Question

Hi I have the following scenario

#define CONSTANT 10

struct structA{
    int var1;
    int var2[CONSTANT];
};

main(){
    structA *varA = NULL;
    int i;

    /* some C code */


    varA = mmap(0,..);
    for(i = 0; i < CONSTANT; i++){
        varA.var2[i] = /* C code */ ;
    }

    /* C code */

}

Here the number of elements in var2 array of structA is constant and hence the varA.var2[i] can be directly referenced . But If the number CONSTANT is to be determined dynamically during runtime , how should the structure definition be modified?

Answer 1

The number CONSTANT cannot be determined dynamically at runtime, since it affects the structure size.

The usual trick for what you’re trying to do (I’m not sure if it’s strictly valid C but it’s very common) is to create

struct structA{
    int var1;
    int var2[0];
};

and when you map or allocate, allocate (sizeof(structA) + sizeof(int) * CONSTANT) bytes.