Why this postfix input calculator in c using stack of array doesn;t work?

Question

I am learning data structures and as I go through I needed to make a postfix input calculator in c.I wanted to make it using stack and stack of array.I wrote some code but it is not giving output instead it gives the first word i input in it.If I dont take input and give value in declaration of string this works but if I ask for input then it doesnt work.And i also tried to print strlength after the scanf it even if my input is 17 length it only prints 1.scan f is not working correctly or strlen is not working correctly.My code is :-

#include <stdio.h>
#include <string.h>
#include "stack_forpostfix.h"

int postfix(char *exp);
int isoperator1(char b);
int isnumericdigit1(char c);
int doevaluation(int oper1,char optr,int oper2);

int main(){
    char *exp1;
    int a;
    printf("Enter the postfix expresssion\n");/*if we dont ask and put input in declaration the code works perfect,because if we ask the strlen is not working*/
    scanf("%s",exp1);       //2 3 * 5 4 * + 9 -
    printf(" , %d , ",strlen(exp1));
    a=postfix(exp1);
    printf("%d\n",a);
}

int postfix(char *exp){
   
    for(int i=0;i<strlen(exp);i++){
        if(exp[i]==' ' || exp[i]==','){
            continue;
        }
        else if(isoperator1(exp[i])){
            int op2=gettop();
            pop();
            int op1=gettop();
            pop();
            int result=doevaluation(op1,exp[i],op2);
            push(result);
        }

        else if(isnumericdigit1(exp[i])){
            int oper=0;

            while(i<strlen(exp) && isnumericdigit1(exp[i])){
                oper=(oper*10)+(exp[i]-'0');
                i++;
            }  //since i++ is there if no i-- exp[i] will escape one further
            i--;
            push(oper); 
        }
    }
    return gettop();    
}

int isnumericdigit1(char c){
    if (c>='0' && c<='9'){
        return 1;
    }
    else return 0;
}

int isoperator1(char b){
    if(b=='+'||b=='-'||b=='*'||b=='/'){
        return 1;
    }
    else {
        return 0;
    }
}

int doevaluation(int oper1,char optr,int oper2){

    if(optr=='+'){
        return oper1+oper2;
    }else if (optr=='-'){
        return oper1-oper2;
    }else if (optr=='*'){
        return oper1*oper2;
    }else if (optr=='/'){
        return oper1/oper2;
    }else {
        printf("not valid");
        return -1;
    }
}

And my header file(stack_forpostfix.h) code is :-

#ifndef stackyfix
#define stackyfix

#define maxsize 111


int a[maxsize];
int top=-1;

void push(int r){
    top++;
    a[top]=r;
}

void pop(){
    top--;
}

int gettop(){
    return a[top];
}
#endif

Answer 1

In infix to postfix function, in place of str[i]=gettop(); there should be str[j]=gettop() so the expression entered inside the brackets can be pprocessed. Also add this piece of code after fgets to remove the '\n' that fgets may append.

fgets(b,sizeof(b),stdin);    

for(int i=0;b[i]!='\0';i++){ // removes \n added by fgets
        if(b[i]=='\0'){
            if(b[i-1]=='\n'){
                b[i-1]='\0';
            }
        }
    }