Gentoo Archives: gentoo-science

From: Marco Matthies <marco-ml@×××.net>
To: gentoo-science@l.g.o
Subject: Re: [gentoo-science] question about signbit
Date: Sat, 22 Oct 2005 21:03:38
In Reply to: [gentoo-science] question about signbit by Darren Dale
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: 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. Marco -- gentoo-science@g.o mailing list


Subject Author
Re: [gentoo-science] question about signbit Darren Dale <dd55@×××××××.edu>
Re: [gentoo-science] question about signbit Marco Matthies <marco-ml@×××.net>
Re: [gentoo-science] question about signbit Darren Dale <dd55@×××××××.edu>