> Hello. > > I'm new to kernel development and have some questions. > > 1. Why can't I divide with regular casting to double ((double)a / > (double)b)? It gives me strange errors when compiling: > > WARNING: "__divdf3" [/root....] undefined! > WARNING: "__addf3" [/root/...] undefined! > WARNING: "__floatsidf" [/root/...] undefined! > > And if I compile with normal integers, I get zero as the result.
As the HOWTO says: "The kernel is written using GNU C and the GNU toolchain. While it adheres to the ISO C89 standard, it uses a number of extensions that are not featured in the standard. The kernel is a freestanding C environment, with no reliance on the standard C library, so some portions of the C standard are not supported. Arbitrary long long divisions and floating point are not allowed. It can sometimes be difficult to understand the assumptions the kernel has on the toolchain and the extensions that it uses, and unfortunately there is no definitive reference for them. Please check the gcc info pages (`info gcc`) for some information on them." The short version is simply that it's very expensive to allow this, adding overhead even in cases where it isn't used, and there is very little benefit. For almost every imaginable case where you would want floating point in the kernel, fixed point works as well or better. Just scale your numbers by shifting them. DS - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
