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c++pngintegercharcasting

how to cast an array of char into a single integer number?

i'm trying to read contents of PNG file.

As you may know, all data is written in a 4-byte manner in png files, both text and numbers. so if we have number 35234 it is save in this way: [1000][1001][1010][0010].

but sometimes numbers are shorter, so the first bytes are zero, and when I read the array and cast it from char* to integer I get wrong number. for example [0000] [0000] [0001] [1011] sometimes numbers are misinterpreted as negative numbers and simetimes as zero!

let me give you an intuitive example:

char s_num[4] = {120, 80, 40, 1};

int  t_num = 0;

t_num = int(s_num);

I wish I could explain my problem well!

how can i cast such arrays into a single integer value?

ok ok ok, let me change my code to explain it better:

char s_num[4] = {0, 0, 0, 13};
int  t_num;


t_num = *((int*) s_num);
cout << "t_num: " << t_num << endl;

here we have to get 13 as the result, ok? but again with this new solution the answer is wrong, you can test on your computers! i get this number:218103808 which is definitely wrong!

Solution

  • You cast (char*) to (int). What you should do is cast to pointer to integer, i.e.

    t_num = *((int*) s_num));

    But really you should extract your code into it's own function and make sure that:

    1. endianness is correct
    2. sizeof(int) == 4
    3. Use C++ casts (i.e. static, dynamic, const, reinterpret)