libc: add strnlen implementation

This is standard function and useful for application writers.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>

lib/libc/minimal/include/string.h

@@ -22,6 +22,7 @@ extern char  *strncpy(char *_MLIBC_RESTRICT d, const char *_MLIBC_RESTRICT s,
 extern char  *strchr(const char *s, int c);
 extern char  *strrchr(const char *s, int c);
 extern size_t strlen(const char *s);
+extern size_t strnlen(const char *s, size_t maxlen);
 extern int    strcmp(const char *s1, const char *s2);
 extern int    strncmp(const char *s1, const char *s2, size_t n);
 extern char *strcat(char *_MLIBC_RESTRICT dest,

lib/libc/minimal/source/string/string.c

@@ -116,6 +116,25 @@ size_t strlen(const char *s)
 	return n;
 }
 
+/**
+ *
+ * @brief Get fixed-size string length
+ *
+ * @return number of bytes in fixed-size string <s>
+ */
+
+size_t strnlen(const char *s, size_t maxlen)
+{
+	size_t n = 0;
+
+	while (*s != '\0' && n < maxlen) {
+		s++;
+		n++;
+	}
+
+	return n;
+}
+
 /**
  *
  * @brief Compare two strings