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\n", 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 for both assemblys:

23,25c23,25
< .globl abs
<   .type   abs, @function
< abs:
---
> .globl abs2
>   .type   abs2, @function
> abs2:
54c54
<   .size   abs, .-abs
---
>   .size   abs2, .-abs2
71c71,74
<   movl    -4(%rbp), %edx
---
>   movl    -4(%rbp), %eax
>   movl    %eax, %edi
>   call    abs2
>   movl    %eax, %edx

From what i understand, the first version (where the function is named abs) is simply discarding the function call, thus using the parameter x instead of abs(x)

So to sum up: why does this happen, especially since i couldn't find a way to get any sort of warning or error about this.

Tested on Debian Squeeze, ggc 4.4.5, and also on gcc 4.1.2

Answer 1

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 .)

Answer 2

gcc optimizes the call to abs() to use its built-in abs() . So if you use the -fno-builtin option (or define your abs() as returning int ), you'll notice you get the correct result. According tothis (quoting):

GCC includes built-in versions of many of the functions in the standard C library. The versions prefixed with _ builtin will always be treated as having the same meaning as the C library function even if you specify the -fno-builtin option. (see C Dialect Options) Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function will be emitted .

Had you included stdlib.h , which declares abs() in the first place, you'd get an error at compile time.

Answer 3

Sounds a lot like this bug , which is from 2007 and noted as being fixed.

You should of course try to compile without GCC's intrinics, ie pass -fno-builtin (or just -fno-builtin-abs to snipe out only abs() ) when compiling.