diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0af8e42..dd2baa6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -73,6 +73,7 @@ set(SOURCES
   src/libc/ctype/isxdigit.c
   src/libc/ctype/tolower.c
   src/libc/ctype/toupper.c
+  src/libc/errno/errno.c
   src/libc/locale/setlocale.c
   src/libc/locale/localeconv.c
   src/libc/stdio/vsnprintf.c
diff --git a/include/errno.h b/include/errno.h
new file mode 100644
index 0000000..41f0201
--- /dev/null
+++ b/include/errno.h
@@ -0,0 +1,12 @@
+#ifndef __ERRNO_H__
+# define __ERRNO_H__
+
+/* Initialized to 0 at startup. Currently not TLS, maybe in the future if
+   threads are supported. */
+extern int errno;
+
+#define EDOM       1
+#define EILSEQ     2
+#define ERANGE     3
+
+#endif /*__ERRNO_H__*/
diff --git a/src/libc/errno/errno.c b/src/libc/errno/errno.c
new file mode 100644
index 0000000..01c99f8
--- /dev/null
+++ b/src/libc/errno/errno.c
@@ -0,0 +1,4 @@
+#include <errno.h>
+
+/* The initial value is 0 at program startup */
+int errno = 0;