sys/string_utils: add memchk()

Check if all bytes in a buffer are set to s certain value - inverse
of memset().

sys/include/string_utils.h

@@ -91,6 +91,18 @@ static inline void explicit_bzero(void *dest, size_t n_bytes)
  */
 ssize_t strscpy(char *dest, const char *src, size_t count);
 
+/**
+ * @brief   Check if the entire buffer is filled with the same byte.
+ *
+ * @param[in]   data    The buffer to probe
+ * @param[in]   c       The byte to check of
+ * @param[in]   len     Size of the buffer
+ *
+ * @return NULL if the entire buffer is filled with @p c
+ * @return pointer to the first non-matching byte
+ */
+const void *memchk(const void *data, uint8_t c, size_t len);
+
 #ifdef __cplusplus
 }
 #endif

sys/libc/string.c

@@ -37,4 +37,17 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
         return -E2BIG;
     }
 }
+
+const void *memchk(const void *data, uint8_t c, size_t len)
+{
+    const uint8_t *end = (uint8_t *)data + len;
+    for (const uint8_t *d = data; d != end; ++d) {
+        if (c != *d) {
+            return d;
+        }
+    }
+
+    return NULL;
+}
+
 /** @} */