Thanks to the two quick responders: I wasn't anticipating such immediate help. I've looked in gccint.pdf, and I was hoping to find but couldn't something like: #include <stdlib.h> If it helps, "wide int" is used in function "choose_multiplier" in expmed.c which is located here for example: gcc-7.3.0/gcc/expmed.c I've only ever managed to use "wide int" by making GCC and inserting a "hook" in "choose_multiplier" which runs some tests and benchmarks comparing using "wide int" with using fixed width integers "unsigned HOST_WIDE_INT". I was hoping there was an easier way to do it!