how does the getint() function from "THE C PROGRAMMING BOOK" works ?

I am going through the "C Programming Book" by K&R.
Now the code for the function "getint()" is as follows :-->

Code:

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#include<stdio.h>
#include"getch.h"


int getint(int *pn) {
    int c, sign;
   
    while(isspace(c = getch()));
   
    if(!isdigit(c) && c != EOF && c != '-' && c != '+') {
        ungetch(c);
        return 0;
    }
   
    sign = (c == '-')?-1:1;
    if(c == '+' || c == '-')
        c = getch();
       
    *pn = 0;
    while(isdigit(c)) {
        *pn = (*pn * 10) + (c - '0');
        c = getch();
    }
   
    *pn *= sign;
   
    if(c != EOF)
        ungetch(c);


    return c;
}


int main(int argc, char** argv) {
   
    int r, i;
   
    while((r = getint(&i)) != EOF)
        if(r != 0)
            printf("res: %d\n", i);
           
    return 0;
}

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Now I don't get the step by step working procedure of this function, even though I tried to run it theoretically on a paper.

And the fact that when I input "23". how does it converted to 23 , I know there is the logic to convert "23" to 23 but c = getch() doesn't store the remaining "3" in the buffer after input then how does it get back the 3, during the conversion.
Does getchar() have it's own buffer where it stores all the inout characters and fetch them 1 by 1.
Any help is highly appriciated.

> Does getchar() have it's own buffer where it stores all the inout characters and fetch them 1 by 1.
Yes.

Or more specifically, the standard input stream is usually line buffered, meaning the OS / standard library will buffer characters outside your program until return is pressed. Once pressed, your program code will read each character in turn up to the newline. At which point, the whole thing begins again.