C Expression must be a modifiable lvalue

Question

I`m struggling with an error in C code.

#define MAX(i, limit) do \
{ \
if (i < limit) \
{ \
i++; \
} \
} while(1)
void main(void)
{ 
MAX(0,3);
}

This leads into the following error.

Error[Pe137]: expression must be a modifiable lvalue

Any ideas? Thank you in advance.

Answer 1

In this use case you try to increment 0 which is not possible.

do {} while in this case makes no sense at all.

I would write it this way :

#define MAX(i, limit)  (((i) < (limit)) ? (i) + 1 : (i))  

void main(void)
{ 
    int i = 5;
    i = MAX(i, 7);
    printf("%d\n", MAX(0,3));
    printf("%d\n", i);
    MAX(0,3);  // this statement has no effect
}

And (probably) you would not try to:

0 = MAX(0,3);

You need to remember that is the macro and i and limit will be evaluated as many times as they are in the macro. Example:

#define MAX(i, limit)  (((i) < (limit)) ? i + 1 : i)

int y = 1;
int increase(void)
{
    y++;
}

void main(void)
{ 
    printf("%d\n", MAX(increase(),3));
}

Some expressions may lead to the undefined behaviour:

#include <stdio.h>
#define MAX(i, limit)  (((i) < (limit)) ? i + 1 : i)

int y = 1;

void main(void)
{ 
    printf("%d\n", MAX(y++,3));
}