For loop compiler bug

[jleblanc]

I'm having a problem with a particular for loop in one of my programs and I think it is a problem with the compiler. The for loop is supposed to count backwards from 4 to 0; however what ends up happening is the program gets stuck in an infinite loop with the integer variable counting down, overflowing, then counting down again, etc.

I've included source code that demonstrates this problem. I have not tried it on another PIC. I have also included the assembly generated when I compile this program. If someone could try this code and confirm or deny my results that would be much appreciated.

[newguy]

Hate to rain on your parade, but your for loop is doing exactly what you're asking it to.

You declare x as an unsigned integer. In the loop you set x = 4, and decrement x on every pass through the loop. Your test for the continuation of the loop is to test if x is greater than or equal to 0.

Let's say that on a particular pass through the loop, x = 0. Since 0 is equal to 0, the loop executes. Once it executes, x is decremented and becomes 255. 255 is indeed greater than 0, so the loop continues.

Change your definition of x to:

[jleblanc]

Addendum:

I noticed what was wrong. I needed to add the "signed" keyword infront of x's declaration. However what I find puzzling is how without this keyword the application would count negative numbers when it ran (which if "int" is unsigned by default it shouldn't do).

It was only when I tried setting x equal to negative 2 to do a test of the ">=" operator and the compiler flaged that as not in the valid range for that variable that I caught this.

So it appears a variable declared only as "int" exhibits properties of both the signed and unsigned types.

Edit: Wait, I just realized that was probably the printf() call interpretting the numbers as signed because of the %d.

Nevermind; nothing to see here. Move along.