Question

I wrote a small code of C.

#include<stdio.h>
int main()
{
    int a  = 0;
    printf("Hello  World %llu is here %d\n",a, 1);
    return 0;
}

It is printing the following ouput

Hello World 4294967296 is here -1216225312

With the following warning on compilation

prog.cpp: In function ‘int main()’:

prog.cpp:5: warning: format ‘%llu’ expects type ‘long long unsigned int’, but argument 2 has type ‘int’

I know i need to cast the int to long long unsigned int, but i could not understand the fact that why the later values got corrupted.

Thanks in advance

Answer 1

%llu expects a 64-bit integer. But you only gave it a 32-bit integer.

The effect is that what printf is reading is “shifted” over by 32-bits with respect to what you’ve passed. Hence your “1” is not being read in the right location.

EDIT:

Now to explain the output:

a and 1 are being stored 32-bits apart because they are both 32-bit integers.
However, printf expects the first argument to be a 64-bit integer. Hence it reads it as a + 2^32 * 1 which is 4294967296 in your case. The second value that is printed is undefined because it is past the 1.