0

Code :

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

typedef struct tree
{
    char data;
    struct tree *left;
    struct tree *right;
}*pos;

pos stack[30];
int top=-1;

pos newnode(char b)
{ 
    pos temp;
    temp=(struct tree*)malloc(sizeof(struct tree));
    temp->data=b;
    temp->left=NULL;
    temp->right=NULL;
    return(temp);
}

void push(pos temp)
{
    stack[++top]=temp;
}

pos pop()
{
    pos p;
    p=stack[top--];
    return(p);
}

void inorder(pos t)
{
    if(t!=NULL)
    {
        inorder(t->left);
        printf("%s",t->data);
        inorder(t->right);
    }
}
void preorder(pos t)
{
    if(t!=NULL)
    {
        printf("%s",t->data);
        preorder(t->left);
        inorder(t->right);
    }
}

void postorder(pos t)
{
    if(t!=NULL)
    { 
        postorder(t->left);
        postorder(t->right);
        printf("%s",t->data);
    }
}

void main()
{
    char *a;
    pos temp,t;
    int j,i;
    puts("Enter the expression :");
    scanf("%s",&a);
    for(i=0;a[i]!='\0';i++)
    {
        if(a[i]=='*' || a[i]=='/' || a[i]=='+' || a[i]=='-')
        {
            temp=newnode(a[i]);
            temp->right=pop();
            temp->left=pop();
            push(temp);
        }
        else
        {
            temp=newnode(a[i]);
            push(temp);
        }
    }
    inorder(temp);
    printf("\n");
    preorder(temp);
    printf("\n");
    postorder(temp);
}

Error : Segmentation Fault

This code is for construction of binary tree traversal and conversion of postfix to infix and prefix. I dont know where am going wrong but it keeps saying the same fault. Can anyone help me with this ?

Evg
  • 25,259
  • 5
  • 41
  • 83
Mohammad Hasan
  • 71
  • 10

4 Answers4

2
    scanf("%s",&a); // is the problem.

scanf accepts pointer and you are passing the address of a pointer. You have to pass only pointer.

    scanf("%s",a); // since a is already pointer, just use a.

And you are not allocating memory. You need to allocate memory to hold the scanned string some thing like this...

    a = (char*)malloc(sizeof(*a) * MAX_SIZE);
  • Besides just pointing to the problematic construct one shall provide a solution: That is `scanf("%s", a)`! – alk Aug 20 '13 at 10:50
  • @alk Sure brother, i will keep this in mind from now on and will correct my answer too. –  Aug 20 '13 at 11:07
  • 1
    +1 also for proposing to use `*a` :-). However in C one should not cast the return value of `malloc/calloc/realloc` as its not necessary nor recommended: http://stackoverflow.com/a/605858/694576 – alk Aug 20 '13 at 12:47
  • Still the program is not running :/ It says Fault access violation at 0x405888 read of address 0x31 – Mohammad Hasan Aug 21 '13 at 02:38
  • It worked ! The problem was here too ---> printf("%s", t->data); I used the format as a string. Changed it to character and it worked flawlessly ! :) – Mohammad Hasan Aug 21 '13 at 02:43
2

You don't use scanf correctly: you give to scanf the address of a pointer to a char, but it is not initialized: it could point to a bad memory address and then you will get the segmentation fault.

You could do something like this:

# define MAX_BUFF_SIZE (64)

void main()
{
 char a[MAX_BUFF_SIZE];
 pos temp,t;
 int j,i;
 puts("Enter the expression :");
 scanf("%s", a);
 /* ... */
 return 0;
}

Or if you prefere dynamic allocation:

# define MAX_BUFF_SIZE (64)

void main()
{
 char *a;
 pos temp,t;
 int j,i;
 a = malloc(sizeof(*a) * MAX_BUFF_SIZE);
 if (a == NULL)
     return -1;
 puts("Enter the expression :");
 scanf("%s", a);
 /* ... */
 free(a);
 return 0;
}

By the way, be aware that using scanf is not safe, read this if you want more informations.

Community
  • 1
  • 1
nouney
  • 4,363
  • 19
  • 31
  • 1
    to complete the answer: not only does scanf expect a char* (i.e. string, becuase of %s formatter) it's also important to reserve some memory for the result and point to it. Otherwise scanf will just go ahead and try to overwrite whatever happens to be at the destination of the pointer, which resultet in a Segmentation Fault. – Tobias Aug 20 '13 at 09:34
  • It is not working again :/ It says Fault access violation at 0x405888 read of address 0x31 – Mohammad Hasan Aug 21 '13 at 02:40
  • It worked ! The problem was here too ---> printf("%s", t->data); I used the format as a string. Changed it to character and it worked flawlessly ! :) – Mohammad Hasan Aug 21 '13 at 02:44
1

This line

printf("%s", t->data);

tries to print a char (t->data) as a 0-terminated char array (commonly referred to as "string"), which does not work.

To fix this use "%c" instead of "%s".

alk
  • 69,737
  • 10
  • 105
  • 255
0

In order:

  • C doesn't have a malloc.h file. The malloc() function and friends are declared in stdlib.h. The malloc.h file, if present, is a system-specific nonstandardness, and should not be used.
  • int top=-1;? Odd way to handle indexing. What's wrong with an index grounded at zero?
  • You're casting the return value of the malloc() call. In C, this is generally recommended against.
  • return is not a function call, it is a statement.
  • Your push() function does no bounds checking. If you push more than thirty items, you'll be overwriting memory that doesn't belong to you.
  • Your pop() function also does no bounds checking. What happens if you try to pop when there's nothing on the stack?
  • You're using the "%s" format specifier to print an element of type char. Undefined Behavior. (All three traversal functions.)
  • Your preorder() function is calling inorder(). Oops.
  • You declare a as being of type char *, but then you pass it to scanf(). Either declare it as a sufficiently large array, or use malloc() to allocate the storage that you'll be passing to scanf. (Either way, you should be passing a, not &a, to the scanf() function.)
  • What happens if my input string starts with */-+?
This isn't my real name
  • 4,869
  • 3
  • 17
  • 30
  • In stacks generally top value is taken as -1 right ? That's why I indexed it with -1. Thanks anyways :) That taught me a lot ! – Mohammad Hasan Aug 22 '13 at 10:57