Signed/Unsigned int, short and char

Question

Signed/Unsigned int, short and char

I am trying to understand the output of the code given at : http://phrack.org/issues/60/10.html

Quoting it here for reference:

#include <stdio.h>

int main(void){
        int l;
        short s;
        char c;

        l = 0xdeadbeef;
        s = l;
        c = l;

        printf("l = 0x%x (%d bits)\n", l, sizeof(l) * 8);
        printf("s = 0x%x (%d bits)\n", s, sizeof(s) * 8);
        printf("c = 0x%x (%d bits)\n", c, sizeof(c) * 8);

        return 0;
}

The output i get on my machine is:-

l = 0xdeadbeef (32 bits)
s = 0xffffbeef (16 bits)
c = 0xffffffef (8 bits)

Here is my understanding:-

The assignments s=l, c=l will result in s and c being promoted to ints and they will have the last 16 bits (0xbeef) and last 8 bits (0xef) of l respectively.

Printf tries to interpret each of the above values (l,s and c) as unsigned integers (as %x is passed as the format specifier). From the output i see that sign extension has taken place. My doubt is that since %x represents unsigned int, why has the sign extension taken place while printing s and c? Should not the output for s be 0x0000beef and for c be 0x000000ef?

c

printf

integer-promotion

asked on Stack Overflow Jan 19, 2015 by

user720694

2 Answers

why has the sign extension taken place while printing s and c

Let's see the following code:

unsigned char ucr8bit; /* Range is 0 to 255 on my machine */
signed char cr8bit; /* Range is -128 to 127 on my machine */
int i32bit;
cr8bit = MINUS_100;  /* (char)(-100) or 0x9C */
i32bit = cr8bit;     /* i32 bit is -100 or 0xFFFFFF9C */

As you can see, althout the number -100 is same, its representation is not mere prepending 0s in wider character but may be prepending the MSB or sign bit of the signed type in 2s complement system and 1s complement system.

In your example you are trying to print s and c as wider type and hence getting the sign bit replication.

Also your code contains many sources of undefined and unspecified behavior and thus may give different output on different compilers. (For instance, you should use signed char instead of char as char may behave as unsigned char on some implementation and as signed char on some other implmentations)

l = 0xdeadbeef; /* Initializing l from an unsigned
                   if sizeof l is 32 bit UB as l is signed */
s = l;  /* Initializing with an undefined value. Moreover
           implicit conversion wider to narrower type */
printf("l = 0x%x (%d bits)\n", l, sizeof(l) * 8);  /* Using %x
               to print signed number and %d to print size_t */

answered on Stack Overflow Jan 19, 2015 by

Mohit Jain • edited Jan 19, 2015 by

Mohit Jain

You are using a 32-bit signed integer. That means that only 31 bits can be used for positive numbers. 0xdeadbeef uses 32 bits. Therefore, assigning it to a 32-bit signed integer makes it a negative number.

When shown with an unsigned conversion specifier, %x, it looks like the negative number that it is (with the sign extension).

When copying it into a short or char, the property of it being a negative number is retained.

To further show this, try setting:

l = 0xef;

The output is now:

l = 0xef (32 bits)
s = 0xef (16 bits)
c = 0xffffffef (8 bits)

0xef uses 8 bits which is positive when placed into a 32-bit or 16-bit variable. When you place an 8-bit number into a signed 8-bit variable (char), you are creating a negative number.

To see the retention of the negative number, try the reverse:

c = 0xef;
s = c;
l = c;

The output is:

l = 0xffffffef (32 bits)
s = 0xffffffef (16 bits)
c = 0xffffffef (8 bits)

answered on Stack Overflow Feb 27, 2015 by

Jumbo

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