Comparision between signed and unsigned int Linux C

barunparichha · 08-15-2014, 11:41 AM

Code:

#include<stdio.h>
int main(){
  signed int a=-1;
  unsigned int b=-1;
  if(a==b)
    printf("%d %d",a,b);
  else
    printf("Not equal");
  return 0;
}

In my linux box, it gives me "-1 -1", but was expecting "Not equal".

metaschima · 08-15-2014, 12:07 PM

Try:

Code:

#include <stdio.h>

int main(void)
{
  signed int a=-1;
  unsigned int b=-1;

  printf("%x %x\n", a, b);

  if(a==b)
  {
    printf("%d %d\n",a,b);
    printf("%u %u\n",a,b);
  }
  else
  {
    printf("Not equal\n");
  }

  return 0;
}

Code:

ffffffff ffffffff
-1 -1
4294967295 4294967295

As you can see '-1' is represented as 0xffffffff in both cases. Also, printf will print a variable however you want.

barunparichha · 08-15-2014, 12:36 PM

How are we able to store negative int in "unsigned int" ?
Looks like it's stored in the same way as signed int.

rtmistler · 08-15-2014, 01:04 PM

Quote:

Originally Posted by barunparichha

How are we able to store negative int in "unsigned int" ?
Looks like it's stored in the same way as signed int.

Never make assumptions, that's why Metaschima wrote additional code to make a point. You should also determine what the size of an integer is:

Code:

printf("Size of an integter is: %d\n", sizeof(int));

You should also be cognizant that just because something compiles doesn't mean it's correct. The default compiler settings are near worthless:

Code:

~/testcode$ 
~/testcode$ gcc -Wextra -o signed signed.c 
signed.c: In function ‘main’:
signed.c:10:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
~/testcode$ 
~/testcode$ gcc -Wsign-compare -o signed signed.c 
signed.c: In function ‘main’:
signed.c:10:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
~/testcode$

Here are some suggestions which I use as part of a Makefile, in fact from this very forum I added one or two to my list of checks. The whole point here is that the art of C programming is not just writing the programs but also understanding how the compiler treats what you have written. That actually was a huge point made by a course instructor years and years ago. The class was being taught commercially to our company and it actually wasn't about C programming, but instead network programming in C, but we Engineers were so self entitled that we started code style fights with the instructor and for him fortunately he was a C master and he pretty much shut us down at every juncture by being 1000% correct. I've been doing C for 25 years and I'm not always correct. Good points to remember are that one about understanding how the compiler interprets what you wrote, to test your hypothesis, and code accordingly so that you're not making any assumptions.

Code:

CFLAGS= -I. -O0 -ggdb -Wformat=2 -Werror -Wall -Wextra -Wswitch-default -Wswitch-enum -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations 
-Wmissing-noreturn

metaschima · 08-15-2014, 01:43 PM

Heh, none of those warning options produces an error in this case, at least not for my version of gcc.

It's best to do logical things in your programs. Assigning a negative number to an unsigned int is just not logical.

In the usual compiler representation a 4 byte unsigned int goes from 0x0 = 0 to 0xffffffff = 4294967295. A signed int is usually represented as going from 0x0 = 0 to 0x7fffffff = 2147483647, and the negative values 0xffffffff = -1 to 0x80000000 = -2147483648. This representation is not mandatory, so don't bet on it.

rtmistler · 08-15-2014, 01:54 PM

Quote:

Originally Posted by metaschima

Heh, none of those warning options produces an error in this case, at least not for my version of gcc.

Not even with -Werror? That's supposed to force a warning to be interpreted as an error and cause compilation to exit.

rtmistler · 08-15-2014, 01:57 PM

Quote:

Originally Posted by metaschima

It's best to do logical things in your programs. Assigning a negative number to an unsigned int is just not logical.

Yeah, I kinda get frustrated when someone decrements an unsigned number and it wraps to be 0xffffffff and they get offended that it grew to a very large value or the way they wrote code it got interpreted as negative and they're all angry about it. Really it's just their personal interpretation which doesn't match the compiler and they're offended that, that nasty compiler dared to do something unexpected.

metaschima · 08-15-2014, 04:05 PM

Quote:

Originally Posted by rtmistler

Not even with -Werror? That's supposed to force a warning to be interpreted as an error and cause compilation to exit.

No warnings and no errors.

keefaz · 08-15-2014, 04:19 PM

Quote:

Originally Posted by metaschima

No warnings and no errors.

Even with -Wextra ?

Code:

$ gcc -o int -Wextra int.c
int.c: In function 'main':
int.c:10:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(a==b)
       ^
$ gcc -v
...
gcc version 4.8.2 (GCC)

metaschima · 08-15-2014, 05:04 PM

oops, ok it does with -Wextra.