Question

A macro definition:

#define HTTP_ERRNO_MAP(XX)                                           \
  /* No error */                                                     \
  XX(OK, "success")                                                  \
                                                                     \
  /* Callback-related errors */                                      \
  XX(CB_message_begin, "the on_message_begin callback failed")       \
  XX(CB_url, "the on_url callback failed")                           \


/* Define HPE_* values for each errno value above */
#define HTTP_ERRNO_GEN(n, s) HPE_##n,
enum http_errno {
  HTTP_ERRNO_MAP(HTTP_ERRNO_GEN)
};
#undef HTTP_ERRNO_GEN

After expand it by “gcc -E”,

enum http_errno {
  HPE_OK, HPE_CB_message_begin, HPE_CB_url,};

How does the macro expand to the result?

Answer 1

HTTP_ERRNO_MAP(XX) will substitute XX with HTTP_ERRNO_GEN and therefore call the HTTP_ERRNO_GEN(n, s) macro 3 times:

HTTP_ERRNO_GEN(OK, "success")
HTTP_ERRNO_GEN(CB_message_begin, "the on_message_begin callback failed")
HTTP_ERRNO_GEN(CB_url, "the on_url callback failed")

HTTP_ERRNO_GEN(n, s) simply takes the actual text of the first argument and concatonates it (##n) with a HPE out the front and a comma at the end. Therefore the three commands above will produce the following three outputs:

HPE_OK,
HPE_CB_message_begin,
HPE_CB_url,

Therefore when expanded by the preprocessor,

enum http_errno {
  HTTP_ERRNO_MAP(HTTP_ERRNO_GEN)
};

becomes (+- whitespace):

enum http_errno {
   HPE_OK,
   HPE_CB_message_begin,
   HPE_CB_url,
};