Question is why does &response_string work where a C char* is expected?

A C char* is not necessarily expected. Whatever you pass is later given as an argument to the WRITEFUNCTION . It could be char*, FILE* , std::string*, or any other pointer type depending on how WRITEFUNCTION is implemented.

Also, why does response_string.c_str() core dump if I use that instead of &response_string?

Probably due to passing a char* where a std::string* is expected.