understanding Macros in C and preprocessor directives

Question

I don't understand those macros, how does it work?, also what is irc_##name ?..i've got this code from insobot IRC bot,here is the code https://github.com/baines/insobot/blob/master/src/insobot.c

#define IRC_CALLBACK_BASE(name, event_type) static void irc_##name ( \
irc_session_t* session, \
event_type     event,   \
const char*    origin,  \
const char**   params,  \
unsigned int   count    \
)

#define IRC_STR_CALLBACK(name) IRC_CALLBACK_BASE(name, const char*)
#define IRC_NUM_CALLBACK(name) IRC_CALLBACK_BASE(name, unsigned int)

This is basic preprocessor usage. Please read some online C tutorials or a book about C: for example "The C programming language" by Ritchie and Kernighan. — ForceBru
– ForceBru, Commented Sep 30, 2018 at 12:02
It is basic C grammar. en.cppreference.com/w/cpp/preprocessor/… — paxbun
– paxbun, Commented Sep 30, 2018 at 12:05

chqrlie · Accepted Answer · 2018-09-30 13:24:59Z

1

## is the token concatenation operator: it is used in these macro definitions to create the identifier for the name of the callback function by prepending irc_ before the value of the first argument of the macro IRC_STR_CALLBACK and/or IRC_NUM_CALLBACK

Look at this macro invocation at line 183:

IRC_STR_CALLBACK(on_join);

this source line gets expanded to

static void irc_on_join ( irc_session_t* session, event_type     event,   const char*    origin,  const char**   params,  unsigned int   count    );

The macro is used to declare the handler in a consistent fashion without the need to write the prototype explicitly, which is handy because there are many handlers in this source file.

edited Sep 30, 2018 at 13:24

answered Sep 30, 2018 at 13:15

chqrlie

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