[lug] math.h Weirdness
Bill Thoen
bthoen at gisnet.com
Tue Mar 13 07:09:50 MDT 2007
Thanks Lori,
The sqrt example listed in that article was exactly the situation I
encountered. Using -lm did the trick. Funny thing though; with some code
I write I don't need to explicitly link to the math library, but on this
module, I had to or it wouldn't resolve sqrt().
Also, I found that the protoype for sqrt _is_ in math.h. It's immplemented
as a macro in bits/mathcalls.h and included.
Thanks for the help!
- Bill
On Mon, Mar 12, 2007 at 08:59:09PM -0600, Lori Reed wrote:
> The sqrt() should certainly be prototyped by including math.h, but many
> header files include other header files, so it may actually be
> prototyped elsewhere.
>
> However, the linker doesn't care about header files, so the problem is
> you're not linking in the appropriate library. Try adding -lm to the
> link command.
>
> This page directly addresses linking sqrt()
>
> http://www.network-theory.co.uk/docs/gccintro/gccintro_17.html
>
> Lori
>
> Bill Thoen wrote:
>
> >Am I mistaken in thinking that in C the sqrt() function is prototyped in
> >math.h? I just tried to build a simple C program that called sqrt() and the
> >linker balked. I then tried:
> >
> >$ grep sqrt /usr/include/math.h
> >define M_2_SQRTPI 1.12837916709551257390 /* 2/sqrt(pi) */
> >define M_SQRT2 1.41421356237309504880 /* sqrt(2) */
> >define M_SQRT1_2 0.70710678118654752440 /* 1/sqrt(2) */
> >define M_2_SQRTPIl 1.1283791670955125738961589031215452L /* 2/sqrt(pi)
> >*/
> >define M_SQRT2l 1.4142135623730950488016887242096981L /* sqrt(2) */
> >define M_SQRT1_2l 0.7071067811865475244008443621048490L /* 1/sqrt(2)
> >*/
> >
> >It's not there! Shouldn't it be? If not, where is it these days, or has the
> >square root been retired?
>
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