Re: sizeof() function result tests < -1

["Kaveh R. Ghazi" <ghazi at caip dot rutgers dot edu>]

 > From: Richard Sharman <richard_sharman@Mitel.COM>
 > 
 > On a sparc and a 386,  although sizeof(int) returns 4 when used in an
 > assignment, the test  sizeof(int) < -1   returns true.
 > 
 > 
 > % gcc -v
 > Reading specs from /usr/local/lib/gcc-lib/sparc-sun-solaris2.6/2.8.1/specs
 > gcc version 2.8.1
 > % uname -a
 > SunOS topaz 5.6 Generic_ sun4u unknown
 > % 
 > 
 > % cat -n x.c
 >      1	#include <stdio.h>
 >      2	
 >      3	int main(void) {
 >      4	  int	x;
 >      5	
 >      6	  /* This works as expected */
 >      7	  x = sizeof(int);
 >      8	  printf("%d\n", x);
 >      9	  if (x < -1) {
 >     10	    printf("x < -1\n");
 >     11	  } else {
 >     12	    printf("ok(1)\n");
 >     13	  }
 >     14	
 >     15	
 >     16	  /* This fails */
 >     17	  printf("%d\n", sizeof(int));
 >     18	  if (sizeof(int) < -1) {
 >     19	    printf("sizeof(int) < -1\n");
 >     20	  } else {
 >     21	    printf("ok(2)\n");
 >     22	  }
 >     23	
 >     24	
 >     25	  /* This, testing against 0, works as expected. */
 >     26	  printf("%d\n", sizeof(int));
 >     27	  if (sizeof(int) < 0) {
 >     28	    printf("sizeof(int) < 0\n");
 >     29	  } else {
 >     30	    printf("ok(3)\n");
 >     31	  }
 >     32	
 >     33	  exit(0);
 >     34	}
 > % gcc -Wall -o x x.c && ./x
 > 4
 > ok(1)
 > 4
 > sizeof(int) < -1
 > 4
 > ok(3)
 > % 

	Try compiling with -Wsign-compare, it'll hint at the problem.

What's going on is that the sizeof() directive is unsigned on your
system.  Your comparison is essentially (4U < -1).  The promotion
rules will convert -1 to unsigned which then becomes a very large
positive number, and greater than 4.

		--Kaveh
--
Kaveh R. Ghazi			Engagement Manager / Project Services
ghazi@caip.rutgers.edu		Qwest Internet Solutions