Odd 'case' of a GCC warning I've come across tonight, the simple classic "comparison between signed and unsigned integer expressions [-Wsign-compare]". No harm, no foul - best I go check my types and make sure I'm not comparing signed and unsigned types. Easy.
Except, not. I'm working in a codebase with a MISRA-C compliance requirement, and so cannot use the standard abs library function to take an absolute value of what could be a maximum negative value (for which the standard lib has undefined behaviour). I have two numbers where I need the difference between them, and to avoid a negative result - we check first which is larger such that we can subtract in the order that always results in a positive value.
Here's a minimum reproduceable example:
#include <stdint.h>
#include <stdio.h>
#define CONSTANT 100u
static struct {
uint16_t example_var_a;
uint16_t example_var_b;
} demo = {
.example_var_a = 2600,
.example_var_b = 2400
};
int main(void)
{
if (demo.example_var_a > demo.example_var_b){
if ((demo.example_var_a - demo.example_var_b) > CONSTANT){
printf("%s\n", "A is greater than B, and the difference is greater than CONSTANT");
}
}
else {
if ((demo.example_var_b - demo.example_var_a) > CONSTANT){
printf("%s\n", "A is less than B, and the difference is greater than CONSTANT");
}
}
}
This compiles and runs on Windows using GCC (MinGW.org GCC-6.3.0-1) - with no warnings or messages, and prints the correct message depending on the values of a and b.
If I compile that same code for an ATSAMC21 (only with the real application calls, in place of printf...), using GCC (gcc version 6.3.1 20170620 (release) [ARM/embedded-6-branch revision 249437] (Atmel build: 508)) - I get a "comparison between signed and unsigned integer expressions [-Wsign-compare]" warning. But why? both example_var_a, and _b are unsigned, and CONSTANT is forced to be unsigned with a u suffix (said suffix also being a MISRA requirement).
Most confusingly, removal of the u suffix get's rid of the warning!
So, what is it about Cortex M0+ data types combined with C's rules of integer promotion that I don't understand that's causing this warning to occur? I would expect #define CONSTANT 100 would default to int, and the u suffix should make CONSTANT an unsigned int. At which point, we're comparing unsigned shorts (uint16_t) with unsigned ints, so the shorts get promoted to ints?
Setting the value of CONSTANT to > 255 to force it to short, makes no difference. Including a cast in the definition of CONSTANT however, as in #define CONSTANT (uint8_t)100, does remove the warning. But then, so does (int8_t)100.
Edit:
- checking the output of the pre-processor, CONSTANT is being correctly substituted for 100u
- compiling with -funsigned-char makes no difference
