[src/trunk]: src/share/misc style(5): Forbid extern in .c files.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/bef0a033cf6a
branches:  trunk
changeset: 374064:bef0a033cf6a
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Thu Mar 30 10:39:30 2023 +0000

description:
style(5): Forbid extern in .c files.

Discussed on tech-userlevel (bcc tech-kern):
https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

diffstat:

 share/misc/style |  27 +++++++++++++++++++++++----
 1 files changed, 23 insertions(+), 4 deletions(-)

diffs (56 lines):

diff -r b401dc85a9bf -r bef0a033cf6a share/misc/style
--- a/share/misc/style  Wed Mar 29 21:44:35 2023 +0000
+++ b/share/misc/style  Thu Mar 30 10:39:30 2023 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: style,v 1.68 2023/02/14 18:56:10 jschauma Exp $ */
+/* $NetBSD: style,v 1.69 2023/03/30 10:39:30 riastradh Exp $ */
 
 /*
  * The revision control tag appears first, with a blank line after it.
@@ -30,7 +30,7 @@
 #include <sys/cdefs.h>
 __COPYRIGHT("@(#) Copyright (c) 2008\
  The NetBSD Foundation, inc. All rights reserved.");
-__RCSID("$NetBSD: style,v 1.68 2023/02/14 18:56:10 jschauma Exp $");
+__RCSID("$NetBSD: style,v 1.69 2023/03/30 10:39:30 riastradh Exp $");
 
 /*
  * VERY important single-line comments look like this.
@@ -56,6 +56,23 @@
  */
 #ifndef _SYS_SOCKET_H_
 #define _SYS_SOCKET_H_
+
+/*
+ * extern declarations must only appear in header files, not in .c
+ * files, so the same declaration is used by the .c file defining it
+ * and the .c file using it, giving the compiler the opportunity to
+ * detect type errors.
+ *
+ * extern function declarations should not use the extern keyword,
+ * which is unnecessary.
+ *
+ * Exception: A subroutine written in assembly in an adjacent .S file,
+ * which is used only in one .c file, may be declared in the .c file.
+ */
+extern int frotz;
+
+int frobnicate(const char *);
+
 /*
  * Contents of #include file go between the #ifndef and the #endif at the end.
  */
@@ -345,9 +362,11 @@ function(int a1, int a2, float fl, int a
         * declarations next to their first use, and initialize
         * opportunistically. This avoids over-initialization and
         * accidental bugs caused by declaration reordering.
+        *
+        * Never declare extern variables in .c files.  Declare them in the
+        * appropriate .h file shared by the .c file where they are defined
+        * and the .c file where they are used.
         */
-       extern u_char one;
-       extern char two;
        struct foo three, *four;
        double five;
        int *six, seven;