[tbirdz]

A 48-bit size_t like I suggested could address up to 256TB of RAM. All modern x86_64 cpus are limited to 48-bits of address space, you're not losing any portability here.

Also consider my strlen example. Say you compute strlen by iterating through the whole string until you find a 0, then you return a size_t for the number of bytes you iterated through. That operation is O(n) in the length of the string. If you were to use my strlen function on a string whose length is greater than would fit into a 32-bit integer, say a 1TB string or something, then the function would take so long to compute it that it would be useless. To be efficient, you'd probably have to redesign your program to do some special things to handle 1TB strings, maybe some special algorithms, or some kind of indexing. Returning a 64-bit integer type does not mean that the function can actually handle working with 64-bit sized quantities. So if you have a lot of datastructures where you keep string length, why store them as a 64-bit size_t when your application would be completely unable to handle strings of that size without keeling over?

Of course you wouldn't really use a 48-bit size_t, because x86_64 cpus don't work well with 48-bit quantities.