EXPR_UNRELATED_PTRCMP
Expression compares unrelated pointers
Problem
Diagnosis (at runtime)
Repair

EXPR_UNRELATED_PTRCMP

Expression compares unrelated pointers

This error is generated whenever an expression tries to compare pointers that do not point into the same memory block. This only applies to the operators >, >=, <, and <=. The operators == and != are exempt from this case.

The ANSI C-language specification declares this construct undefined except in the special case where a pointer points to an object one past the end of a block.

Problem

The following code illustrates the problem by comparing pointers to two data objects.

	1:	/*
	2:	 * File: expucmp.c
	3:	 */
	4:	#include <stdlib.h>
	5:	
	6:	main()
	7:	{
	8:		char a[10], *b;
	9:
	10:		b = (char *)malloc(10);
	11:
	12:		if(a > b) a[0] = 'x'; 
	13:		else a[0] = 'y';
	14:		return (0);
	15:	}

Note that the error in this code is not that the two objects a and b are of different data types (array vs. dynamic memory block), but that the comparison in line 12 attempts to compare pointers which do not point into the same memory block. According to the ANSI specification, this is an undefined operation.

Diagnosis (at runtime)

1.	[expucmp.c:12] **EXPR_UNRELATED_PTRCMP**
	>> 		if(a > b) a[0] = 'x';

2.		Expression compares unrelated pointers: a > b

		Left hand side	: 0xf7fffb8c
3.		In block	: 0xf7fffb8c thru 0xf7fffb95 (10 bytes)
				a, declared at expucmp.c, 8

		Right hand side	: 0x00013870
		In block	: 0x00013870 thru 0x00013879 (10 bytes)
				block allocated at:
				malloc() (interface)
				  main() expucmp.c, 10

4.		Stack trace where the error occurred:
				  main() expucmp.c, 12

1. Source line at which the problem was detected.
2. Description of the problem and the expression that is in error.
3. Description of the two pointers involved in the comparison. For each pointer, the associated block of memory is shown together with its size and the line number at which it was declared or allocated.
4. Stack trace showing the function call sequence leading to the error.

Repair

While this construct is technically undefined according to the ANSI C specification, it is supported on many machines and its use is fairly common practice. If your application genuinely needs to use this construct, you can suppress error messages by adding the option

	insure++.suppress EXPR_UNRELATED_PTRCMP

to the .psrc file.

EXPR_UNINIT_PTR

EXPR_UNRELATED_PTRDIFF