From aeb12302660810e8e4d5f7ac5604a604a68c1422 Mon Sep 17 00:00:00 2001
From: Juergen Fitschen <me@jue.yt>
Date: Tue, 12 May 2020 17:39:25 +0200
Subject: [PATCH] sys/fmt: add fmt_hex_bytes size probing

---
 sys/fmt/fmt.c                         | 5 +++++
 tests/unittests/tests-fmt/tests-fmt.c | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/sys/fmt/fmt.c b/sys/fmt/fmt.c
index 0d012ed1cc46..ae47fbf3875d 100644
--- a/sys/fmt/fmt.c
+++ b/sys/fmt/fmt.c
@@ -160,6 +160,11 @@ size_t fmt_hex_bytes(uint8_t *out, const char *hex)
     }
 
     size_t final_len = len >> 1;
+
+    if (out == NULL) {
+        return final_len;
+    }
+
     for (size_t i = 0, j = 0; j < final_len; i += 2, j++) {
         out[j] = fmt_hex_byte(hex + i);
     }
diff --git a/tests/unittests/tests-fmt/tests-fmt.c b/tests/unittests/tests-fmt/tests-fmt.c
index e321dc99cc1b..75138a986c40 100644
--- a/tests/unittests/tests-fmt/tests-fmt.c
+++ b/tests/unittests/tests-fmt/tests-fmt.c
@@ -176,6 +176,9 @@ static void test_fmt_hex_bytes(void)
     TEST_ASSERT_EQUAL_INT(0, val);
     TEST_ASSERT_EQUAL_INT(0, bytes);
 
+    bytes = fmt_hex_bytes(NULL, "ABCDEF");
+    TEST_ASSERT_EQUAL_INT(3, bytes);
+
     char hex2[3] = "00";
     uint8_t val1[1] = { 0 };
     bytes = fmt_hex_bytes(val1, hex2);