CODEWITHSUNDEEP
c++pointers
Ayberk Eser
Ayberk Eser

Reputation: 13

What does this code do ? (About pointers)

I have this code and im stuck at second line 

unsigned long check_password(const char* p){
    int* ip = (int*)p;
    int i;
    int res=0;
    for(i=0; i<5; i++){
        res += ip[i];
    }
    return res;
}

int* ip = (int*)p; What does this line mean ?

Upvotes: 1

Views: 210

Answers (2)

haslersn
haslersn

Reputation: 585

unsigned long check_password(const char* p){
    int* ip = (int*)p; // line 2, non-const not required
    int i;
    int res=0;
    for(i=0; i<5; i++){
        res += ip[i]; // line 6
    }
    return res;
}

Line 2 declares ip as a pointer to integer and initializes it to (int*)p. (int*)p is a C-style cast, which is in this case resolved to reinterpret_cast<int*>(const_cast<char*>(p)). See

When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?

ip now points to the memory location of p. When using ip, the memory at that location is interpreted as holding an integer, even though it holds a char; and it can be modified, even though it was initially declared const.

Line 6 treats ip as if it was pointing to the first element of an array of int and adds the ith element of that array to res. It's likely that ip actually points to the first element of an array of char. That means, that each iteration sizeof(int) consecutive elements of that array are interpreted as the representation of a single int.

The intention of this code is probably, that a sequence of 5 ints is passed as an array of char of length sizeof(int)*5.

Note that this code is UB if p doesn't actually point to memory of a size of at least sizeof(int)*5, since the memory in that region is read. I.e. the char-array must be at least of length sizeof(int)*5.

Casting away the const in your code is not required

unsigned long check_password(const char* p){
    const int* ip = reinterpret_cast<const int*>(p);
    int res = 0;
    for (int i = 0; i < 5; ++i){
        res += ip[i];
    }
    return res;
}

Upvotes: 1

Richard Hodges
Richard Hodges

Reputation: 69864

int* ip = (int*)p; What does this line mean ?

It means, "create a pointer to integers which starts at the same address as p."

The function is expecting p to be pointing to a sequence of bytes that represents 5 integers.

The behaviour of this function is only defined if p really was pointing to a sequence of integers before being cast to a const char*

In practice, the author of this code is assuming:

  1. That the sequence of bytes has been encoded in such a way that they truly represent 5 integers on this machine (taking into account word size and endian-ness of the architecture).

  2. that the alignment of the address represented by p is correct for addressing memory as integers (some processors have restrictions in this regard).

It's probably not code you want to copy or learn from, other than as a cautionary tale.

Upvotes: 2

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