git: f6ed9cd68c28 - main - cdefs.h: Document the _XOPEN_SOURCE - 0 construct

The branch main has been updated by imp:

URL: https://cgit.FreeBSD.org/src/commit/?id=f6ed9cd68c28b18082773c6f77cee4b8492dcfe6

commit f6ed9cd68c28b18082773c6f77cee4b8492dcfe6
Author:     Warner Losh <imp@FreeBSD.org>
AuthorDate: 2024-11-14 23:53:13 +0000
Commit:     Warner Losh <imp@FreeBSD.org>
CommitDate: 2024-11-15 02:01:12 +0000

    cdefs.h: Document the _XOPEN_SOURCE - 0 construct
    
    Various System V Interface Definition editions, as well as the X/Open
    group portability guide issue 4, recommend defining _XOPEN_SOURCE and
    broadly intimating it means the same thing as _POSIX_SOURCE == 2.
    
    Starting in X/Open issue 5 (1995), _XOPEN_SOURCE needs to be defined to
    be 500 to bring in the newer interfaces. However, it is still common hat
    sources define _XOPEN_SOURCE to be blank. To deal with that, we subtract
    0 from _XOPEN_SOURCE to make the other expressions well formed.
    
    While here, document that we should define _POSIX_C_SOURCE to be 199209
    based on the SVID, the first version of the Single Unix Specification,
    and X/Open CAE issue 4, version 2. Also document that historically this
    has been a NOP. Any value of _XOPEN_SOURCE < 500 (including it being
    blank) was not viewed as a request for a restricted namespace.
    
    Reviewed by:            brooks (earlier version)
    Differential Revision:  https://reviews.freebsd.org/D47584
    Sponsored by:           Netflix
---
 sys/sys/cdefs.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 4100cc0de7a5..0f53ff816527 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -513,7 +513,16 @@
 #define	_POSIX_C_SOURCE		199209
 #endif
 
-/* Deal with various X/Open Portability Guides and Single UNIX Spec. */
+/*
+ * Deal with various X/Open Portability Guides and Single UNIX Spec. We use the
+ * '- 0' construct so software that defines _XOPEN_SOURCE to nothing doesn't
+ * cause errors. X/Open CAE Specification, August 1994, System Interfaces and
+ * Headers, Issue 4, Version 2 section 2.2 states an empty definition means the
+ * same thing as _POSIX_C_SOURCE == 2. This broadly mirrors "System V Interface
+ * Definition, Fourth Edition", but earlier editions suggest some ambiguity.
+ * However, FreeBSD has histoically implemented this as a NOP, so we just
+ * document what it should be for now to not break ports gratuitously.
+ */
 #ifdef _XOPEN_SOURCE
 #if _XOPEN_SOURCE - 0 >= 800
 #define	__XSI_VISIBLE		800
@@ -531,6 +540,8 @@
 #define	__XSI_VISIBLE		500
 #undef _POSIX_C_SOURCE
 #define	_POSIX_C_SOURCE		199506
+#else
+/* #define	_POSIX_C_SOURCE		199209 */
 #endif
 #endif