Accessing pointers within a struct

Question

At the moment I have the following code:

typedef struct _hexagon {
    int *vertice[6];
    int *path[6];
    int resourceType;
} hexagon;


typedef struct _game {
    hexagon hexagons[5][5];
} Game;

and in the main I have:

Game g;
// This is the line that fails
g.hexagons[0][0].vertice[0] = 0;

This compiles fine but gives a segmentation fault. I have tried many variations, such as

g.hexagons[0][0].*vertice[0] = 0;

which doesn't compile. How do I access a pointer's memory from within a struct?

Why is vertices an array of int* instead of an array of int? — Doug Richardson
– Doug Richardson, Commented Apr 25, 2012 at 6:58
Also, the code you said seg faults for you doesn't for me on Mac OS X. What system are you running on? — Doug Richardson
– Doug Richardson, Commented Apr 25, 2012 at 6:59
How do you say g.hexagons[0][0].vertice[0] = 0; line fails? Are you checking the stacktrack on gdb? You should be using a gdb in this case - correct?! — Sangeeth Saravanaraj
– Sangeeth Saravanaraj, Commented Apr 25, 2012 at 7:24

Sangeeth Saravanaraj · Accepted Answer · 2012-04-25 07:15:40Z

5

As vertice is a array-of-pointes-to-integers, to access vertice[0], you need to do *g.hexagons[0][0].vertice[0]

Sample program:

#include <stdio.h>

typedef struct _hexagon {
    int *vertice[6];
    int *path[6];
    int resourceType;
} hexagon;


typedef struct _game {
    hexagon hexagons[5][5];
} Game;

int main()
{
    int i1 = 1;
    int i2 = 2;
    int i3 = 3;
    int i4 = 4;
    int i5 = 5;
    int i6 = 6;

    Game g;
    g.hexagons[0][0].vertice[0] = &i1;
    g.hexagons[0][0].vertice[1] = &i2;
    g.hexagons[0][0].vertice[2] = &i3;
    g.hexagons[0][0].vertice[3] = &i4;
    g.hexagons[0][0].vertice[4] = &i5;
    g.hexagons[0][0].vertice[5] = &i6;

    printf("%d \n", *g.hexagons[0][0].vertice[0]);
    printf("%d \n", *g.hexagons[0][0].vertice[1]);
    printf("%d \n", *g.hexagons[0][0].vertice[2]);
    printf("%d \n", *g.hexagons[0][0].vertice[3]);
    printf("%d \n", *g.hexagons[0][0].vertice[4]);
    printf("%d \n", *g.hexagons[0][0].vertice[5]);

    return 0;   
}

Output:

$ gcc -Wall -ggdb test.c 
$ ./a.out 
1 
2 
3 
4 
5 
6 
$

Hope it helps!

UPDATE: as pointed out by Luchian Grigore

The reason for the segmentation fault is explained by the following small program. In short, you are de-referencing a NULL pointer.

#include <stdio.h>

/*
int *ip[3];
+----+----+----+
|    |    |    |
+----+----+----+
   |    |    |
   |    |    +----- points to an int *
   |    +---------- points to an int *
   +--------------- points to an int *

ip[0] = 0;
ip[1] = 0;
ip[2] = 0;

+----+----+----+
|    |    |    |
+----+----+----+
   |    |    |
   |    |    +----- NULL
   |    +---------- NULL
   +--------------- NULL

*ip[0] -> dereferencing a NULL pointer ---> segmantation fault
*/

int main()
{
    int * ip[3];
    ip[0] = 0;
    ip[1] = 0;
    ip[2] = 0;

    if (ip[0] == NULL) {
        printf("ip[0] is NULL \n");
    }

    printf("%d \n", *ip[0]);
    return 0;
}

Now you can co-relate int *ip[] with your g.hexagons[0][0].vertice[0]

edited Apr 25, 2012 at 7:15

answered Apr 25, 2012 at 7:02

Sangeeth Saravanaraj

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4 Comments

Luchian Grigore Over a year ago

Why does g.hexagons[0][0].vertice[0] = 0; fail?

efrey Over a year ago

also you have missed some parantheses. You write *g.hexagons[0][0].vertice[0] where you mean (*g.hexagons[0][0].vertice)[0] i believe.

Luchian Grigore Over a year ago

He doesn't have that, his code is crashing at g.hexagons[0][0].vertice[0] = 0;, effectively ip[0] = 0; in your example.

Lobe Over a year ago

thanks for the help, the explanation you added was particularly insightful.

Tony The Lion · Accepted Answer · 2012-04-25 06:59:52Z

0

I think you may have misunderstood what you have declared in _hexagon. *vertice[6] and your other array members are all arrays of pointers, so you have to treat each element like a pointer.

int x = 10;
g.hexagons[0][0].vertice[0] = &x;

Store the address of x into pointer at position 0 of your array of pointers.

answered Apr 25, 2012 at 6:59

Tony The Lion

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