Converting an infix to a prefix using stacks in C - c

Question

I am trying to use stacks with linked lists to convert an infix expression to a prefix expression. Here is my code for the same:
#include <stdio.h>
#include <stdlib.h>
#define M 100
struct Stack{
char ele;
struct Stack *next;
};
struct Stack* next_node(char element){
struct Stack *node=(struct Stack *)malloc(sizeof(struct Stack));
node->ele=element;
node->next=NULL;
return node;
}
int isEmpty(struct Stack *node){
return node==NULL;
}
void push(struct Stack **node, char element){
struct Stack *temp=next_node(element);
temp->next=*node;
*node=temp;
}
char pop(struct Stack **node){
if(isEmpty(*node)){
return 0;
}
struct Stack *temp=*node;
*node=(*node)->next;
char revc=temp->ele;
free(temp);
return revc;
}
char* rev(char str[]){
int i, n;
for(n=0;str[n]!='\0';n++);
struct Stack *s=(struct Stack *)malloc(sizeof(struct Stack));
for(i=0;i<n;i++)
push(&s, str[i]);
for(i=0;i<n;i++)
str[i]=pop(&s);
if(str[i]=='(')
str[i]=')';
else if(str[i]==')')
str[i]='(';
return str;
}
int isVariable (char ch){
return (ch>='a' && ch<='z')||(ch>='A'&&ch<='Z');
}
int precedence(char ch){
switch(ch){
case '+':
case '-': return 1;
case '*':
case '/': return 2;
}
return -1;
}
char* postfix(char str[]){
int i, j=0;
struct Stack *s=NULL;
for(i=0;str[i]!='\0';i++){
if(isVariable(str[i]))
str[j++]=str[i];
else if(str[i]==')')
push(&s, str[i]);
else if(str[i]=='('){
while(!isEmpty(s)&&s->ele!=')')
str[j++]=pop(&s);
if(!isEmpty(s)&&s->ele!=')')
return 0;
else
pop(&s);
}
else{
while(!isEmpty(s)&&precedence(str[i])<=precedence(s->ele))
str[j++]=pop(&s);
push(&s, str[i]);
}
}
while(!isEmpty(s))
str[j++]=pop(&s);
str[j++]='\0';
return str;
}
void prefix(char str[]){
str=rev(str);
str=postfix(str);
str=rev(str);
printf("The prefix equivalent is: %s\n", str);
}
int main()
{
char string[M], op[1];
do{
printf("Enter the infix expression: ");
scanf("%s", string);
prefix(string);
printf("Do you want to go again?(Y/N): ");
scanf("%s", op);
}while(op[0]=='Y');
}
While the code works fine for expressions with no parantheses, it fails with expressions that do.
For example: when I input something like "(a+b-c) *d-(e+f)", my output is "- * a-b+cd+ef", when the output should be "- * -+abcd+ef".
Why is it happening? Any help is welcome! :-)