why printf can not print string of a pointer?

I write this program for print content of a string in c language. but dont print string!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define GROW_BY 10
int main(){
    char *ptr_s;
    int c;
    ptr_s= (char *) malloc(GROW_BY);

      printf("please enter a string\n");
     while((c=getchar() ) !='\n'){
         *ptr_s++=c;
     }
     *ptr_s=0;
     printf("the string is : \n %s \n", ptr_s);
}

result is : the string is:

do not print content of string in ptr_s.

where is problem?

the problem is use of original value of ptr_s in increment

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define GROW_BY 10
int main(){
    char *ptr_s, *copy_ptrs;
    int c;
    ptr_s= (char *) malloc(GROW_BY);
    copy_ptrs=ptr_s;

      printf("please enter a string\n");
     while((c=getchar() ) !='\n'){
         *copy_ptrs++=c;
     }
     *copy_ptrs=0;
     printf("the string is : \n %s \n", ptr_s);
}