convert infix to postfix

[chemical]

Hi everyone.I got problem about my homework which contains converting to infix to postfix.
For example : if the user enters 5*6+8/4 as infix, its postfix will be 57*84/+
Here is my code bu it doesn't work and I couldn't understand where my fault is.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
struct yapi{
       char data;
       struct yapi *next;
       };
typedef struct yapi *stack;
void push(stack ptr,char x);
char pop(stack ptr);
int priority(char x) ;
void infixtopostfix(char *infix,char *postfix,stack ptr);
int main()
       { 
                        char in[50],post[50];
                        stack header;
                        header=(stack)malloc(sizeof(struct yapi));
                        header->next=NULL;
                        printf("Enter infix expression\n");
                        scanf("%s",in);
                        infixtopostfix(&in[0],&post[0],header);
                        printf("Postfix expression is %s\n",post);
                        system("pause");
                        return 0;
                        }
void push(stack ptr,char x){ 
            stack newnode;
            newnode=(stack)malloc(sizeof(struct yapi));
            newnode->next=ptr->next;
            newnode->data=x;
            ptr->next=newnode;
            }                                  
char pop(stack ptr){ 
     stack current; 
     char value;    
     ptr->next=current;
     value=current->data;
     ptr->next=ptr->next->next;
     free(current);
     return value;
     }                        
int priority(char x){
     int pri=0;
     if(x=='+' || x=='-'){
                  pri=1;
                      }  
     if(x=='*' || x=='/'){
              pri=2;
              }   
              return pri;
              }                     
void infixtopostfix(char *infix,char *postfix,stack ptr){
             char *i,*p;
             char character;
             i=&infix[0];
             p=&postfix[0];
             while(*i!='\0')
             {
                   if(isdigit(*i) || isalpha(*i)){
                                  *p=*i;
                                   p++;
                                   i++;
                                                  }
                   if(*i=='+' || *i=='-' || *i=='*' || *i=='/')
                   {
                              if(ptr->next==NULL)
                                      {
                                      push(ptr,*i);
                                      }
                        else 
                        {
                   character=pop(ptr);
                   if(priority(character)>=priority(*i)) 
                   {
                   *p=character;
                   p++;
                   push(ptr,*i);
                   }
                          else{
                                 push(ptr,character); 
                                 push(ptr,*i);    
                              }                                        
                        }   
                     i++;                 
                   }
             }
             while(ptr->next!=NULL){
                                   *p=pop(ptr);
                                   p++;
                                   }
                                   *p='\0';
                                    
             }

[tabstop]

Your compiler is giving you a big hint:

$ gcc -o i2p i2p.c -Wall -Wextra
i2p.c: In function ‘pop’:
i2p.c:36: warning: ‘current’ is used uninitialized in this function