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cinthexbit-fieldsnegative-number

Bit Fields in C

I have the following code:

#include <stdio.h>
struct test 
{
   int x: 2;
   int y: 2;
};
int main()
{
   test t;
   t.x = -1;
   printf("%d", t.x);
   return 0;
}

This snippet prints -1 which I can understand,

If the same code, %dis replaced with %x format specifier like below:

#include <stdio.h>
struct test 
{
   int x: 2;
   int y: 2;
};
int main()
{
   test t;
   t.x = -1;
   printf("%x", t.x);
   return 0;
}

The output becomes ffffffff.

Please explain why this is the case.

Solution

  • %x prints the hex representation of the value of the given argument. The two's complement representation of -1, in hex, gives you that ffffffff.

    FWIW: This outcome is not particularly related to use of bit-filed variable here, as printf() is a variadic function.