C99 supports the integral data type long long. This type is 64 bits wide in the ARM compilation tools. For example:

long long int j = 25902068371200;                // length of light day, meters
unsigned long long int i = 94607304725808000ULL; // length of light year, meters

long long is also available in C90 when not using --strict.

__int64 is a synonym for long long. __int64 is always available.