diff --git a/include/ctype.h b/include/ctype.h
index dee281da..b0c7889f 100644
--- a/include/ctype.h
+++ b/include/ctype.h
@@ -31,6 +31,7 @@
 #include <endian.h>
 int isdigit (int);
 int isxdigit (int);
+int isspace (int);
 #endif // ! (__GNUC__ && POSIX)
 
 #endif // __MES_CTYPE_H
diff --git a/lib/libc.c b/lib/libc.c
index 0e889feb..bfef3eec 100644
--- a/lib/libc.c
+++ b/lib/libc.c
@@ -41,6 +41,12 @@ isxdigit (int c)
   return isdigit (c) || (c>='a') && (c<='f');
 }
 
+int
+isspace (int c)
+{
+  return (c == '\t' || c == '\n' || c == '\v' || c == '\f' || c == '\r' || c == ' ');
+}
+
 int
 isnumber (int c, int base)
 {