GCC function name conflict

Question

I was having some problems with a sample code i was testing, since my abs function was not returning the correct result. abs(-2) was outputing -2 (this, by the way, is suposed to be the absolute value function, if that was unclear) After getting a bit desperate, i eventually had the following code ``` #include <stdio.h> unsigned int abs(int x) { return 1; } int main() { printf("%d
", abs(-2)); return 0; } ``` This does nothing useful but it serves to show my problem. This was outputing -2, when it was expected to output 1. if i change the function name to something else (abs2 for example), the result is now correct. Also, if i change it to receive two arguments instead of one, it also fixes the problem. My obvious guess: a conflict with the standart abs function. But this still doesn't explain why the output is -2 (it should be 2, if using the standart abs function). I tried checking the assembly output of both versions (with the function named abs and abs2) Here's the diff output fo…

Accepted Answer

GCC is playing tricks on you due to the interplay of the following:

- `abs` is a built-in function;

- you're declaring `abs` to return `unsigned int` while the standard (and built-in) `abs` returns `signed int`.

Try compiling with `gcc -fno-builtin`; on my box, that gives the expected result of `1`. Compiling without that option but with `abs` declared as returning `signed int` causes the program to print `2`.

(The real solution to this problem is to not use library identifiers for your own functions. Also note that you shouldn't be printing an `unsigned int` with `%d`.)

GCC function name conflict

Solution

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