Using macro with variables in c

Question

I have a flat C file including ctype.h where i cant figure out how a macro works. There is this macro

#define da_dim(name, type)  type *name = NULL;          \
                            int _qy_ ## name ## _p = 0;  \
                            int _qy_ ## name ## _max = 0

I thought it should define the type of a given value. So for example i could write

int a;
da_dim(a,"char");

to convert it to a char but it doesnt do that. I can imagine what '## name ##' is/does (like a placeholder) but i dont understand what 'qy' is and where it came from. So what is this macro for, how tu use it and (maybe) how does it work?

I suggest you run the code through the preprocessor to see what the preprocessor replaces the macro invocation with (or attempt to do the replacement yourself on paper). — Some programmer dude
– Some programmer dude, Commented Dec 31, 2018 at 16:59
As for what the macro is for, have you asked the author? Or read the documentation of the code you saw the macro in? — Some programmer dude
– Some programmer dude, Commented Dec 31, 2018 at 17:00
Lastly, your example usage of the macro is wrong and would lead to build errors. If you expand the macro (either on paper or using the preprocessor) you would see why. — Some programmer dude
– Some programmer dude, Commented Dec 31, 2018 at 17:01

Pablo Santa Cruz · Accepted Answer · 2018-12-31 21:55:00Z

4

A macro, in C is just a simple token replacement mechanism.

Your example:

int a;
da_dim(a,"char");

Will expand to:

int a;
"char" *a = NULL;
int _qy_a_p = 0;
int _qy_a_max = 0;

So, if will expand to errors because you will have two a identifiers and "char" is not expected where you are placing it.

If you are using gcc, you can "see" macro expansions by doing:

$ gcc -E your_program.c

edited Dec 31, 2018 at 21:55

answered Dec 31, 2018 at 17:01

Pablo Santa Cruz

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2 Comments

DoJo Over a year ago

Thank you very much i understand it :)

Pablo Santa Cruz Over a year ago

Thanks @EricPostpischil. Fixed that.

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