Help with segmentation fault

boris · #1 02-Feb-2005, 13:59

Hi guys , im working on a calculator programn and i kept getting this segmentation fault when i run it , This is my code , can someone give me a hand on what is wrong with it , thank you.

stack.h

CPP / C++ / C Code:

// Stack data structure
typedef struct stackNode
{
    int value;
    struct stackNode *next;
} stackElement;

typedef stackElement *stackPtr;


// Functions
stackPtr newElement(void);
void setElement(stackPtr element, int val);
stackPtr push(stackPtr stack, stackPtr element);
stackPtr pop(stackPtr stack);
stackPtr top(stackPtr stack);
void printStack(stackPtr stack);
int isdigit(char c);
int isspace(char c);

stack.c:

CPP / C++ / C Code:

#include <stdio.h>
#include <stdlib.h>
#include "stack.h"

stackPtr newElement(void)
{
    stackPtr element;

    element = (stackPtr) malloc(sizeof(stackElement));
    element->next = NULL;
    return(element);
}

void setElement(stackPtr element, int val)
{
    element->value = val;
}

stackPtr push(stackPtr stack, stackPtr element)
{
    element->next = stack;
    return(element);
}

stackPtr pop(stackPtr stack)
{
    stackPtr tmp;

    if (stack == NULL)
    return(NULL);
    tmp = stack->next;
    free(stack);
    return(tmp);
}

stackPtr top(stackPtr stack)
{
    return(stack);
}

void printStack(stackPtr stack)
{
    stackPtr tmp;

    printf("The stack contains:\n");
    for(tmp = stack; tmp != NULL; tmp = tmp->next)
    printf("\tStack element: %d\n", tmp->value);
    printf("\n");
    return;
}

int isdigit(char c)
{
    return(c >= '0' && c <= '9');
}

int isspace(char c)
{
    // space and tab characters are considered 'space'
    return(c == ' ' || c == '\t');
}

calc.c: 
#include <stdio.h>
#include <stdlib.h>
#include "stack.h"

int main(void)
{
    stackPtr tmp, stack = NULL;
    char c;
    int number = 0, op1, op2;

    c = getchar();
    while(c != EOF)
    {
        while(isspace(c)) // skip whitespace
        c = getchar();
        if (c == '\n') // we're done - print the result which is at the top of the stack
        {
            printf("Expression evaluates to %d\n", top(stack)->value);
            c = getchar();
            continue;
        }
        else if(isdigit(c)) // we need to get and store the integer
        {
            number = c -'0';
            while(isdigit(c = getchar()))
                number = (number * 10) + c - '0';
            tmp = newElement();
            setElement(tmp, number);
            stack = push(stack, tmp);
        }
            else // it must be an operator - we assume correct input
        {
            // get the first two numbers from the stack and
            // perform necessary operation storing it back on stack
            op1 = top(stack)->value;
            stack = pop(stack);
            op2 = top(stack)->value;
            stack = pop(stack);
            tmp = newElement();
            switch(c)
            {
                case '+':
                    setElement(tmp, op1 + op2);
                    break;
                case '*':
                    setElement(tmp, op1 * op2);
                    break;
                case '-':
                    setElement(tmp, op2 - op1);
                    break;
            }
            stack = push(stack, tmp);
            c = getchar();
        }
    }

    return 0;
}

davekw7x · #2 02-Feb-2005, 15:25

Quote:

Originally Posted by boris

Hi guys , im working on a calculator programn and i kept getting this segmentation fault when i run it , This is my code , can someone give me a hand on what is wrong with it , thank you.

For reasonable expectation of help, you should tell us:

1. What is the program supposed to do (I know, you said it is a calculator, but I mean specifically: what kind of expressions it is supposed to process; infix notation? postfix? --- what?)

2. What input(s) did you give it? Show us exactly what you entered.

3. What did you expect the program to do?

4. What did it do? (I know --- you said it crashed, but did it do anything before it crashed? Did it always crash, or just sometimes?)

Now, here's a method that can (sometimes) let you get to the bottom of things instead of having to wait for someone else to respond:

Put printf() statements at various places in the program. This does two things:

(a) It might let you know where the problem is. (That is how far did it get before the crash.)

(b) You can make the printf() statements tell you what the program is working on at any given step (sometimes it's not seeing what you think it is).

For example

CPP / C++ / C Code:

    while(c != EOF)
    {
        while(isspace(c)) // skip whitespace
        c = getchar();
        printf("End of whitespace, c = %c (0x%02x hex)\n", c, c);

and

CPP / C++ / C Code:

            while(isdigit(c = getchar()))
                number = (number * 10) + c - '0';
            tmp = newElement();
            printf("After newwElement(), calling setElement(%p, %d)\n", 
                    tmp, number);
            setElement(tmp, number);
            printf("After setElement(), calling push(%p, %p)\n",
                    stack, tmp);

These are just a couple of places to put the printf() statements. Put lots of printf() until you see exactly where things are going wrong.

Regards,

Dave

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