Problems with has_log2.c and has_exp2.c

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

Hello all,

I had a problem compiling sage-4.1.1 with gcc-4.5.0 and it could be 
traced back to a problem in PARI's Configure programs has_log2.c and 
has_exp2.c (which are the same for pari-2.3.3 and pari-SVN).

The short story is that I would propose to change these configure 
programs to something like:

#include <math.h>
int main()
{
  double x = log2(2.0);
  return 0;
}


Now the long story of why:

On my system, the functions log2() and exp2() only exist when compiling 
C99 or C++ code.  Not C89, which is still the default for gcc:

$ gcc -lm -o has_exp2 has_exp2.c
has_exp2.c:2: error: 'exp2' undeclared here (not in a function)

$ gcc -std=c99 -lm -o has_exp2 has_exp2.c
has_exp2.c:2: warning: initialization from incompatible pointer type
has_exp2.c: In function 'main':
has_exp2.c:3: warning: comparison of distinct pointer types lacks a cast

$ g++ -lm -o has_exp2 has_exp2.c
has_exp2.c:2: error: invalid conversion from 'double (*)(double)throw 
()' to 'char (*)()'
has_exp2.c: In function 'int main()':
has_exp2.c:3: error: comparison between distinct pointer types 'char 
(*)()' and 'double (*)(double)throw ()' lacks a cast

Note that the configure program really does a bad job on C++ code, 
because the function exp2() exists but Configure thinks that it does not 
exist.  This causes paricom.h to declare exp2() in a different way from 
<math.h> which then gives an error:

In file included from ../src/headers/pari.h:50:0,
                 from ../src/gp/gp.c:21:
../src/headers/paricom.h: In function 'double exp2(double)':
../src/headers/paricom.h:94:33: error: new declaration 'double exp2(double)'
/usr/include/bits/mathcalls.h:142:1: error: ambiguates old declaration 
'double exp2(double)'
../src/headers/paricom.h: In function 'double log2(double)':
../src/headers/paricom.h:102:33: error: new declaration 'double 
log2(double)'
/usr/include/bits/mathcalls.h:145:1: error: ambiguates old declaration 
'double log2(double)'
../src/gp/gp.c: In function 'char* what_readline()':
../src/gp/gp.c:776:7: warning: deprecated conversion from string 
constant to 'char*'

The problem with sage is even more subtle, because it mixes C and C++ 
code.  I have not analyzed exactly what changed in gcc-4.5.0.


I would also propose that config/get_cc checks for a "-std=gnu99" or 
"-std=c99" option for the C compiler and adds it when compiling PARI.


Cheers,
Jeroen.