C++ linux gdb c

#include <stdio.h>
#include <string.h>
#include <ctype.h>
char * index( const char *s, int c);
/*
** State machine for romantoint
**
**        M       D       C       L       X       V       I
**  0: +1000/0  +500/0  +100/1   +50/0   +10/2    +5/0    +1/3
**  1:  +800/0   --     +100/0   +50/0   +10/2    +5/0    +1/3
**  2:   --      --      +80/0   +30/0   +10/2    +5/0    +1/3
**  3:   --      --      --      --       +8/0    +3/0    +1/3
**
*/

int FromRoman(char *s)
{
    int n=0;
    int prev=0;

    while (*s!=' '&&*s!=NULL) {
        switch (toupper(*s))
        {
            case 'M':   n += 1000 - prev*2;
                        prev=0;
                        break;
            case 'D':   n +=  500 - prev*2;
                        prev=0;
                        break;
            case 'C':   n +=  100;
                        if (prev < 100) n -= 2*prev;
                        prev=100;
                        break;
            case 'L':   n +=   50;
                        if (prev <  50) n -= 2*prev;
                        prev=0;
                        break;
            case 'X':   n +=   10;
                        if (prev <  10) n -= 2*prev;
                        prev=10;
                        break;
            case 'V':   n +=    5;
                        if (prev <   5) n -= 2*prev;
                        prev=0;
                        break;
            case 'I':   n +=    1;
                        prev=1;
                        break;
        }
        s++;
    }
    return n;
}

void PrintRoman(int n)
{

    if (n<=0 || n>4999)
     {
        printf("out of range exception\n");
        return;
    }

    while (n/1000)
    {
        printf("M");
        n -= 1000;
    }

    switch(n/100)
    {
        case 0: break;
        case 1: printf("C");    break;
        case 2: printf("CC");   break;
        case 3: printf("CCC");  break;
        case 4: printf("CD");   break;
        case 5: printf("D");    break;
        case 6: printf("DC");   break;
        case 7: printf("DCC");  break;
        case 8: printf("DCCC"); break;
        case 9: printf("CM");   break;
    }
    n %= 100;

    switch(n/10)
     {
        case 0: break;
        case 1: printf("X");    break;
        case 2: printf("XX");   break;
        case 3: printf("XXX");  break;
        case 4: printf("XL");   break;
        case 5: printf("L");    break;
        case 6: printf("LX");   break;
        case 7: printf("LXX");  break;
        case 8: printf("LXXX"); break;
        case 9: printf("XC");   break;
    }
    n %= 10;
    switch(n) {
        case 0: break;
        case 1: printf("I");    break;
        case 2: printf("II");   break;
        case 3: printf("III");  break;
        case 4: printf("IV");   break;
        case 5: printf("V");    break;
        case 6: printf("VI");   break;
        case 7: printf("VII");  break;
        case 8: printf("VIII"); break;
        case 9: printf("IX");   break;
    }

    printf("\n");
}

#define STACKSIZE 1024
int nitems=0;
int stack[STACKSIZE];

int Pop()
{
    if (!nitems) {
        printf("stack underflow\n");
        return 0;
    }

    return stack[--nitems];
}

void Push(int n)
{
    if (nitems == STACKSIZE-1) {
        printf("stack overflow\n");
        return;
    }

    stack[nitems++] = n;
}

int PrintTop(void)
{
    if (!nitems) {
        printf("stack underflow\n");
        return -1;
    }
    else {
        PrintRoman(stack[nitems-1]);
        return 0;
    }
}

int PerformAdd(void)
{
    int x,y;
    if (nitems < 2) {
        printf("stack underflow\n");
        return -1;
    }

    y = Pop();
    x = Pop();
   
    Push(x+y);

    return 0;
}

int PerformSub(void)
{
    int x,y;
    if (nitems < 2) {
        printf("stack underflow\n");
        return -1;
    }

    y = Pop();
    x = Pop();
   
    Push(x-y);

    return 0;
}

int PerformMul(void)
{
    int x,y;
    if (nitems < 2) {
        printf("stack underflow\n");
        return -1;
    }

    y = Pop();
    x = Pop();
   
    Push(x*y);

    return 0;
}

int PerformDiv(void)
{
    int x,y;
    if (nitems < 2) {
        printf("stack underflow\n");
        return -1;
    }

    y = Pop();
    x = Pop();
   
    if (y!=0)
        Push(x/y);
    else {
        printf("division by zero exception\n");
        Push(x);
    }

    return 0;
}

main()
{
    char line[256];
 //freopen("H.in","r",stdin);
// freopen("H.out","w",stdout);
    while (fgets(line,sizeof(line),stdin)) {
        if (line[0] == '=')
            PrintTop();
        else if (line[0] == '+')
            PerformAdd();
        else if (line[0] == '-')
            PerformSub();
        else if (line[0] == '*')
            PerformMul();
        else if (line[0] == '/')
            PerformDiv();
        else {
            int n = FromRoman(line);
            if (n > 0) {
                Push(n);
            }
        }
    }
}