The Sign of Char in ARM

The following below has a value that is vague:

char i = -1;

The issue with the above line is that the value of i is not immediately obvious as compilers for different architectures could treat this as signed or unsigned. The signedness of a data type can be simply thought as whether or not there is a dedicated sign bit that indicates whether or not the number is postive or negative.

In Robert Love’s section on “Signedness of Chars” (Chapter 19 - Portability) of his book on the Linux Kernel Development, he notes that on some systems such as in ARM would treat char as unsigned which goes against the logic of us AMD64 (x86-64) programmers. Effectively, the value of i will be stored as 255 rather than -1. The reason for this is apparently due to performance.

Let’s verify this on my Raspberry Pi 4 machine running Linux:

char i = -1;
if (i == 255) {
    printf("char is unsigned\n");
}
if (i == -1) {
    printf("char is signed\n");
}

Result: char is unsigned

Therefore to make your code portable, one should ensure to explicitly state whether or not char is signed or unsigned instead of making assumptions if they know their char will lie outside of 0 to 127.

I heard rumours that Apple may treat the signedness of char as signed instead but I unfortunately do not have access to an Apple ARM development machine to verify this myself.

Maybe I’ll examine this more in my blog by examining what is going on under the hood and compare this with amd64 and whether or not QNX makes this change as well.

Note: I purposely omitted how signed bit works and the 2’s complement