Would you like signs with those chars?

>> #define tolower(c) ((_tolower_tab_ + 1)[c])
>> Also, EOF is a permitted value and typically -1 (thus the +1 in the above), though that would typically only be an issue for non-C locales.

> The argument is of type int (according to the standard, not that untyped macro), not char, so it's completely unambiguous: You are allowed to pass negative numbers to those routines, because int is always signed, and if it is implemented as a macro, it has to accept signed values in the int range. Of course, if you pass negatives other than EOF (or whatever EOF is #define'd to), then the standard presumably gives you UB (which is why it's OK for the array implementation to walk off the end in that case).

The problem is that this will only work for values between -1 and 127 for an array of 129 bytes. A value of -2 (or any other non-ascii value other than EOF with signed char) would access memory before the array and a value of 255 (EOF mistakenly stored in an unsigned char or anything non ascii) would access memory after the array.

Looking at the source link in the other comments the BSD code seems to assume chars are unsigned. The test for ascii doesn't work with signed chars at all.

So I assume that "_tolower_tab_" is actually 257 bytes long to cover all unsigned chars and EOF (which is -1 when stored as int).