append 1 character to the end of a string

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

char* f(char *s,char c)
{
  int len=strlen(s);
  s[len]=c;
  s[len+1]='\0';

  return s;

}



int main(void)
{
 char s[20]="Gummybear";
 char c='S';
 puts(f(s,c));




  return 0;
}