Some nice things about C - unsigned long long ints are impressively large. I don't know if these are standard in most programming languages yet (irrelevant for languages such as Python and Ruby, for obvious reasons). And using gmp with C is a dream. It's a C library, after all.
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The width of integers in C is implementation defined, strictly speaking. And so sizeof(long) is only 4 on MSVC, for example. In practice established C codebases like Linux and Postgres use typedefs that specify a width explicitly.