C language removes white space at the end of string

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Remove trailing white space   Include \t \n \r 
/*  Standard white space characters include ： ' ' (0x20) space (SPC)  Space character  '\t' (0x09) horizontal tab (TAB)  Horizontal tabs  '\n' (0x0a) newline (LF)  A newline  '\v' (0x0b) vertical tab (VT)  Vertical tabs  '\f' (0x0c) feed (FF)  Page identifier  '\r' (0x0d) carriage return (CR)  A carriage return  //windows \r\n linux \n mac \r */
char *rtrim(char *str)
{
    
    if (str == NULL || *str == '\0')
        {
    
              return str;
        }

    int len = strlen(str);
    char *p = str + len - 1;
    while (p >= str  && isspace(*p))
    {
    
         *p = '\0';
         --p;
     }
        return str;
 }

int main(){
    
                char dbsname[] = "abc1 23 ";
                char *c;
                int m ;
                for ( m=0; m<20 ;m++){
    
                        c=&dbsname[m];
                        printf("%c,%d-------\n",*c,*c);
                }
        printf("%s",dbsname);
        char *p = rtrim(dbsname);
        printf("%s\n",p);
                for ( m=0; m<20 ;m++){
    
                        printf("%c,%d-------\n",*p,*p);
                        p++;
                }

}