Question

I’m trying to access a returned 2d array of structs i create. I can access them with in the func they were created in but seem to lose the reference as i pass it out.
I have tired a thousand ways to do it, but it doesn’t seem to make a difference.
Always with the seg faults.

my current run at it looks like:

a simple struct:

typedef struct{
    int weight;
} myStruct_t;

a function that creates a 2-d array for myStruct_t’s.
My current attempt to get it out is to make a new variable to act as a pointer to “map”, as seen in the last 2 lines.

myStruct_t*** makeGrid(int sizeX, int sizeY)
{  
    int i;

    myStruct_t **map = malloc(sizeX * sizeof(*map));  

    if(map == NULL){
        printf("Unable to set memory, Exiting\n");
        exit(1);
    }else{  
        for ( i = 0; i < sizeX; i++)
        {
            map[i] = malloc(sizeY*sizeof(myStruct_t));

            if(map[i] == NULL)
            {
                printf("Unable to set memory, Exiting\n");
                exit(1);
            }else{
                printf("success!\n");
            }
        }
    }

    myStruct_t ***mapPtr = &map;
    return mapPtr;  
}  

then in another file:

void initSys(){
    myStruct_t** map = *myStruct_t(100,100);    
    map[0][0].weight = 7;

   //or i also just tried
   //myStruct_t*** map = myStruct_t(100,100);
   //(*map)[0][0].weight = 7;

}

output is a SegFault.
its amazing how quickly i have totally forgoten C memory management and i cant seem to get it back!

any suggestions would be amazing.

Answer 1

The bug is on the line

myStruct_t ***mapPtr = ↦

It should be:

 return map;

And you’ll need to change your return type to a double pointer.

When you return “&map” you are returning the address of the variable map. As the variable map is stored on the stack, you are returning an address to invalid memory.