CODEWITHSUNDEEP
cpointersstructpointer-arithmetic
Henrik Hillestad Løvold
Henrik Hillestad Løvold

Reputation: 1281

C pointer address changes without assignment

I am working on a Uni assignment here, and I've run into a problem. I am attempting to store a string input at a point inside a struct using a for-loop. Later on I intend to use the pointer to the place where the data was stored to fetch the string. Now the problem is, as I move on inside my for-loop, the address of the point changes as well. This code:

printf("B: %p\n", txt->point);
for(i = 0; i < input_sz; i++)
{
    txt->point[i] = input[i];
}
printf("A: %p\n", txt->point);

gives the output:

B: 0x7fc111803200
A: 0x7fc111803265

where B is before-value and A is after-copying value.

Any help debugging this would be very appreciated!

EDIT: Here's some more code:

The struct:

struct text_storage {
    char* start;
    char* point;
    char* end;
} typedef text_t;

Initialization function:

text_t* text_init(void *memory, size_t size)
{
    text_t* to_return;
    if(size < sizeof(text_t))
    {
        return NULL;
    }
    to_return = (text_t*) memory;
    to_return->start = to_return;

    to_return->end = to_return->start + size;
    to_return->point = to_return->start;

    printf("Start: %p, point: %p, end: %p, end-start: %d\n", to_return->start, to_return->point, to_return->end, (to_return->end - to_return->start));


    return to_return;
}

The text-store method in which the error occurs:

int text_store_entry(text_t *txt, const char *input, size_t input_sz)
{
    int to_return;
    char* begin = txt->point;
    int i;

    if(input_sz > (txt->end - txt->point))
    {
        return -1;
    }

    printf("Start: %p, point: %p, end: %p, end-start: %d\n", txt->start, txt->point, txt->end, (txt->end - txt->start));


    printf("B: %p\n", txt->point);
    for(i = 0; i < input_sz; i++)
    {
        txt->point[i] = input[i];
    }
    printf("A: %p\n", txt->point);

}

Main-function (testing purposes only):

int main(int argc, char* argv[])
{
    void* memory = malloc(10000);
    char* a = "hei pa deg din trekkbasun";
    text_t* txt;
    int memoverwritten;

    txt = text_init(memory, 10000);

    memoverwritten = text_store_entry(txt, a, (size_t)26);


    printf("got through\n");
    return 0;
}

Upvotes: 2

Views: 2219

Answers (2)

nickie
nickie

Reputation: 5808

The problem most probably is due to the initialization of structures of type struct text_storage. Such structures contain three pointers to text. Each pointer should be initialized, possibly with a malloc. Your text_init function does not do that properly. In fact, the place where the start pointer is stored overlaps with the first bytes of the memory that you want to use.

I'm guessing that you need a structure like this:

typedef struct text_storage {
    char* start;
    char* point;
    char* end;
    char* data;
} text_t;

initialized with a function like this:

text_t text_init(void *memory, size_t size)
{
  text_t to_return;
  to_return.data = (char *) memory;
  to_return.start = to_return.data;
  to_return.end = to_return.start + size;
  to_return.point = to_return.start;
  return to_return;
}

Upvotes: 3

Charles J. Daniels
Charles J. Daniels

Reputation: 542

Print txt->point in the loop and see the point at which it changes. I'm guessing it changes when assigning to txt->point[0]. I'm not fully familiar with printf, so I'm not sure what it's printing out for you, but the name of an array references the first location. If printf is printing out a pointer, txt->point[i] is always a char pointer, and printf may be dereferencing txt->point, which will get it the first entry, and then showing the address there, which you do assign when you change the point to input[i].

Upvotes: 0

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