klibc: add some helper

core/klibc.h

@@ -1,6 +1,14 @@
 #pragma once
 #include "stdarg.h"
 
+#define islower(c)  (('a' <= (c)) && ((c) <= 'z'))
+#define isupper(c)  (('A' <= (c)) && ((c) <= 'Z'))
+#define isdigit(c)  (('0' <= (c)) && ((c) <= '9'))
+#define isspace(c)  (((c) == ' ') || ((c) == '\t') || \
+                    ((c) == '\f') || ((c) == '\n') || \
+                    ((c) == '\r') || ((c) == '\v'))
+#define isprint(c)  ((' ' <= (c)) && ((c) <= '~'))
+
 void *memcpy(void *dest, const void *src, size_t n);
 void *memset(void *s, int c, size_t n);
 char *itoa(int value, char *str, int base);