Gentoo Archives: gentoo-science

From: Darren Dale <dd55@×××××××.edu>
To: gentoo-science@l.g.o
Subject: Re: [gentoo-science] question about signbit
Date: Thu, 27 Oct 2005 12:53:28
Message-Id: 200510270852.35524.dd55@cornell.edu
In Reply to: Re: [gentoo-science] question about signbit by Marco Matthies
Hi Marco,

On Saturday 22 October 2005 5:05 pm, Marco Matthies wrote:
> Darren Dale wrote: > > On my system, SciPy's signbit function reports that the sign bit is not > > set for any number, positive or negative. Could someone here help me > > understand how to test the libc signbit function? I have to admit I have > > no experience with C programming. > > Hi Darren, > > the signbit fuction is actually a macro (as the manpage says) defined in > math.h that in turn calls the right inline function (for the type > needed) which is defined in mathinline.h --- so as far as i can see, > libc should not be involved, only header files. I have attached a small > example below on how to use the function. Please note the use of > -std=c99 (you may also use -std=gnu99) as the macro is only activated > when in C99 mode and gcc's default mode is C89 ("ANSI C"). If you're > interested in the differences between the two standards the wikipedia > entry on c has some info: > http://en.wikipedia.org/wiki/C_programming_language > > > the program (save it under signbit_test.c): > [cut] > #include <math.h> > #include <stdio.h> > > int main() { > printf("sign of 1.7 is %d\n", signbit(1.7)); > printf("sign of -1.1 is %d\n", signbit(-1.1)); > printf("sign of -0.0 is %d\n", signbit(-0.0)); > printf("sign of 0.0 is %d\n", signbit(0.0)); > return 0; > } > [/cut] > > compile with: > gcc -Wall -std=c99 -lm signbit_test.c -o signbit_test > > run with: > ./signbit_test > > should produce this output: > sign of 1.7 is 0 > sign of -1.1 is -2147483648 > sign of -0.0 is -2147483648 > sign of 0.0 is 0 > > This was run with gcc 3.4.4 on amd64, if you want to i can try on a x86 > install in qemu.
Here is another test: #include <math.h> #include <stdio.h> int main() { printf("signbit(-1): %d\n", signbit(-1)); printf("isnan(0.0/0): %d\n", isnan(0.0/0)); printf("isinf(1.0/0): %d\n", isinf(1.0/0)); return 0; } which yields: signbit(-1): -2147483648 isnan(0.0/0): 1 isinf(1.0/0): 1 Do you know why signbit doesn't yield 1? I wonder if this might be the source of the problem in Scipy. Thanks, Darren -- gentoo-science@g.o mailing list

Replies

Subject Author
Re: [gentoo-science] question about signbit Miguel Barao <mjsb@×××××××××××××××.pt>
Re: [gentoo-science] question about signbit Marco Matthies <marco-ml@×××.net>