How to copy the content of a structure​ in another structure in C

Question

I have a C program where I have two structures

struct one{
  int a;
  int b;
  char *s;
  int c;
};

struct two{
  int a;
  int e;
  char *s;
  int d;
};

Is possible to write a function that copies the values of the variables with the same type and name from struct one to struct two ? For example, in this case the function should do this

two.a = one.a;
two.s = one.s;

Answer 1

There's no way to automatically grab fields of a given name from a struct. While you could do something like this in Java with reflection, it can't be done in C. You just need to manually copy the relevant members.

Answer 2

You may write function macro like this:

#define CAT(A, B) A ## B
#define ARGS_ASSIGN(N, L, R, ...) CAT(_ARGS_ASSIGN, N) (L, R, __VA_ARGS__)
#define _ARGS_ASSIGN1(L, R, M0) L.M0 = R.M0;
#define _ARGS_ASSIGN2(L, R, M0, M1) L.M0 = R.M0; L.M1 = R.M1;
/* ... define sufficiently more */

and use in such way:

ARGS_ASSIGN(2, two, one, a, s)

Answer 3

In theory you could do this with a simple block copy function for your example above using the code below if you are certain that your compiler sequences the structure as sequenced in its type definition. However, I don't think it's a great idea. Block copy would be safer with two data structures of the same type as defined in one of the answers proposed above.

Example using block copy function:

void main(void)
{

    struct one{
        int a;
        int b;
        char *s;
        int c;
    };

    struct two{
        int a;
        int e;
        char *s;
        int d;
    };

    // Place code that assigns variable one here

    memcpy(&two, &one, sizeof(one));
}