Question

I am having a tough time understanding the following piece of code:

int stride = 512;
int max_len = 1024 * stride; 
char *addr = (char *)malloc(max_len);
for (int i=stride; i<max_len; i += stride)
    *(char **)&addr[i-stride] = (char*)&addr[i];
*(char **)&addr[i-stride] = (char*)&addr[0];

Looking at the code it seems this is trying to create some kind of circular link list. But I have no clue what those casts are actually doing.

Answer 1

IMHO the code you present is poor taste.

*(char **)&addr[i-stride] = (char*)&addr[0]; is taking the pointer addr to some dynamically allocated zone (right part (char*)&addr[0] could have been written addr which is simpler), then copying that address to the memory located at position addr[i-stride].

This actually could be wrong (undefined behavior), e.g. when &addr[i-stride] (which could be written addr+i-stride) is not word aligned on some processors. In your case i happens to be max_len and stride happens to be 512, so that address is word aligned (since malloc gives a well aligned zone).

The original author of the code should perhaps have declared a struct (or maybe a union) like e.g.

 struct buffer_st {
    struct buffer_st* next;
    char content[];
 };