference.com/w/c/string/byte/atoi.html [2]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/atoi.html https://pubs.opengroup.org/onlinepubs/9799919799/functions/a... [3]: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf

acidx 11mo ago
The Linux man page (https://man7.org/linux/man-pages/man3/atoi.3.html#VERSIONS https://man7.org/linux/man-pages/man3/atoi.3.html#VERSIONS) says that POSIX.1 leaves it unspecified. As you found out, it's really something that should be avoided as much as possible, because pretty much everywhere disagrees how it should behave, especially if you value portability. sscanf() is not a good replacement either! It's better to use strtol() instead. Either do what Lwan does (https://github.com/lpereira/lwan/blob/master/src/lib/lwan-config.c#L156-L174 https://github.com/lpereira/lwan/blob/master/src/lib/lwan-co...), or look (https://cvsweb.openbsd.org/src/lib/libc/stdlib/strtonum.c?rev=1.8&content-type=text/x-cvsweb-markup https://cvsweb.openbsd.org/src/lib/libc/stdlib/strtonum.c?re...) at how OpenBSD implemented strtonum(3). For instance, if you try to parse a number that's preceded by a lot of spaces, sscanf() will take a long time going through it. I've been hit by that when fuzzing Lwan. Even cURL is avoiding sscanf(): https://daniel.haxx.se/blog/2025/04/07/writing-c-for-curl/ https://daniel.haxx.se/blog/2025/04/07/writing-c-for-curl/
- MathMonkeyMan 11mo ago
  If your use case can have C++, then [std::from_chars][1] is ideal. Here's gcc's [implementation][2]; a lot of it seems to be handling different bases. [1]: https://en.cppreference.com/w/cpp/utility/from_chars.html https://en.cppreference.com/w/cpp/utility/from_chars.html [2]: https://github.com/gcc-mirror/gcc/blob/461fa63908b5bb1a44f12303676d9eb83f5dbdd3/libstdc%2B%2B-v3/include/std/charconv#L562 https://github.com/gcc-mirror/gcc/blob/461fa63908b5bb1a44f12...

acidx 11mo ago

The Linux man page (https://man7.org/linux/man-pages/man3/atoi.3.html#VERSIONS https://man7.org/linux/man-pages/man3/atoi.3.html#VERSIONS) says that POSIX.1 leaves it unspecified. As you found out, it's really something that should be avoided as much as possible, because pretty much everywhere disagrees how it should behave, especially if you value portability. sscanf() is not a good replacement either! It's better to use strtol() instead. Either do what Lwan does (https://github.com/lpereira/lwan/blob/master/src/lib/lwan-config.c#L156-L174 https://github.com/lpereira/lwan/blob/master/src/lib/lwan-co...), or look (https://cvsweb.openbsd.org/src/lib/libc/stdlib/strtonum.c?rev=1.8&content-type=text/x-cvsweb-markup https://cvsweb.openbsd.org/src/lib/libc/stdlib/strtonum.c?re...) at how OpenBSD implemented strtonum(3). For instance, if you try to parse a number that's preceded by a lot of spaces, sscanf() will take a long time going through it. I've been hit by that when fuzzing Lwan. Even cURL is avoiding sscanf(): https://daniel.haxx.se/blog/2025/04/07/writing-c-for-curl/ https://daniel.haxx.se/blog/2025/04/07/writing-c-for-curl/

MathMonkeyMan 11mo ago
If your use case can have C++, then [std::from_chars][1] is ideal. Here's gcc's [implementation][2]; a lot of it seems to be handling different bases. [1]: https://en.cppreference.com/w/cpp/utility/from_chars.html https://en.cppreference.com/w/cpp/utility/from_chars.html [2]: https://github.com/gcc-mirror/gcc/blob/461fa63908b5bb1a44f12303676d9eb83f5dbdd3/libstdc%2B%2B-v3/include/std/charconv#L562 https://github.com/gcc-mirror/gcc/blob/461fa63908b5bb1a44f12...