CODEWITHSUNDEEP
cpointer-arithmetic
Hanan
Hanan

Reputation: 1179

Access Violation in pointer arithmetic

With the code:

int nsize;
int * buffer;
char TargetBuffer[4096];
const SIZE_T buffersize = (320*240) * sizeof(int);


buffer = (int *) malloc(bufferSize);
// fill buffer with data

nsize = 0;
while(nsize < buffersize)
{
    // HERE after some loops i get Access Violation
    memcpy(TargetBuffer, buffer + nsize, 4096);


    // do stuff with TargetBuffer
    nsize += 4096;
}

Why am I get the Access Violation? What should I change?

Upvotes: 3

Views: 165

Answers (3)

Mats Petersson
Mats Petersson

Reputation: 129504

I would do something like this:

SIZE_T left = buffersize;
nsize = 0;

while(left)
{
    SIZE_T block = (left >= 4096)?4096:left;
    // HERE after some loops i get Access Violation
    memcpy(TargetBuffer, buffer, block);
    buffer += (left) / sizeof(*buffer);

    // do stuff with TargetBuffer
    left -= block;
}

I'm pretty sure the problem you see is that you are going over the edge because your code isn't taking care of sizes that are not multiples of the 4K.

Upvotes: 3

Rohan
Rohan

Reputation: 53386

Update your while loop as 

while(nsize < (buffersize/sizeof(int))

buffer is for ints and not char. int takes multiple bytes, you are over counting and copying from memory which is not part of buffer.

Upvotes: 0

Pablo Santa Cruz
Pablo Santa Cruz

Reputation: 181430

When you add buffer + nsize you have to realize that you are actually adding buffer + (nsize * (sizeof(int)) since it's a int * when you are doing pointer arithmetic.

So it probably has something to do with it. Try incrementing nsize by nsize += 4096/sizeof(int) or something more clever.

Upvotes: 9

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