I believe cos is defined in mathcalss.h which is included by math.h.

If you are having difficulty linking, make sure you use -lm on your gcc 
command line to link to the math library.

Hope this helps.....

Bob

Shawn T. Rutledge wrote:

>I'm trying to compile a program which depends on sin and cos, and they're
>not in math.h!  I don't believe it.  The man page for cos even says it's
>in math.h.  I'm running Debian of course.
>