comparison between signed and unsigned char - c

Question

I'm pretty much assuming this is a stupid question... but I can't really find the answer for it. So I'm asking this here.
For the purpose of learning about implicit type casting, I'm running the following code on C.
#include <stdio.h>
int main()
{
unsigned char i;
char cnt = -1;
int a[255];
for (int k = 0; k < 255; k++)
{
a[k] = k;
}
for (i = cnt - 2; i < cnt; i--)
{
a[i] += a[i + 1];
printf("%d\n", a[i]);
}
return 0;
}
When I ran this program, nothing happened.
I was able to found out that the loop condition of for-loop was false at the first iteration, so the program exited the for-loop right away.
However, I don't get the reason why.
As far as I know, C does implicit casting when assigning or comparing variables with different types. So I thought that on i = cnt - 2, the minus operation makes the value -3, and then implicit casting assigns i with a value 253.
Then, shouldn't the condition i < cnt be true since (by another implicit casting of cnt because of comparison of signed and unsigned char) 253 is smaller than 255?
If it isn't, why is this false? Is there something that I missed or is there some exception that I don't know?

Answer 1

Your question is not stupid at all. You were close to the solution: i is assigned the value -3 but the implicit conversion to the type of i, unsigned char, changes the value to 253.
For a more precise explanation, there are multiple issues in your test code:
char may be signed or unsigned depending on the platform and compiler configuration, so char cnt = -1; may store the value -1 or 255 into cnt, or even some other value if char is unsigned and has more than 8 bits.
The behavior of for (i = cnt - 2; i < cnt; i--) also depends on whether char is signed or unsigned by default:
in all cases, the test i < cnt is evaluated with both operands converted to int (or unsigned int in the rare case where sizeof(int)==1). If int can represent all values of types char and unsigned char, this conversion does not change the values.
if char is unsigned and has 8 bits, cnt has the value 255 so i is initialized with the value 253 and the loop runs 254 times with i from 253 down to 0, then i-- stores the value 255 again into i, for which the test i < cnt evaluates to false. The loop prints 507, then 759, ... 32385.
if char is signed and has 8 bits, as is probably the case on your system, cnt has the value -1 and i is initialized with the value -3 converted to unsigned char, which is 253. The initial test i < cnt evaluates as 253 < -1, which is false, causing the loop body to be skipped immediately.
You can force char to be unsigned by default by giving the compiler the appropriate flag (eg: gcc -funsigned-char) and test how the behavior changes. Using Godbolt's compiler explorer, you can see that gcc generates just 2 instructions to return 0 in the signed (default) case and the expected output in the unsigned case.

Answer 2

For starters let's assume that the type char behaves as the type
signed char.
In this condition
i < cnt
the both operands are implicitly converted to the type int due to the integer promotions.
From the C Standard (6.5.8 Relational operators)
3 If both of the operands have arithmetic type, the usual arithmetic
conversions are performed.
and (6.3.1.8 Usual arithmetic conversions)
1 Many operators that expect operands of arithmetic type cause
conversions and yield result types in a similar way. The purpose is to
determine a common real type for the operands and result. For the
specified operands, each operand is converted, without change of type
domain, to a type whose corresponding real type is the common real
type. Unless explicitly stated otherwise, the common real type is also
the corresponding real type of the result, whose type domain is the
type domain of the operands if they are the same, and complex
otherwise. This pattern is called the usual arithmetic conversions:
...
Otherwise, the integer promotions are performed on both operands. Then
the following rules are applied to the promoted operands:
If both operands have the same type, then no further conversion is
needed
and (6.3.1.1 Boolean, characters, and integers)
...If an int can represent all values of the original type (as restricted
by the width, for a bit-field), the value is converted to an int;
otherwise, it is converted to an unsigned int. These are called the
integer promotions.
Thus the positive value of i represented like 0000 0000 1111 1101 after the integer promotions will be greater than the negative value 1111 1111 1111 1111.
So the condition of the for loop at once evaluates to logical false because the positive value 253 of the type int is greater than the negative value -1 of the type int.
Here is a demonstrative program.
#include <stdio.h>
int main(void)
{
char cnt = -1;
unsigned char i = cnt - 2;
printf( "cnt = %x\n", ( unsigned int )cnt );
printf( "i = %x\n", ( unsigned int )i );
printf ( "i < cnt is %s\n", i < cnt ? "true" : "false" );
return 0;
}
The program output is
cnt = ffffffff
i = fd
i < cnt is false