int ERR_GET_LIB(unsigned long e);
int ERR_GET_FUNC(unsigned long e);
int ERR_GET_REASON(unsigned long e);
The library number and function code describe where the error occurred, the reason code is the information about what went wrong.
Each sub-library of OpenSSL has a unique library number; function and reason codes are unique within each sub-library. Note that different libraries may use the same value to signal different functions and reasons.
ERR_R_... reason codes such as ERR_R_MALLOC_FAILURE are globally unique. However, when checking for sub-library specific reason codes, be sure to also compare the library number.
ERR_GET_LIB(), ERR_GET_FUNC() and ERR_GET_REASON() are macros.