Cannot use math.h - undefined reference to `sqrt'

[Warren Tang]

When I try to compile the following program, the gcc complains that:

main.c.text+0x35): undefined reference to `sqrt'
collect2: ld returned 1 exit status

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

int main(void)
{

double x = 10;

double y;

y = sqrt(x);

return 0;

}

I've checked that the math.h file is indeed in the following folder:
/usr/include

But why can't gcc find it? How can I use math.h?

Thanks for any help!

[Günther Jehle]

> But why can't gcc find it? How can I use math.h?

including math.h is not the problem. gcc is just missing the code for the
math functions. You can achieve this by adding "-lm" to your gcc command
line. Now, the math library is added.

gcc main.c -lm

hth
Guenther

[Warren Tang]

By searching the internet I found I need to use the "-lm" option to
include "math library". But I don't know why I have to do this.. Isn't
math.h just an ordinary header file just as stdio.h? What makes the
difference?

[Warren Tang]

Thank you!

[Günther Jehle]

> By searching the internet I found I need to use the "-lm" option to
> include "math library". But I don't know why I have to do this.. Isn't
> math.h just an ordinary header file just as stdio.h? What makes the
> difference?

the header files only have the declarations of the functions, not the source
itself. The functions in stdio.h are in the glibc library, which is linked
by default. The math functions are not defined in glibc, they are in the
math library. And to link this library you have to "-lm".

hth
Guenther

[Nikos Chantziaras]

Warren Tang wrote:
> By searching the internet I found I need to use the "-lm" option to
> include "math library". But I don't know why I have to do this.. Isn't
> math.h just an ordinary header file just as stdio.h? What makes the
> difference?

The compiler automatically links against libc (the C library) so that
you don't have to specify "-lc" every time. The math functions are in
libm though, and since the compiler doesn't link against it
automatically, you need "-lm".

[sri.gem...@gmail.com]

e1 after using '-lm' for math.h progs i m getting

/tmp/ccrKnLP1.o: In function `main':
triangle.c:(.text+0xfd): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status

i donno wt to do

[b...@coolfone.comze.com]

What OS are you on?

I was able to compile/build this with just "gcc test.c" on Mac Os Mountain Lion:

[valdez...@gmail.com]

thank you ;)

[k.rak...@gmail.com]

when compliling gcc filename.c -lm