David Kirkby <[email protected]> added the comment:
Hi Mark,
Since 'copysign' is in the maths library, I would not expect the link phase to
fail. Solaris does not ship with different maths libraries for C99 (one just
links to libm).
However, I would not be surprised if the behavior was ill defined if the
compiler is not C99. Certainly header files behave differently on Solaris
depending on the mode of the compiler. For example, trying to use the INFINITY
macro when the compiler is not C99 seems to work on Linux, but fails on Solaris
unless you force C99 mode with gcc -std=c99.
The following bit of code gives the same results whether one uses 'gcc' or 'gcc
-std=c99' on OpenSolaris or Linux. However, if one uses 'gcc -ansi' then the
behavior is totally different.
drkir...@hawk:~$ cat cs.c
#include <stdio.h>
#include <math.h>
int main(int argc, char **argv) {
double x, y;
/* Set x and y differently if a command line arguement is given.
This will avoid the compiler optimising the values out, as they
will not be known in advance. */
if (argc==1) { /* This will stop compiler optimising 0.0 out x */
x=1.0;
y=0.0;
} else {
x=2.0;
y=-0.0;
}
printf("copysign(%lf,%lf)=%lf\n", x, y, copysign(x, y));
}
drkir...@hawk:~$ gcc -lm cs.c
drkir...@hawk:~$ ./a.out
copysign(1.000000,0.000000)=1.000000
drkir...@hawk:~$ ./a.out z
copysign(2.000000,-0.000000)=-2.000000
drkir...@hawk:~$ gcc -lm -std=c99 cs.c
drkir...@hawk:~$ ./a.out
copysign(1.000000,0.000000)=1.000000
drkir...@hawk:~$ ./a.out z
copysign(2.000000,-0.000000)=-2.000000
Note how -ansi screws it up completely
drkir...@hawk:~$ gcc -lm -ansi cs.c
drkir...@hawk:~$ ./a.out
copysign(1.000000,0.000000)=0.000000
drkir...@hawk:~$ ./a.out z
copysign(2.000000,-0.000000)=0.000000
I also tried it on a Sun SPARC running a recent version of Solaris (2009
release). Again the results are the same.
I then tried it on a Solaris box running the first release of Solaris 10
(03/2005). Then one gets even stranger behavior if one defines -ansi, where the
results are almost right, but with poor rounding errors.
drkir...@redstart:~$ gcc -ansi -lm cs.c
drkir...@redstart:~$ ./a.out
copysign(1.000000,0.000000)=1.000001
drkir...@redstart:~$ ./a.out d
copysign(2.000000,-0.000000)=-2.000002
But in C99 mode, it works fine.
drkir...@redstart:~$ gcc -std=c99 -lm cs.c
drkir...@redstart:~$ ./a.out
copysign(1.000000,0.000000)=1.000000
drkir...@redstart:~$ ./a.out d
copysign(2.000000,-0.000000)=-2.000000
So I draw two conclusions.
1) 'copysign' is in the maths library, so a program which tries to link to
'copysign' will succeed.
2) The behavior of 'copysign' is ill defined unless the compiler is a C99
compiler.
I don't think you should use copysign unless the compiler is C99. Trying to
come up with a test for 'copysign' working is probably an impossible task, as
it undefined. So you could try 99 different values of x and y and they all
work, but its anyone guess what will happen with the 100th set of values.
Dave
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