C memcpy中的分割错误

Question

I have a basic problem using memcpy and don't understand where the problem is. I show below the relevant parts of the code. The code seg. faults in the last right iteration of the loop. Why can't I index in to an memory area that is reserved?

Thank you in advance.

mystr->data = malloc(2048);

unsigned char buf[8500];
for (i=0;i<32;i++){
    offset = i*256;
    memcpy(&mystr->data[64*i],&buf[8+offset],64);
}

Answer 1

From the comments it'cs clear that my suspicion was right:

if sizeof( *mystr->data ) > 1 (because eg it's unsigned long long *data; ) then you run beyond the end of the buffer because the offsets calculated by expressions like &mystr->data[64*i] are relative to the type, here it is mystr->data + 64*i*sizeof(*mystr->data) bytes which was up to 64*31*8 in your code.

You could either change the type, as you have done, or change the offsets (to &mystr->data[8*i] in your case) depending on what what seems 'right' semantically in your context