Converting infix to postfix not showing proper symbols

I was trying to write a code to convert infix expression into postfix expression , I have the source code here and when I compile and give the input say 'a+b' it returns 'ab?' as the answer it does not show the operator symbols at all. Can you guys please tell me where I have gone wrong.!!

It also shows this warning when I compile

infpos.c:74:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]

PS: I got to know what is the error.. Thank you

(The error was in the line 58 I had to use prefix increment but I had used postfix increment)

    #include<stdio.h>
    #include<ctype.h>
    #define max 30

    void main()
    {
        char inf[30],post[30];
        void convert(char [],char []);
                printf("Enter the infix exxpression\n");
                     scanf("%s",inf);

                convert(inf,post);

                printf("Postfix expression is\n");
                printf("%s\n",post);
    }

  void convert(char inf[],char post[])
  {
        int i,j=0,top=-1,f=1,test;
        char stack[30],ch,x;
        int check(char);
        int pre(char);

          for(i=0;inf[i]!='\0';i++)
          {
              ch = inf[i];
              test = check(ch);

              switch(test)
              {
                 case 1:
                    post[j++] = ch;
                    break;
                 case 2:
                    stack[++top] = ch;
                    break;
                 case 3:
                    while((x=stack[top--])!='(')
                        post[j++] = x;
                    break;
                 case 4:
                   do
                   {
                    if(top==-1)
                        f = 1;
                    else if(stack[top] == '(')
                        f = 1;
                    else if(pre(ch)>pre(stack[top]))
                        f = 1;
                    else
                    {
                        post[j++] = stack[top--];
                        f = 0;
                    }
                  }while(f==0);
                  stack[top++] = ch;
                  break;
           }
       }
          while(top!=-1)
              post[j++] = stack[top--];
         post[j] = '\0';
    }

     int pre(char op)
    {
        if(op == '+' || op=='-')
           return 1;
        else if(op == '/' || op=='*' || op =='%')
          return 2;
        else if(op=='^')
          return 3;
     }

      int check(char ch)
      {
            if(isalnum(ch))
              return 1;
           else if(ch=='(')
               return 2;
           else if(ch == ')')
               return 3;
           else if(ch == '+' || ch =='-' || ch=='/' || ch =='*' || ch=='%' || ch=='^')
             return 4;
          else 
               return 5;
    }