Add rt_strnstr function and its configuration options to support finding substrings within a specified maximum length

Author: youzuwei

include/klibc/kstring.h

@@ -25,6 +25,7 @@ rt_int32_t rt_memcmp(const void *cs, const void *ct, rt_size_t count);
 char *rt_strdup(const char *s);
 rt_size_t rt_strnlen(const char *s, rt_ubase_t maxlen);
 char *rt_strstr(const char *str1, const char *str2);
+char *rt_strnstr(const char *s1, const char *s2, rt_size_t maxlen);
 rt_int32_t rt_strcasecmp(const char *a, const char *b);
 char *rt_strcpy(char *dst, const char *src);
 char *rt_strncpy(char *dest, const char *src, rt_size_t n);

src/klibc/Kconfig

@@ -178,6 +178,12 @@ menu "klibc options"
         endif
     endmenu # rt_strstr options
 
+    menu "rt_strnstr options"
+        config RT_KLIBC_USING_USER_STRNSTR
+            bool "Enable rt_strnstr to use user-defined version"
+            default n
+    endmenu # rt_strnstr options
+
     menu "rt_strcasecmp options"
         config RT_KLIBC_USING_USER_STRCASECMP
             bool "Enable rt_strcasecmp to use user-defined version"

src/klibc/kstring.c

@@ -313,6 +313,62 @@ char *rt_strstr(const char *s1, const char *s2)
 #endif /* RT_KLIBC_USING_USER_STRSTR */
 RTM_EXPORT(rt_strstr);
 
+/**
+ * @brief  This function will return the first occurrence of a string, without the
+ * terminator '\0'.
+ *
+ * @param  s1 is the source string.
+ *
+ * @param  s2 is the find string.
+ *
+ * @param  maxlen is the max size.
+ *
+ * @return The first occurrence of a s2 in s1, or RT_NULL if no found.
+ */
+#ifndef RT_KLIBC_USING_USER_STRNSTR
+char *rt_strnstr(const char *s1, const char *s2, rt_size_t maxlen)
+{
+    rt_size_t l1 = 0, l2 = 0;
+    int max_pos = 0;
+    
+    l2 = rt_strlen(s2);
+    if (!l2) 
+    {
+        return (char *)s1;
+    }
+
+    l1 = rt_strlen(s1);
+    if (!l1)
+    {
+        return RT_NULL;
+    }
+    
+    max_pos = maxlen < l1 ? maxlen : l1;
+    if (max_pos < l2)
+    {
+        return RT_NULL;
+    }
+
+    max_pos = max_pos - l2;
+    while (max_pos >= 0)
+    {
+        if (*s1 == *s2) 
+        {
+            if (!rt_strncmp(s1, s2, l2)) 
+            {
+                return (char *)s1;
+            }
+        }
+
+        s1++;
+        max_pos--;
+    }
+
+    return RT_NULL;
+}
+#endif /* RT_KLIBC_USING_USER_STRNSTR */
+RTM_EXPORT(rt_strnstr);
+
 /**
  * @brief  This function will compare two strings while ignoring differences in case
  *