Converting Infix 2 Postfix

[Unregistered]

Can someone pls help me?
I cant get my program to work
Here's the code

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

extern void InitializeStack (stack *S);
extern int FullStack (stack *S);
extern int EmptyStack (stack *S);
extern void Pop (stack *S,int *X);
extern void Push (stack *S, int X);

void main(){
stack s;
char str[100],PFstring[100],a;
int i=0;
PFstring[0]='\0';
printf("Enter infix expression with parantheses: ");
scanf("%s", str);
InitializeStack(&s);
while(str[i]!='\0'){
if(str[i]>='0'&&str[i]<='9')
strncat(PFstring,&str[i],1);
else if(str[i]='(')
Push(&s,str[i]);
else if(str[i]=')'){
Pop(&s,&a);
while(a!='('){
strncat(PFstring,&a,1);
Pop(&s,&a);
}
}
else if(!EmptyStack(&s)){
Pop(&s,&a);
switch(str[i]){
case'-':
case'+':if(a=='(')
Push(&s,a);
else
strncat(PFstring,&a,1);
break;
case'*':
case'/':if(a=='+'||a=='-'||a=='(')
Push(&s,a);
else
strncat(PFstring,&a,1);
break;
case'^':if(a=='^')
strncat(PFstring,&a,1);
else
Push(&s,a);
break;
}
Push(&s,str[1]);
}
i++;
}
while(!EmptyStack(&s)){
Pop(&s,&a);
strncat(PFstring,&a,1);
}
printf("The postfix expression is %s\n", PFstring);
}

Its only a portion of the program.I didnt include the function declaration but the prototype.The error i keep on getting is cannot convert from int* to char*.How do I change the code without changing the structure of my stack functions?I can affort to change the stack functions cause it will cause problem in my other functions.

[QuestionC]

Ung.... this is tough. I would normally suggest just writing up a different stack function to handle chars, but the following MAY work...

Pop(&s,&(int)a);

or...

Pop(&s, (int *) &a);