Conversion from Infix to Postfix in C

#include<stdio.h>
#include<conio.h>
#include<process.h>
#include<string.h>
#include<ctype.h>
#define max 30

struct stack
{
char opstack[max]; int tos;
};

void push(struct stack *,char); char pop(struct stack *); int prece(char );

void main()
{
int i,j=0,k,l; char ch,infix[max], postfix[max]; stack pq; pq.tos=-1; A: clrscr(); printf("\nEnter the infix string:\t"); gets(infix); for(i=0;infix[i]!='\0';i++)
{
if (infix[i]=='(') push(&pq,infix[i]); else if (isalpha(infix[i])) { postfix[j]=infix[i]; j++; } else if (infix[i]=='+'||infix[i]=='-'||infix[i]=='/'||infix[i]=='$'||infix[i]=='*') { if(pq.tos!=-1 && prece(pq.opstack[pq.tos])>=prece(infix[i])) {ch=pop(&pq); postfix[j]=ch; j++; } push(&pq,infix[i]); } else if (infix[i]==')') { for(;pq.opstack[pq.tos]=='(';) l=pq.tos; for(k=-1;k<l;k++) { postfix[j]=pop(&pq); j++; } } else { printf("\nError: Your infix contains invalid characters. Press any key to try again.\t"); getch(); goto A; }
}
while (pq.opstack[pq.tos]!=NULL) { postfix[j]=pop(&pq); j++; } postfix[j]='\0'; for(i=0;i<j;i++)
{
if(postfix[i]!='(') printf("%c",postfix[i]); } getch(); }

int prece(char a)
{
int q; switch (a) { case '$': q=4; break;

case '*': case '/': q=3; break;

case '+': case '-': q=2; break;

case '(': q=1; } return(q); }

void push(struct stack *ps,char b)
{
ps->tos++; ps->opstack[ps->tos]=b;
}

char pop(struct stack *ps)
{
char p; p=ps->opstack[ps->tos]; ps->tos--; return(p);