The strncpy function copies at most len characters from src to dst. Characters are copied until a null character ('\0') is copied or until len characters have been copied. If the length of src is less than len the remaining bytes in dst are padded with null characters ('\0').

The strncpy function returns dst.

strcat, strcpy, strlen, strncat

#include <string.h>
#include <stdio.h> /* for printf */

void tst_strncpy (char *s) {
  char buf [21];

  strncpy (buf, s, sizeof (buf));
  buf [sizeof (buf) -1] = '\0';
}