How to modify a struct content through 2 layer of functions? - c

Question

Suppose I have the following code:
typedef otherStruct_t struct_t[X][Y];
void foo1(struct_t * mystruct); //declaration in main()
main()
{
struct_t mystruct = {0};
foo1(&mystruct);
...
}
void foo1(struct_t * mystruct)
{
foo2(&mystruct, number);
...
}
in other .c file.
void foo2(struct_t ** mystruct, int number)
{
(*mystruct[x][y])-> number = number;
}
I read some posts regarding pointer of pointer in struct and this what I came up with.
It compiles, but the caller struct is not modified, and sometime cause seg. fault as well.
Where is my mistake? Thank you!

Answer 1

There are couple of ways to do it. In the following code you can remove the comment around
#define NO_PTR_USE 1
to do it without using pointer(s). If using pointers, then you have to remember as #Oli Charlesworth pointed out, [] has higher precedence than *.
/* #define NO_PTR_USE 1 */
#define X 10
#define Y 10
#define ax 1
#define ay 5
struct foo {
int number;
};
typedef struct foo struct_t[X][Y];
void
#if NO_PTR_USE
foo1(struct_t mystruct),
foo2(struct_t mystruct, int x, int y, int n);
#else
foo1(struct_t *mystruct),
foo2(struct_t *mystruct, int x, int y, int n);
#endif
main()
{
struct_t mystruct = {0};
#if NO_PTR_USE
foo1(mystruct);
printf("No pointer was used.\n");
#else
foo1(&mystruct);
printf("Pointer was used.\n");
#endif
printf("mystruct[%d][%d].number = %d\n",
ax, ay, mystruct[ax][ay].number);
}
void
#if NO_PTR_USE
foo1(struct_t mystruct)
#else
foo1(struct_t *mystruct)
#endif
{
foo2(mystruct, ax, ay, 1234);
}
void
#if NO_PTR_USE
foo2(struct_t mystruct, int x, int y, int n)
#else
foo2(struct_t *mystruct, int x, int y, int n)
#endif
{
#if NO_PTR_USE
mystruct[x][y].number = n;
#else
(*mystruct)[x][y].number = n;
#endif
}

Answer 2

Normally, once you've got a pointer, you don't pass it by address. Think of FILE *fp = fopen(...);. You don't use fprintf(&fp, ...);, do you?
Same here:
typedef struct foo struct_t[X][Y]; // Nasty piece of work - don't do it!
extern void foo1(struct_t *t);
extern void foo2(struct_t *t);
int main(void)
{
struct_t data = {0};
...
foo1(&data);
...
return 0;
}
void foo1(struct_t *t)
{
...
foo2(t);
...
}
Other file:
void foo2(struct_t *t)
{
...
stuct foo x = (*t)[i][j];
...
}
Note that a struct_t is a 2D array of structures, so a struct_t * is a pointer to an array of structures. Therefore, you have to get the access notation correct, as shown in foo2() above. And that's one reason why you probably don't want to use that type. You'd be better off, almost certainly, using:
typedef struct bar
{
struct foo data[X][Y];
} bar;
And you can then pass bar *bp values around, and access the data using bp->data[i][j].