Make _REENTRANT and _THREAD_SAFE aliases for _POSIX_C_SOURCE=199506L.

For many years, the only effect of these macros has been to make
unistd.h declare getlogin_r.  _POSIX_C_SOURCE >= 199506L also causes
this function to be declared.  However, people who don't carefully
read all the headers might be confused into thinking they need to
define _REENTRANT for any threaded code (as was indeed the case a long
time ago).

Therefore, remove __USE_REENTRANT, and make _REENTRANT and _THREAD_SAFE
into synonyms for _POSIX_C_SOURCE=199506L.  This will only affect
programs that don't select a higher conformance level some other way.
For instance, -std=c89 -D_REENTRANT will see a change in visible
declarations, but -std=c99 -D_POSIX_C_SOURCE=200809L -D_REENTRANT won't,
and -D_REENTRANT all by itself also won't, because _DEFAULT_SOURCE
implies _POSIX_C_SOURCE > 199506.

	* include/features.h: Remove __USE_REENTRANT.  Treat _REENTRANT
	and _THREAD_SAFE the same as _POSIX_C_SOURCE=199506L, if a higher
	POSIX conformance level has not been selected by other macros.
	* NEWS, manual/creature.texi: Document this change.

	* posix/unistd.h, posix/bits/unistd.h: Don't check __USE_REENTRANT.
	* include/libc-symbols.h: Don't define _REENTRANT.
	* scripts/check-installed-headers.sh: Don't undefine _REENTRANT.

7 files changed, 47 insertions, 28 deletions

diff --git a/NEWS b/NEWS
index 922a5004ff..4fe8886e83 100644
--- a/NEWS
+++ b/NEWS
@@ -22,6 +22,19 @@ Version 2.25
   from that TS.  Note that most features from that TS are not supported by
   the GNU C Library.
 
+* The nonstandard feature selection macros _REENTRANT and _THREAD_SAFE are
+  now treated as compatibility synonyms for _POSIX_C_SOURCE=199506L.
+  Since the GNU C Library defaults to a much newer revision of POSIX, this
+  will only affect programs that specifically request an old conformance
+  mode.  For instance, a program compiled with -std=c89 -D_REENTRANT will
+  see a change in the visible declarations, but a program compiled with
+  just -D_REENTRANT, or -std=c99 -D_POSIX_C_SOURCE=200809L -D_REENTRANT,
+  will not.
+
+  Some C libraries once required _REENTRANT and/or _THREAD_SAFE to be
+  defined by all multithreaded code, but glibc has not required this for
+  many years.
+
 * The inclusion of <sys/sysmacros.h> by <sys/types.h> is deprecated.  This
   means that in a future release, the macros “major”, “minor”, and “makedev”
   will only be available from <sys/sysmacros.h>.
diff --git a/include/features.h b/include/features.h
index 650d4c5e12..4d8bda1bc6 100644
--- a/include/features.h
+++ b/include/features.h
@@ -24,10 +24,13 @@
    __STRICT_ANSI__	ISO Standard C.
    _ISOC99_SOURCE	Extensions to ISO C89 from ISO C99.
    _ISOC11_SOURCE	Extensions to ISO C99 from ISO C11.
-   __STDC_WANT_LIB_EXT2__ Extensions to ISO C99 from TR 27431-2:2010.
-   __STDC_WANT_IEC_60559_BFP_EXT__ Extensions to ISO C11 from TS 18661-1:2014.
-   __STDC_WANT_IEC_60559_FUNCS_EXT__ Extensions to ISO C11 from
-			TS 18661-4:2015.
+   __STDC_WANT_LIB_EXT2__
+			Extensions to ISO C99 from TR 27431-2:2010.
+   __STDC_WANT_IEC_60559_BFP_EXT__
+			Extensions to ISO C11 from TS 18661-1:2014.
+   __STDC_WANT_IEC_60559_FUNCS_EXT__
+			Extensions to ISO C11 from TS 18661-4:2015.
+
    _POSIX_SOURCE	IEEE Std 1003.1.
    _POSIX_C_SOURCE	If ==1, like _POSIX_SOURCE; if >=2 add IEEE Std 1003.2;
 			if >=199309L, add IEEE Std 1003.1b-1993;
@@ -45,10 +48,12 @@
    _GNU_SOURCE		All of the above, plus GNU extensions.
    _DEFAULT_SOURCE	The default set of features (taking precedence over
 			__STRICT_ANSI__).
-   _REENTRANT		Select additionally reentrant object.
-   _THREAD_SAFE		Same as _REENTRANT, often used by other systems.
-   _FORTIFY_SOURCE	If set to numeric value > 0 additional security
-			measures are defined, according to level.
+
+   _FORTIFY_SOURCE	Add security hardening to many library functions.
+			Set to 1 or 2; 2 performs stricter checks than 1.
+
+   _REENTRANT, _THREAD_SAFE
+			Obsolete; equivalent to _POSIX_C_SOURCE=199506L.
 
    The `-ansi' switch to the GNU C compiler, and standards conformance
    options such as `-std=c99', define __STRICT_ANSI__.  If none of
@@ -86,7 +91,6 @@
    __USE_MISC		Define things from 4.3BSD or System V Unix.
    __USE_ATFILE		Define *at interfaces and AT_* constants for them.
    __USE_GNU		Define GNU extensions.
-   __USE_REENTRANT	Define reentrant/thread-safe *_r functions.
    __USE_FORTIFY_LEVEL	Additional security measures used, according to level.
 
    The macros `__GNU_LIBRARY__', `__GLIBC__', and `__GLIBC_MINOR__' are
@@ -130,7 +134,6 @@
 #undef	__USE_MISC
 #undef	__USE_ATFILE
 #undef	__USE_GNU
-#undef	__USE_REENTRANT
 #undef	__USE_FORTIFY_LEVEL
 #undef	__KERNEL_STRICT_NAMES
 
@@ -253,6 +256,7 @@
 # undef  _POSIX_C_SOURCE
 # define _POSIX_C_SOURCE	200809L
 #endif
+
 #if ((!defined __STRICT_ANSI__					\
       || (defined _XOPEN_SOURCE && (_XOPEN_SOURCE - 0) >= 500))	\
      && !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE)
@@ -269,6 +273,19 @@
 # define __USE_POSIX_IMPLICITLY	1
 #endif
 
+/* Some C libraries once required _REENTRANT and/or _THREAD_SAFE to be
+   defined in all multithreaded code.  GNU libc has not required this
+   for many years.  We now treat them as compatibility synonyms for
+   _POSIX_C_SOURCE=199506L, which is the earliest level of POSIX with
+   comprehensive support for multithreaded code.  Using them never
+   lowers the selected level of POSIX conformance, only raises it.  */
+#if ((!defined _POSIX_C_SOURCE || (_POSIX_C_SOURCE - 0) < 199506L) \
+     && (defined _REENTRANT || defined _THREAD_SAFE))
+# define _POSIX_SOURCE   1
+# undef  _POSIX_C_SOURCE
+# define _POSIX_C_SOURCE 199506L
+#endif
+
 #if (defined _POSIX_SOURCE					\
      || (defined _POSIX_C_SOURCE && _POSIX_C_SOURCE >= 1)	\
      || defined _XOPEN_SOURCE)
@@ -351,10 +368,6 @@
 # define __USE_GNU	1
 #endif
 
-#if defined _REENTRANT || defined _THREAD_SAFE
-# define __USE_REENTRANT	1
-#endif
-
 #if defined _FORTIFY_SOURCE && _FORTIFY_SOURCE > 0
 # if !defined __OPTIMIZE__ || __OPTIMIZE__ <= 0
 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index 1c91f2e6a3..4238d7930b 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -52,8 +52,6 @@
 
 /* Enable declarations of GNU extensions, since we are compiling them.  */
 #define _GNU_SOURCE	1
-/* And we also need the data for the reentrant functions.  */
-#define _REENTRANT	1
 
 #include <config.h>
 
diff --git a/manual/creature.texi b/manual/creature.texi
index 257f8718b1..5048886292 100644
--- a/manual/creature.texi
+++ b/manual/creature.texi
@@ -219,15 +219,11 @@ cause them to be disabled.
 @comment GNU
 @defvr Macro _REENTRANT
 @defvrx Macro _THREAD_SAFE
-If you define one of these macros, reentrant versions of several functions get
-declared.  Some of the functions are specified in POSIX.1c but many others
-are only available on a few other systems or are unique to @theglibc{}.
-The problem is the delay in the standardization of the thread safe C library
-interface.
-
-Unlike on some other systems, no special version of the C library must be
-used for linking.  There is only one version but while compiling this
-it must have been specified to compile as thread safe.
+These macros are obsolete.  They have the same effect as defining
+@code{_POSIX_C_SOURCE} with the value @code{199506L}.
+
+Some very old C libraries required one of these macros to be defined
+for basic functionality (e.g.@: @code{getchar}) to be thread-safe.
 @end defvr
 
 We recommend you use @code{_GNU_SOURCE} in new programs.  If you don't
diff --git a/posix/bits/unistd.h b/posix/bits/unistd.h
index 0105f04f20..4a5f42e120 100644
--- a/posix/bits/unistd.h
+++ b/posix/bits/unistd.h
@@ -302,7 +302,7 @@ __NTH (ttyname_r (int __fd, char *__buf, size_t __buflen))
 }
 
 
-#if defined __USE_REENTRANT || defined __USE_POSIX199506
+#ifdef __USE_POSIX199506
 extern int __getlogin_r_chk (char *__buf, size_t __buflen, size_t __nreal)
      __nonnull ((1));
 extern int __REDIRECT (__getlogin_r_alias, (char *__buf, size_t __buflen),
diff --git a/posix/unistd.h b/posix/unistd.h
index 625ba77da7..392dc0e004 100644
--- a/posix/unistd.h
+++ b/posix/unistd.h
@@ -849,7 +849,7 @@ extern int tcsetpgrp (int __fd, __pid_t __pgrp_id) __THROW;
    This function is a possible cancellation point and therefore not
    marked with __THROW.  */
 extern char *getlogin (void);
-#if defined __USE_REENTRANT || defined __USE_POSIX199506
+#ifdef __USE_POSIX199506
 /* Return at most NAME_LEN characters of the login name of the user in NAME.
    If it cannot be determined or some other error occurred, return the error
    code.  Otherwise return 0.
diff --git a/scripts/check-installed-headers.sh b/scripts/check-installed-headers.sh
index a245fe6be2..89e1514c7b 100644
--- a/scripts/check-installed-headers.sh
+++ b/scripts/check-installed-headers.sh
@@ -143,7 +143,6 @@ EOF
    inappropriate for this test.  */
 #undef _LIBC
 #undef _GNU_SOURCE
-#undef _REENTRANT
 /* The library mode is selected here rather than on the command line to
    ensure that this selection wins. */
 $expanded_lib_mode