Why is '-lm' used explicitly only when passing variables to 'math.h' functions?

Question

First of all, I have read this post Why do you need an explicit `-lm` compiler option & this gcc: why is the -lm flag needed to link the math library?. I wanna know why It doesn't happen in case of constants (when I say constants, I mean random floats/doubles)? If you're confused, call it floating-point literals.

Why do we have to use -lm to tell the linker to use math.h functions only when using variables as parameters but not constants? If I use sqrt(N)(N is some number), it compiles fine without any errors but when I pass some variable, let's say sqrt(var), it doesn't. It says:

/usr/bin/ld: /tmp/cc5P9o72.o: in function `main':
sq.c:(.text+0x1b): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status

It should behave the same all the time (I think so, but I am wrong, of-course) as I am using the same function from the same library. Either its variable or constant. I first thought its some kind of compiler optimization (if it's the same value every time, why not calculate it while compiling by some other way, i.e not using the library, as it's not working) but it's doesn't work even if I pass some variable that has a fixed value from beginning to the end. So, I was wrong there. What is actually happening here?

Following is the snippet I tried:

#include <stdio.h>
#include <math.h>

int main () {
    float a=9;
    printf("%f",sqrt(a));
    return 0;
}

because the linker does not link by default with math library. this is not a question for so. — alinsoar, Mar 04 '19 at 07:51
The math functions are expensive and was traditionally not commonly used. Therefore to save space they were put into their own library. — Some programmer dude, Mar 04 '19 at 07:52
I read that on the linked posts. Question is why only with variables? — , Mar 04 '19 at 07:54
floating point constants & variables are supported natively in C. Also new compilers don't require -lm anymore with math functions. — Jean-François Fabre, Mar 04 '19 at 07:57

0___________ · Accepted Answer · 2019-03-04T08:19:04.667

3

It is very simple. When you pass the constants many compilers will evaluate it (in such a trivial example when the result is not float inaccuracies and implementation differences prone) compile time without calling the math.h functions.

Even if you do not pass the constants values and compile it with no math error checks and fast math, the compiler will generate the direct float machine code instructions without calling the library ones

Before asking check the generated code for example using the godbolt.org, and usually it will answer all of your questions

edited Mar 04 '19 at 08:19

answered Mar 04 '19 at 08:00

0___________

60,014
4
34
74

It is not the optimization. – 0___________ Mar 04 '19 at 08:03
@Jean-FrançoisFabre read carefully my answer. **trivial** and the number **9** a're key points. Result of sqrt(9) is always the same and float implementation differences do not matter – 0___________ Mar 04 '19 at 08:15
@P__J__ your answer is the best here. So, I am kinda satisfied but when I use `var=9` which always has the same value just like _just 9_. Why it's different? – Mar 04 '19 at 08:26
I have explained it in my answer – 0___________ Mar 04 '19 at 08:27

Why is '-lm' used explicitly only when passing variables to 'math.h' functions?

1 Answers1