I wrote this originally on https://emacs.stackexchange.com/questions/38710/why-does-format-x-some-large-number-produces-incorrect-results and a poster recommended I mention this here.I wanted the hexadecimal string for a large integer such as below:

(format "%x" 2738188573457603759)

This returns 2600000000f95c00 which is incorrect, it should be 2600000000f95caf.

The value of most-positive-fixnum on my box is 0x1fffffffffffffff which is less than the number I'm supplying above.

As a user I'm a bit baffled what is happening. The manual indicates integers larger than this range are converted to a floating-point number which is a concern for precision but I suspect this is what is biting me here?

I should have known there was an issue with this number since normally I evaluate them directly using eval-last-sexp and it didn't show the octal/hex variants.. :)

I wonder why Emacs Lisp doesn't support bignums by default, so precision would not be an issue?