Assigning integer to an enum using macro in C

Question

I am working on a big project in which I need to use a generic macro to assign my values from a big payload. the values may be int, uint, bool or enum. all other macros are working fine only for the enum I need help with. Consider this piece of code as something I want to achieve.

#include <stdio.h>

enum reasons { REASON1 = 1, REASON2 = 2, MAX_REASON = 3 };

#define SET_ENUM(dest, data) dest = data;

int main(void)
{
        int a = 2;
        enum reasons rs; 
        SET_ENUM(rs, a); 

        printf("REASON is -> %d\n", rs);
        return 0;
}

Here it works everything fine but in my Coverity run it shows

Event mixed_enum_type: enumerated type mixed with another type

I want to typecast the data to the dest but need to do it from the macro. Any suggestions?

simply cast both to `int`. Remove `;` and even better wrap it into `do {} while(0)` — 0___________, Mar 15 '23 at 21:34

score 1 · Accepted Answer · answered Mar 15 '23 at 20:37

1

You could do a three-arg SET_ENUM, specifying the type:

#define SET_ENUM(dest, type, data) dest = (enum type) data
...
SET_ENUM(rs, reasons, a);

Or with certain compilers (gcc) you could use the typeof keyword:

#define SET_ENUM(dest, data) dest = (typeof(dest)) data
...
SET_ENUM(rs, a);

The typeof keyword is currently a non-standard extension, it seems to be planned for the C23 standard.

answered Mar 15 '23 at 20:37

teapot418

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all your typecasts simple convert from `int` to `int`. Enum in C is just an syntactic sugar – 0___________ Mar 15 '23 at 21:11
the cast is just to inform the compiler, don't see any harm in that. – Andrés Alcarraz Mar 15 '23 at 21:27
1

@0___________ sure, they don't add any meaning; I think the point of OP's question was to get rid of some coverity complaints – teapot418 Mar 15 '23 at 21:36
This is exactly what I was looking for. I have 3 different types of enum and need to use the same macro. – Aravind Ekan Mar 15 '23 at 22:57

Andrés Alcarraz · Answer 2 · 2023-03-16T12:05:18.603

1

The preprocessor will replace SET_ENUM(rs, a) with rs = a.

So you'll need to add the cast so that it is replaced by rs = (reasons)a.

If you are only setting that enum you could just define the macro as:

#define SET_ENUM(dest, data) dest = (reasons)data;

If you want it to be for other enums, I'm afraid you'll have to add the enum as parameter to the macro:

#define SET_ENUM(dest, data, T) dest = (T)data;

And then call it like this:

SET_ENUM(rs, a, reason);

edited Mar 16 '23 at 12:05

answered Mar 15 '23 at 20:38

Andrés Alcarraz

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a very bad macro `if(x) SET_ENUM(a,b,c); else printf("Error");` will not compile – 0___________ Mar 15 '23 at 20:48
can you say why (if `c` is an enum?) and how you would fix it? Also, it's just an example developing over what the OP already had. – Andrés Alcarraz Mar 15 '23 at 21:09
This macro isn't thought to be perfect, just the simplest way, with minimal changes to what the OP asked, so that it works with correct params obviously. – Andrés Alcarraz Mar 15 '23 at 21:26

Assigning integer to an enum using macro in C

2 Answers2