What do # and ## operators do in C? [duplicate]

Answer 1

From the wikipedia page describing the C preprocessor:

The ## operator concatenates two tokens into one token, as in this example:

#define DECLARE_STRUCT_TYPE(name) typedef struct name##_s name##_t
DECLARE_STRUCT_TYPE(g_object); // Outputs typedef struct g_object_s g_object_t;

The # operator signals other directives to the C preprocessor, for example: #include, #define, #undef, #error, #if, #ifdef, #ifndef, #else, #elif, #endif