The software is licensed under GPL-2.0-or-later

The software is licensed under GPL-2.0-or-later. But list.c and list.h
contain source code I edited from book "C Primer Plus" Chapter17
practice problem 2 which contain codes from its example codes, and the
author of "C Primer Plus" does not have a license for list.c and list.h,
so the author of "C Primer Plus" has the copyright on list.c and list.h,
so list.c and list.h can be considered non-free close souce proprietary
code.

I choose GPL-2.0-or-later. Because I prefer GPL over LGPL. And because
libssh's LGPL-2.1-or-later, plplot's LGPL-2.0-or-later, and gtk's
LGPL-2.1-or-later licenses are compatible with GPL-2.0-or-later.

This commit also clarify licensing by copy the GPL-2.0-or-later license
to LICENSE file, adding notes in README.md, and adding
SPDX-License-Identifier and other licensing comment in source code
files.

1 files changed, 5 insertions, 2 deletions

diff --git a/list.h b/list.h
index d5aef17..f83471d 100644
--- a/list.h
+++ b/list.h
@@ -1,5 +1,8 @@
-// references:
-// edited from "C Primer Plus" Chapter17 practice problem 2
+// list.c and list.h contain source code I edited from book "C Primer Plus"
+// Chapter17 practice problem 2 which contain codes from its example codes, and
+// the author of "C Primer Plus" does not have a license for list.c and list.h,
+// so the author of "C Primer Plus" has the copyright on list.c and list.h, so
+// list.c and list.h can be considered non-free close souce proprietary code.
 
 /*list.h--简单列表类型的头文件*/
 #ifndef LIST_H_