How to get value macro in C?

Question

I'm having trouble with macros in C. Is there any way to call and get the value of a parameter in macro to call a function?

The example code below generates an error:

#define order(i) fruits_i##_banana()

void fruits_1_banana()
{
    printf("Order banana\n");
}

int main()
{
   int a = 1;
   order(a);
}

There is a literal replacement and the preprocessor doesn't translate `a` to 1, you can use an array of pointers to functions. — David Ranieri, Nov 03 '21 at 06:20
Thank you very much, could you mind telling more clearly how to use array of pointers to function in macro??? — Son, Nov 03 '21 at 06:38
Sorry because the previous question is so confusing, so that i update a little bit the information that can understand more about my situation — Son, Nov 03 '21 at 06:40
You have an example here: https://stackoverflow.com/a/37394374/1606345 of course you don't need to reverse, just use the prototypes and the array definition and you are done. — David Ranieri, Nov 03 '21 at 06:50
Macros cannot be used for run-time purposes, simple as that. `a` is not an integer constant and therefore can't be used in this manner. There is no way to fix the code, consider a different solution to whatever problem you are trying to solve with this. — Lundin, Nov 03 '21 at 08:37
Even though you've gotten a valid answer, it would probably be a good idea to avoid this kind of code generation if you can. It complicates your program and makes static analysis and debugging much more difficult. — einpoklum, Nov 03 '21 at 13:19

score 2 · Accepted Answer · edited Nov 03 '21 at 07:33

2

You need to use ## before and after i.

#define order( i ) fruits_##i##_banana()

void fruits_1_banana()
{
    printf("Order banana\n");
}

int main()
{
   order(1);
}

Note that you cannot pass a to order because macro expansion doesn't take the value of a variable, it just uses the variable name as it is.

References: https://learn.microsoft.com/en-us/cpp/preprocessor/token-pasting-operator-hash-hash?view=msvc-160

Instead, you can use an array of function pointers:

#include <stdio.h>

static void fruits_0_banana(void)
{
    printf("Order banana 0\n");
}

static void fruits_1_banana(void)
{
    printf("Order banana 1\n");
}

static void (*order[])(void) = {
    fruits_0_banana, // Or NULL if you don't need it
    fruits_1_banana,
    // ...
};

int main(void)
{
    int a = 1;

    order[a]();
    return 0;
}

edited Nov 03 '21 at 07:33

David Ranieri

39,972
7
52
94

answered Nov 03 '21 at 06:28

kiner_shah

3,939
7
23
37

Thank you very much, i updated my code again but still get the error , because the macro not understand the value a is 1 :( – Son Nov 03 '21 at 06:32
Yes, you cannot pass `a` to `order`. – kiner_shah Nov 03 '21 at 06:33
so what i need to do so the macro can understand the value ? please help :( – Son Nov 03 '21 at 06:36
1

@Son, you need to update the question with your exact requirements, why do you need to pass a variable to a macro which will call the respective function? I think you may need to follow DavidRanieri's suggestion of using function pointers ([See this](https://stackoverflow.com/questions/840501/how-do-function-pointers-in-c-work)) – kiner_shah Nov 03 '21 at 06:40
i really appreciate for your help – Son Nov 03 '21 at 06:42
1

You can not, preprocessor doesn't work at runtime but at compile time, and the value of `a` is unknown at this stage, and even knowing the value (i.e using an `enum`) there is a literal replacement. Read about pointers to function, you can achieve what you want using `order()[1];` – David Ranieri Nov 03 '21 at 06:43
Sorry, you want `order[1]()` not `order()[1]` – David Ranieri Nov 03 '21 at 06:51
Okay i will give it a shoot – Son Nov 03 '21 at 06:53
@Son, give me half an hour and I provide an example in this answer (I'm on the mobile) – David Ranieri Nov 03 '21 at 06:59
@David I appreciate your help so much. – Son Nov 03 '21 at 07:03
1

Since it seems that @kiner_shah agrees with the use of function pointers, you have an edit, please, accept the answer if it was useful ;) – David Ranieri Nov 03 '21 at 07:30
Thank you a lot – Son Nov 03 '21 at 13:20

How to get value macro in C?

1 Answers1