undefined reference to `sqrt'

[warrentang]

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

What's the problem? I am using Fedora 8 by the way.


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

int main(void)
{

double x = 10;

double y;

y = sqrt(x);

return 0;

}

Thanks for any help!

[warrentang]

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

But why cann't gcc find it?

[warrentang]

I need to add the "-lm" option for the gcc

[gerard4143]

For some reason I have to do this as well(add the -lm option to the gcc compile line)

[Cabhan]

Correct. You need to add "-lm" to gcc.

Most of the C Standard Library exists in a library called libc.so. Math functions, however, exist in libm.so. gcc will automatically link to libc: any other linking must be specified. To tell it what library to link to, you remove the '.so' from the end and the 'lib' from the beginning. Therefore, "-lm" means "link against libm.so".

I hope this makes sense.

[gerard4143]

Isn't the math header supposed to be part of the standard library, according to the ansi standard?

[Cabhan]

Both the header and the library are considered a part of the C Standard Library. However, this is not a compiletime error ("I don't know what sqrt is"). This is a linktime error ("I can't find the precompiled sqrt function").

I admittedly don't know the history behind this decision, or what the ANSI standard says about it, but that's the way it is. If you find out why it is this way, I'd be interested.

[warrentang]

Thanks for the useful information, Cabban.

I just knew that one still needs to add "-lm" to Eclipse CDT linker (gcc) options.

Correct. You need to add "-lm" to gcc.

Most of the C Standard Library exists in a library called libc.so. Math functions, however, exist in libm.so. gcc will automatically link to libc: any other linking must be specified. To tell it what library to link to, you remove the '.so' from the end and the 'lib' from the beginning. Therefore, "-lm" means "link against libm.so".

I hope this makes sense.

[hondacodonbk]

So, -lm for math.h,but if these is anything similar to math.h,where can I know to add it for gcc

[c.pergiel]

Old info, but still useful. I just ran into the same problem. Adding m to the libraries in Eclipse solved the problem. Thanks.