overflow exception

[Andrea Latina <andrea dot latina at to dot infn dot it>]

hi all,

I think there is a problem with the trapping of the overflow exception
in g++ (maybe in gcc too?).

I'm using gcc version 3.0.3 on a Intel RedHat Linux 7.2, compiled giving
no options to the configure script.

When I run the attached program, which compute the factorial of an
integer number using a recursive (integer) routine and a non recursive
"exact" computation (through an exponentiation and a logarithm
extraction), I get this output:


   n! =         integer           exact

   0! =               1               1
   1! =               1               1
   2! =               2               2
   3! =               6               6
   4! =              24              24
   5! =             120             120
   6! =             720             720
   7! =            5040            5040
   8! =           40320           40320
   9! =          362880          362880
  10! =         3628800         3628800
  11! =        39916800        39916800
  12! =       479001600       479001600
  13! =      1932053504      6227020800
  14! =      1278945280     87178291200
  15! =      2004310016   1307674368000
  16! =      2004189184  20922789888000
  17! = Aborted

compiling the source with:

    g++ fact.cc -ftrapv -o fact
	

That's strange! The integer factorials over 12 are clearly wrong ('cause
of the overflow, being 13!=6227020800 > 2^32), but the program dies only
at 17!.

Shouldn't it die during the computation of 13! ?


If I compile the program without the option "-ftrapv" simply the program
goes ahead ignoring any problem, as I can expect.

I hope this report can be useful.

Thanks to all of you.

cheers,
Andrea Latina
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;



static int fact(int n )
{
	if (n == 0)	return 1;
	
	return n * fact(n -1);
}


static double exp_fact(int n )
{
	double ln_fact = 0;

	for (int i = 1; i <= n; i++)
	{
		ln_fact += log(double(i));
	}
	
	return exp(ln_fact);
}



int main()
{
	cout << setprecision(14);
	
	cout	<< setw(4)  << "   n! = " 
			<< setw(15) << "integer" << '\t'
			<< setw(15) << "exact" << endl << endl;

	for (int n = 0; n < 20; n++)
	{
		cout	<< setw(4)  << n << "! = " 
				<< setw(15) << fact(n) << '\t' 
				<< setw(15) << exp_fact(n)
				<< endl;
	}
	
	return 0;
}