c program to infix expression to postfix expression

#include<stdio.h>
#include<ctype.h>
#include<string.h>
#include<conio.h>
static char str[30];
int top=-1;
int priority(char c);
void push(char c);
int pop();
void main()
{
char in[30],post[30],ch;
int i,j,l;
clrscr();
printf("enter the string");
gets(in);
l=strlen(in);
printf("%d\n",l);

for(i=0,j=0;i<l;i++)
{
  if(isalpha(in[i]))
 post[j++]=in[i];
 else
{
  if(in[i]=='(')
push(in[i]);
else if(in[i]==')')
while((ch=pop())!='(')
post[j++]=ch;
else
  {
while(priority(in[i])<=priority(str[top]))
 post[j++]=pop();
 push(in[i]);
  }  // End else
 }   // End else
}   // End for loop

while(top!=-1)
post[j++]=pop();
 post[j]='\0';
 printf("\n equivalent infix to postfix is:%s",post);
 getch();
}  //End main
int priority (char c)
{
 switch(c)
 {
case'+':
case'-': return 1;
case'*':
case'/': return 2;
case '^':return 3;
  }
  return 0;
}
void push(char c)
{
str[++top]=c;
}
int pop()
{
  return(str[top--]);
}