Stack problem in C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct stackNode
{
  char *data;
  struct stackNode *nextPtr;
};

typedef struct stackNode STACKNODE;
typedef STACKNODE *STACKNODEPTR;

void push(STACKNODEPTR *, char*);
char* pop(STACKNODEPTR *);
int isEmpty(STACKNODEPTR);
void printStack(STACKNODEPTR);

int main()
{
  FILE *file;
  STACKNODEPTR top = NULL;
  char c, *temp, str[9]="", filename[100], infix[302], postfix[302];
  int i,j,k;
  
  do
  {
    printf("Enter the name of the file containing the infix expression: ");
    scanf("%s",filename);
    file= fopen(filename,"r");
    
    if(file == NULL)
      printf("File does not exist, enter again.\n");
  }while(file == NULL);
  
  i=0;
  while ((c= fgetc(file)) != EOF)
  {
    if (c != ' ')
    {
      printf("%c",c);
      infix= c;
      i++;
    }
  }

  printf("The original infix expression is:\n%s\n",infix);
  push(⊤,"(");
  printStack(top);
  infix= ')';
  infix[i+1]= '\0';
  printf("%s\n",infix);

  i=0;
  j=0;
  while (!isEmpty(top))
  {
    if(infix >= '0' && infix <= '9')
    {
      while(infix >= '0' && infix <= '9')
      {
        postfix[j]= infix;
        i++;
        j++;
      }
    
      postfix[j]= ' ';
      j++;
    }
    else if(infix == '(')
    {
      str[0]= infix;
      temp= str;
      push(⊤, temp);
      i++;
      printStack(top);
    }
    else if(infix == '+' || infix == '-' || infix == '*' || infix == '/' ||
            infix == '^')
    {
      switch(infix)
      {
        case '+':
        case '-':
          while(top->data[0] == '*' || top->data[0] == '/' || top->data[0] == '^')
          {
            temp= pop(⊤);
            printStack(top);
            postfix[j]= temp[0];
            j++;
            postfix[j]= ' ';
            j++;
          }
          k= sprintf(str,"%c",infix);
          temp= str;
          printf("%s\n");
          printStack(top);
          printf("Push\n");
          push(⊤, temp);
          printStack(top);
          i++;
          printStack(top);
          break;
        case '*':
        case '/':
          while(top->data[0] == '^')
          {
            temp= pop(⊤);
            printf("Operator1.5\n");
            printStack(top);
            postfix[j]= temp[0];
            j++;
            postfix[j]= ' ';
            j++;
          }
          
                           ********** 
          str[0]=infix; ***Here***
                           **********

          temp= str;
          push(⊤, temp);
          i++;
          printStack(top);
          break;
        default:
          break;
      }
    }
    else if(infix == ')')
    {
      while(top->data[0] != '(')
      {
        temp= pop(⊤);
        printStack(top);
        postfix[j]= temp[0];
        j++;
        postfix[j]= ' ';
        j++;
      }
      
      temp= pop(⊤);
      printStack(top);
    }
  }
  postfix[j]= '\0';
  printf("The expression in postfix notation is:\n%s\n",postfix);


  return 0;
}

/* Insert a node at the stack top */
void push(STACKNODEPTR *topPtr, char *info)
{   STACKNODEPTR newPtr;

   newPtr = malloc(sizeof(STACKNODE));
   if (newPtr != NULL) {
      newPtr->data = info;
      newPtr->nextPtr = *topPtr;
      *topPtr = newPtr;
   }
   else
      printf("%d not inserted. No memory available.\n", info);
}

/* Remove a node from the stack top */
char* pop(STACKNODEPTR *topPtr)
{
   STACKNODEPTR tempPtr;
   char *popValue;

   tempPtr = *topPtr;
   popValue = (*topPtr)->data;
   *topPtr = (*topPtr)->nextPtr;
   free(tempPtr);
   return popValue;
}

/* Print the stack */
void printStack(STACKNODEPTR currentPtr)
{
   if (currentPtr == NULL)
      printf("The stack is empty.\n\n");
   else {
      printf("The stack is:\n");

      while (currentPtr != NULL) {
         printf("%s --> ", currentPtr->data);
         currentPtr = currentPtr->nextPtr;
      }

      printf("NULL\n");
   }
}

/* Is the stack empty? */
int isEmpty(STACKNODEPTR topPtr)
{
   return topPtr == NULL;
}