I'm using the avr toolchain 3.5.4_468 from ATMel.
fabs() is nowhere to be found.
I've tried building with and without -lm and I get
undefined reference to `fabs'
at link time.
I mean, I can use a #define to make it (x<0)?-x:x, but this is supposed to just work, right?
Include the h file with fabs in it? (math.h? stdlib.h?) Use the newest toolchain? Use a commercial compiler that is ansi c89 compliant?
Which micro are you building for? In other words which version of the 17 versions of libm.a is it linking with?
EDIT: actually I just picked one at random (avr5) and found:
C:\Program Files (x86)\Atmel\Studio\7.0\toolchain\avr8\avr8-gnu-toolchain\avr\lib\avr5>avr-nm libm.a | findstr "fabs"
C:\Program Files (x86)\Atmel\Studio\7.0\toolchain\avr8\avr8-gnu-toolchain\avr\lib\avr5>avr-nm libm.a | findstr "sin"
asin.o:
00000000 T asin
U __fp_sinus
fp_sinus.o:
00000000 T __fp_sinus
isinf.o:
00000000 T isinf
sinh.o:
00000000 T sinh
sin.o:
U __fp_sinus
00000000 T sin
So, yes, something like sin() is there but you are right, it looks like fasb() is missing - curious.
I built a test program in Studio 6.2, and it builds cleanly. But I can't get it to invoke fabs().
#include <avr/io.h>
#include <math.h>
volatile float x;
volatile float y;
volatile float z;
int main(void)
{
y = 4.567;
z = fabs(x*y);
PORTB = floor(z);
while (1)
{
}
}z = fabs(x*y); 122: 60 91 00 02 lds r22, 0x0200 126: 70 91 01 02 lds r23, 0x0201 12a: 80 91 02 02 lds r24, 0x0202 12e: 90 91 03 02 lds r25, 0x0203 132: 20 91 08 02 lds r18, 0x0208 136: 30 91 09 02 lds r19, 0x0209 13a: 40 91 0a 02 lds r20, 0x020A 13e: 50 91 0b 02 lds r21, 0x020B 142: bf d0 rcall .+382 ; 0x2c2 <__mulsf3> 144: dc 01 movw r26, r24 146: cb 01 movw r24, r22 148: bf 77 andi r27, 0x7F ; 127 14a: 80 93 04 02 sts 0x0204, r24 14e: 90 93 05 02 sts 0x0205, r25 152: a0 93 06 02 sts 0x0206, r26 156: b0 93 07 02 sts 0x0207, r27 PORTB = floor(z); 15a: 60 91 04 02 lds r22, 0x0204 15e: 70 91 05 02 lds r23, 0x0205 162: 80 91 06 02 lds r24, 0x0206 166: 90 91 07 02 lds r25, 0x0207 16a: 2f d0 rcall .+94 ; 0x1ca <floor> 16c: 02 d0 rcall .+4 ; 0x172 <__fixunssfsi> 16e: 65 b9 out 0x05, r22 ; 5 170: ff cf rjmp .-2 ; 0x170 <main+0x66>
I believe the "andi" instruction removes the sign bit - the equivalent to fabs().
To answer someone else's question, -mmcu=atmega328pb. Which is the same set of libraries as atmega328p.
I figured it out.
I was using -ffreestanding, which allows me to declare main() as void, but also disappears all of the compiler's built-ins, which include fabs().
The correct workaround is
#define fabs(n) __builtin_fabs(n)
As documented: https://gcc.gnu.org/onlinedocs/g... (look under the -fno-builtin section).
Doing that instead of declaring a replacement static inline function saves quite a few bytes, it turns out.
Why on earth are you using -ffreestanding ? Offtopic here, but with that switch you are removing any knowledge about library functions from the compiler — both about correct prototypes and inner working of these functions.
For example, it will no more optimize strlen ("foo") to 3, etc.
I thought that was recommended.
From what I can tell, the only immediate negative from not using it is that insists that main() returns int, but that's largely moot because my main() doesn't return.
That's no issue. If the compiler detects that main never returns, e.g. due to an endless loop, it won't generate no return statement anyway. Hence there is no issue with the final "return 0;".
@nsayer, have a look at attribute OS_main:
https://gcc.gnu.org/onlinedocs/gcc/AVR-Function-Attributes.html
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If GCC specific extensions are no issue, you might also have a look at attribute((noreturn)) and __builtin_unreachable().
