Segmentation Fault - Core dumped

Question

This is my program to convert an expression from postfix to infix using stacks. I am getting no errors but on running, the compiler says Segmentation Fault - Core Dumped. I think it has something to do with a pointer pointing to a garbage value. How do I tracebck to this rogue pointer? Also, is there any better method to convert postfix to infix?

#include<iostream>
#include<string.h>
#include<stdio.h>
#include<math.h>
class StringStack
{
    public:
    char arr[20][20];
    int top;
    StringStack()
    {top=-1; arr[20][20]={NULL};}
    void push(char* a)
    {strcpy(arr[++top],a);}
    char* pop()
    {return arr[top--];}
};
void paranthesise(char a[])
{
    int l = strlen(a);
    a[l+2] = '\0';
    for(int i=l; i>=0; i++)
        a[i] = a[i-1];
    a[0] = '(';
    a[l+1] = ')';

}
using namespace std;
int main()
{
    char temp[1];
    char postfix[20] = "";
    char temp2[10] = "";
    char temp3[10] = "";
    StringStack s1;
    cout<<"Enter Postfix Expression: "<<endl;
    gets(postfix); int l = strlen(postfix);
    for(int i=0; i<l;   i++)
    {
        if(postfix[i]!='*'&&postfix[i]!='/'&&postfix[i]!='+'&&postfix[i]!='-')
        {
            temp[0]=postfix[i];
            s1.push(temp);
        }
        else
        {
            strcpy(temp2,s1.pop());
            strcpy(temp3,s1.pop());
            switch(postfix[i])
            {
                case'*':
                    temp[0]='*'; break;
                case'/':
                    temp[0]='/'; break;
                case'+':
                    temp[0]='+'; break;
                case'-':
                    temp[0]='-'; break;

                default: cout<<"Error"; break;
            }
            strcat(temp3,temp);
            strcat(temp3,temp2);
            paranthesise(temp3);
            s1.push(temp3);
        }
    }
    strcpy(temp2,s1.pop());
    cout<<"\nInfix:";
    puts(temp2); 
    return 0;
}

Possible duplicates: [#1](http://stackoverflow.com/q/20612172), [#2](http://stackoverflow.com/q/22546079), [#3](http://stackoverflow.com/q/4363133). — Kerrek SB, Sep 14 '15 at 01:00
In `paranthesise`: `for (int i = l; i >= 0; i++)` I'm guessing you wanted `i--` here. — user657267, Sep 14 '15 at 01:01
*"Also, is there any better method to convert postfix to infix?"* Too broad. — Baum mit Augen, Sep 14 '15 at 01:01
"on running, the compiler says" - No it doesn't. The compiler is the part of C++ which happens _before_ running. While running, the OS is in charge, and the C++ runtime library is in support. A SegFault is an OS message, usually caused by bugs in your code. — MSalters, Sep 14 '15 at 01:14

R Sahu · Answer 1 · 2015-09-14T01:28:37.517

Problems that I found:

One

arr[20][20]={NULL};

is wrong. This assigns NULL to arr[20][20], which is not a valid element of the array. You are setting the value of memory that you are not supposed to. Valid elements of arr are arr[0][0] - arr[19][19].

That in itself is sufficient for the program to exhibit undefined behavior.

I would change the constructor to:

StringStack() : arr{}, top(-1) {}

Two

When you use

char temp[1];

the only valid string it can hold is the empty string. Since you mean to use it hold a string consisting of one character, you need to use:

char temp[2] = ""; // Doesn't have to be 2 but it has to be 
                   // greater than 1.

Three

In paranthesise, you have:

for(int i=l; i>=0; i++)

That needs to be:

for(int i=l; i>=0; i--)

Otherwise, you keep on modifying the array without ever meeting the criterion to end the loop. Not only that, you also end up modifying the array beyond the valid limits.

Segmentation Fault - Core dumped

1 Answers1