diff --git a/chapter-5/5-5.c b/chapter-5/5-5.c
new file mode 100644
index 0000000..507b920
--- /dev/null
+++ b/chapter-5/5-5.c
@@ -0,0 +1,51 @@
+/* Exercise 5-5. Write versions of the library functions strncpy,
+strncat, and strncmp, which  operate on at most the first n
+characters of their argument strings. For example,
+strncpy(s,t,n) copies at most n characters of t to s. Full
+descriptions are in Appendix B.  */
+
+#include <stdio.h>
+
+void my_strncpy(char *s, char *t, int n) {
+  while (n-- > 0 && (*s++ = *t++))
+    ;
+}
+
+void my_strncat(char *s, char *t, int n) {
+  while (*s)
+    s++;
+  while (n-- > 0 && (*s++ = *t++))
+    ;
+}
+
+int my_strncmp(char *s, char *t, int n) {
+  while (n-- > 0 && (*s == *t)) {
+    s++;
+    t++;
+  };
+  return *s - *t;
+}
+
+int main() {
+  char s1[30] = "Hello, Gentlemen!";
+  char t1[30] = "1234567890:D";
+
+  // this is for printf
+  char *s_old = "Hello, Gentlemen!";
+
+  my_strncpy(s1, t1, 5);
+  printf("strncpy(s = %s,t = %s, n = %d ) = %s \n", s_old, t1, 5, s1);
+
+  //
+  char a[30] = "Hello, ";
+  char *b = "World!lol, not included";
+  my_strncat(a, b, 6);
+  printf("%s\n", a);
+
+  // tested on more cases.
+  char *smol = "123445";
+  char *larg = "123455";
+  printf("strncmp(s = %s,t = %s, n = %d ) = %d \n", smol, larg, 5,
+         my_strncmp(smol, larg, 5));
+  return 0;
+}