Access struct member by an argument of function in C - c

Question

int f(){
struct NUMBER {
int A;
int B;
};
struct NUMBER *num = malloc(sizeof(struct NUMBER));
num->A = 1;
num->B = 2;
int x = num->B;
return x;
}
int main(){
int z = f();
printf("%d\n", z);
}
Obviously, ./a.out will show 2.
My question: Can I access struct member by an argument of function? i.e. f(A) return 1, and f(B) return 2. Thanks a lot.

Answer 1

Not at all elegant, but I think it shows you what you need to do/know.
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
struct NUMBER {
int A;
int B;
};
struct NUMBER extNumber = {1, 2};
int f(int offset) {
int iRet = -1;
if (offset == offsetof(struct NUMBER, A)) {
iRet = extNumber.A;
} else if (offset == offsetof(struct NUMBER, B)) {
iRet = extNumber.B;
}
return iRet;
}
int main (int argc, char **argv) {
struct NUMBER number;
int iVal;
iVal = f(offsetof(struct NUMBER, A));
printf ("A : %d\n", iVal);
iVal = f(offsetof(struct NUMBER, B));
printf ("B : %d\n", iVal);
}

Answer 2

Can I access struct member by an argument of function?
I think you are also assuming struct is not visible outside the function. Now if the person who wrote main does not have visibility into the function (say it is part of a library), then the answer is NO.
Otherwise if author of main can see internals of the function, then: Can someone come up with a fancy way to access struct member inside the function via function argument? Towards that lets think what does f(A) mean? Here, A is a name of struct member, so do you mean passing char A to f and translating that to struct member inside f [e.g. result = *(int *)num+(inchar-'A') where inchar is aninput parameter to the function of type char]. Similarly, one can imagine other ways.

Answer 3

As far as I understand your problem, the cleanest and most "C-like" solution is to pass a pointer to your struct to f(), where you populate it:
typedef struct _NUMBER
{
int A;
int B;
}
NUMBER;
void f (NUMBER* pvNumber)
{
pvNumber->A = 1;
pvNumber->B = 2;
return;
}
Call it like this:
int main ()
{
NUMBER vNumber;
int z;
f (&vNumber);
z = vNumber.A; // or vNumber.B
return 0;
}
So you don't select the desired member inside f(), but outside of it.

Answer 4

Yes. you need to define some way to access to member desired, this is usually done with constants, or an enum. As an aside, you should always check the pointer returned by malloc() before using it.
#define GET_A (0)
#define GET_B (1)
int f(int selector){
struct NUMBER {
int A;
int B;
};
int result;
struct NUMBER *num = malloc(sizeof(struct NUMBER));
if (!num)
return -1; // or some other error code...
num -> A = 1;
num -> B = 2;
switch(selector)
{
case GET_A: result = num->A; break;
case GET_B: result = num->B; break;
// etc... if you have more members in your struct.
default: result = -1; break; // some error code.
}
free(num)
return result;
}
int main(){
int z = f(GET_B);
printf( "%d\n" , f(GET_B));
}