Assigning and accessing pointer to string within struct

Question

I'm trying to store a string in an array contained within a struct, and access it, but I'm having a hard time. The struct looks like this:

typedef struct {
    void **storage;
    int numStorage;
} Box;

Box is initialized as such:

    b->numStorage = 1000000; // Or set more intelligently
    Box *b = malloc(sizeof(Box));
    // Create an array of pointers
    b->storage = calloc(b->numStorage,sizeof(void *));

In order to set the string, I use this function:

void SetString(Box *b, int offset, const char * key)
{
    // This may seem redundant but is necessary
    // I know I could do strcpy, but made the following alternate
    // this isn't the issue
    char * keyValue = malloc(strlen(key) + 1);
    memcpy(keyValue, key, strlen(key) + 1);

    // Assign keyValue to the offset pointer
    b->storage[offset*sizeof(void *)] = &keyValue;

    // Check if it works
    char ** ptr = b->storage[offset*sizeof(void *)];

    // It does
    printf("Hashcode %d, data contained %s\n", offset, *ptr);

}

The problem lies when I try to retrieve it again, with the exact same offset:

// Return pointer to string
void *GetString(const Box *b, int offset, const char *key)

    char ** ptr = b->storage[offset*sizeof(void *)];
    if (ptr != NULL) {
        printf("Data should be %s\n", *ptr);
        return *ptr;
    } else {
     return NULL;
    }

The returned pointer is gibberish. What could be amiss?

user142162 · Accepted Answer · 2011-04-24 17:25:02Z

2

You don't have to specify the actual memory offset when accessing arrays. Simply give it the index and you will get the correct element.

So, in your third code block:

b->storage[offset] = keyValue;

And in your fourth:

char *ptr = b->storage[offset];
if (ptr != NULL) {
    printf("Data should be %s\n", ptr);
    return ptr;
} else {
 return NULL;
}

Also, in the second block of code, has b->numStorage already been set?

answered Apr 24, 2011 at 17:25

user142162

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Comments

Alok Singhal · Accepted Answer · 2011-04-24 17:25:47Z

2

b->storage[offset*sizeof(void *)] = &keyValue;

This stores the address of the local variable keyValue in the array. Once the function completes, this address becomes invalid. I think you want:

b->storage[offset*sizeof(void *)] = keyValue;

and then make the corresponding change while retrieving.

answered Apr 24, 2011 at 17:25

Alok Singhal

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Comments

Catamorphisms · Accepted Answer · 2011-04-24 17:33:37Z

2

Doesn't this:

b->storage[offset*sizeof(void *)] = &keyValue

set storage[offset*sizeof(void*)] to point to the address of the local variable keyValue? i.e. no longer valid after function returns

edited Apr 24, 2011 at 17:33

answered Apr 24, 2011 at 17:27

Catamorphisms

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