diff --git a/include/string.h b/include/string.h
index 2f5953196..83970493e 100644
--- a/include/string.h
+++ b/include/string.h
@@ -78,6 +78,7 @@ int __fastcall__ stricmp (const char* s1, const char* s2);    /* DOS/Windows */
 int __fastcall__ strcasecmp (const char* s1, const char* s2); /* Same for Unix */
 int __fastcall__ strnicmp (const char* s1, const char* s2, size_t count);     /* DOS/Windows */
 int __fastcall__ strncasecmp (const char* s1, const char* s2, size_t count);  /* Same for Unix */
+size_t __fastcall__ strnlen (const char* s, size_t maxlen);     /* POSIX.1-2008 */
 char* __fastcall__ strlwr (char* s);
 char* __fastcall__ strlower (char* s);
 char* __fastcall__ strupr (char* s);
diff --git a/libsrc/common/strnlen.s b/libsrc/common/strnlen.s
new file mode 100644
index 000000000..eb2c16006
--- /dev/null
+++ b/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
+        ; Get maximum length.
+        tay
+        iny
+        sty     tmp3
+        inx
+        stx     tmp4
+
+        ; Fetch string pointer.
+        jsr     popax
+        sta     ptr1
+        stx     ptr1+1
+
+        ; Clear return value.
+        ldy     #0
+        sty     tmp1
+        sty     tmp2
+
+
+        ;;; Loop over string.
+        ; Decrement maximum length.
+next:   dec     tmp3
+        bne     l2
+        dec     tmp4
+        beq     done
+l2:
+
+        lda     (ptr1),y
+        beq     done
+
+        ; Step to next character.
+        iny
+        bne     l1
+        inc     ptr1+1
+l1:
+
+        ; Increment return value.
+        inc     tmp1
+        bne     next
+        inc     tmp2
+
+        bne     next        ; (jmp)
+
+
+done:   lda     tmp1
+        ldx     tmp2
+        rts
+.endproc
diff --git a/targettest/Makefile b/targettest/Makefile
index f3694335c..44933949a 100644
--- a/targettest/Makefile
+++ b/targettest/Makefile
@@ -160,6 +160,7 @@ EXELIST_c64 = \
 	scanf-test \
 	ser-test \
 	strdup-test \
+	strnlen \
 	stroserror-test \
 	strqtok-test \
 	tinyshell \
@@ -190,6 +191,7 @@ EXELIST_vic20 = \
 	rename-test \
 	scanf-test \
 	strdup-test \
+	strnlen \
 	stroserror-test \
 	strqtok-test \
 	tinyshell \
@@ -222,6 +224,7 @@ EXELIST_apple2 = \
 	seek \
 	ser-test \
 	strdup-test \
+	strnlen \
 	stroserror-test \
 	strqtok-test \
 	tinyshell \
@@ -257,6 +260,7 @@ EXELIST_atari = \
 	seek \
 	ser-test \
 	strdup-test \
+	strnlen \
 	stroserror-test \
 	strqtok-test \
 	tinyshell \
diff --git a/targettest/strnlen.c b/targettest/strnlen.c
new file mode 100644
index 000000000..ac39f2396
--- /dev/null
+++ b/targettest/strnlen.c
@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+const char * str = "0123456789";
+
+void
+check (size_t result, size_t expected)
+{
+    if (result != expected) {
+        printf ("Expected strnlen() to return %u, got %u.\n",
+                expected, result);
+        exit (EXIT_FAILURE);
+    }
+}
+
+int
+main (void)
+{
+    size_t  maxlen = strlen (str);
+    size_t  result;
+    size_t  expected;
+
+    for (expected = 0; expected < maxlen; expected++)
+        check (strnlen (str, expected), expected);
+
+    check (strnlen (str, maxlen << 1), maxlen);
+
+    printf ("strnlen() OK.\n");
+
+    return EXIT_SUCCESS;
+}