There is a problem trying to use the 64-bit integer options on x86_64 Linux versions as the json/config.h file defines Int64 as "long long int". However, x86_64 Linux uses only "long int" to define 64-bit integers. An option should probably be added to the define section when it detects a Linux system to be x86_64. The Int64 typedef should be set as "long int" instead. The UInt64 should follow the same pattern.