Add standard C library function strnlen().

include/string.h

@@ -50,6 +50,7 @@ char* __fastcall__ strcpy (char* dest, const char* src);
 size_t __fastcall__ strcspn (const char* s1, const char* s2);
 char* __fastcall__ strerror (int errcode);
 size_t __fastcall__ strlen (const char* s);
+size_t __fastcall__ strnlen (const char* s, size_t maxlen);
 char* __fastcall__ strncat (char* s1, const char* s2, size_t count);
 int __fastcall__ strncmp (const char* s1, const char* s2, size_t count);
 char* __fastcall__ strncpy (char* dest, const char* src, size_t count);

libsrc/common/strnlen.s

@@ -0,0 +1,54 @@
+; size_t __fastcall__ strnlen (const char* s, size_t maxlen);
+
+.export     _strnlen
+.import     popax
+.importzp   ptr1, tmp1, tmp2, tmp3, tmp4
+
+.proc _strnlen
+        ; Fetch string pointer.
+        sta     ptr1
+        stx     ptr1+1
+
+        ; Clear return value.
+        lda     #0
+        sta     tmp1
+        sta     tmp2
+
+        ; Get maximum length.
+        jsr     popax
+        sta     tmp3
+        inc     tmp3
+        inx
+        stx     tmp4
+
+        ;;; Loop over string.
+        ldy     #0
+
+        ; Decrement maximum length.
+next:   dec     tmp3
+        bne     l2
+        dec     tmp4
+        beq     done
+l2:
+
+        lda     (ptr1),y
+        beq     done
+
+        ; Step to next character.
+        inc     ptr1
+        bne     l1
+        inc     ptr1+1
+l1:
+
+        ; Increment return value.
+        inc     tmp1
+        bne     next
+        inc     tmp2
+
+        jmp     next
+
+
+done:   lda     tmp1
+        ldx     tmp2
+        rts
+.endproc