Кому интересно. Вот код:

#include <time.h>
#include <stdio.h>
#include <malloc.h>
#include <math.h>

long B=10000; 
long LB=4;   
long MaxDiv=450;  

void SetToInteger (long n, long *x, long Integer) 
{
	long i;
	for (i=1; i<n; i++) x[i] = 0;
	x[0] = Integer;
}

long IsZero (long n, long *x) 
{
	long i;
	for (i=0; i<n; i++)  
    	if (x[i])   
    		return 0;
        return 1;
}

void Add (long n, long *x, long *y) 
{
	long carry=0, i;
	for (i=n-1; i>=0; i--) 
	{
	    x[i] += y[i]+carry;
	    if (x[i]<B) 
	    	carry = 0;
	    else 
	    {
    		carry = 1;
    		x[i] -= B;
    	}
  	}  
}

void Sub (long n, long *x, long *y) 
{
	long i;
	for (i=n-1; i>=0; i--) 
	{
    	x[i] -= y[i];
		if (x[i]<0) 
		{
	    	if(i) 
   		 	{      
    	    	x[i] += B;
    	   		x[i-1]--;
    	  	}
  		}
  	}  
}

void Mul (long n, long *x, long q) 
{
	long carry=0, xi, i;
	for (i=n-1; i>=0; i--) 
	{
	    xi  = x[i]*q;               
	    xi += carry;                
	    if (xi>=B) 
	    {
      		carry = xi/B;
      		xi -= (carry*B);
    	}
   	 	else 
      		carry = 0;
    	x[i] = xi;
	}  
}

void Div (long n, long *x, long d, long *y) 
{
	long carry=0, xi, q, i;
  	for (i=0; i<n; i++) 
  	{
		xi = x[i]+carry*B;
   		q = xi/d;
    	carry = xi-q*d;   
    	y[i] = q;        
  	}  
}

void arccot (long p, long n, long *x, long *buf1, long *buf2) 
{
	long p2=p*p, k=3, sign=0;
	long *uk=buf1, *vk=buf2;
	SetToInteger (n, x, 0);
  	SetToInteger (n, uk, 1);    
  	Div (n, uk, p, uk);
  	Add (n, x, uk);              
  	while (!IsZero(n, uk)) 
  	{
    	if (p<MaxDiv)
      		Div (n, uk, p2, uk); 
    	else 
    	{
      		Div (n, uk, p, uk);   
      		Div (n, uk, p, uk);  
    	}
    	Div (n, uk, k, vk);       
    	if (sign) 
    		Add (n, x, vk); 
    	else 
    		Sub (n, x, vk);      
    	k+=2;
    	sign = 1-sign;
  	}	
}

void Print (long n, long *x) 
{
  	long i; 
  	printf ("%d.", x[0]);
  	for (i=1; i<n; i++) 
  	{
    	printf ("%.4d", x[i]);
    	if (i%25==0) printf ("%8d\n", i*4);
  	}                          
  	printf ("\n");
}
int main () {  
  	clock_t endclock, startclock; 
  	long NbDigits=10000/*вот тут сколько строк будет сгенерировано(Строка длинною 25 символов)*/, NbArctan;
  	long p[10], m[10];
  	long size=1+NbDigits/LB, i;
  	long *Pi = (long *)malloc(size*sizeof(long));
  	long *arctan  = (long *)malloc(size*sizeof(long));
  	long *buffer1 = (long *)malloc(size*sizeof(long));
  	long *buffer2 = (long *)malloc(size*sizeof(long)); 
  	startclock = clock();    
  	NbArctan = 3;
  	m[0] = 12; m[1] = 8;  m[2] = -5;
  	p[0] = 18; p[1] = 57; p[2] = 239; 
  	SetToInteger (size, Pi, 0);
  	for (i=0; i<NbArctan; i++) 
  	{
    	arccot (p[i], size, arctan, buffer1, buffer2);
    	Mul (size, arctan, abs(m[i]));
    	if (m[i]>0) 
    		Add (size, Pi, arctan);  
    	else 
    		Sub (size, Pi, arctan);  
	}
  	Mul (size, Pi, 4);
  	endclock = clock ();
  	Print (size, Pi); 
  	printf ("time: %9.2f seconds\n",(float)(endclock-startclock)/(float)CLOCKS_PER_SEC ); 
  	free (Pi);
  	free (arctan);
    free (buffer1);
    free (buffer2);
}