Excellent!

> it's in the manual

It's not in MSDN: https://msdn.microsoft.com/en-us/library/yd5xkb5c.aspx

It's not in the manpages: https://linux.die.net/man/3/atoi

Cppreference understates it has having an undefined return value, rather than undefined behavior outright: http://en.cppreference.com/w/cpp/string/byte/atoi

Tutorialspoint defines the behavior as returning 0, and fresh2refresh makes no mention of undefined behavior.

My eighth google hit for atoi finally, finally, gets it right: http://pubs.opengroup.org/onlinepubs/9699919799/functions/at...

If you buy or pirate a copy of e.g. the C89 standard, or refer to one of the free draft versions, it's of course properly documented there too. Neither shows up in the first 50 google results, naturally.

And, of course, by google result 9, we're back to square one - incorrectly defining the behavior as being "returning 0": https://en.wikibooks.org/wiki/C_Programming/stdlib.h/atoi