Clean up conditionals for declaration of gets.

gets has the dubious honor of being the only C89 library feature that
has been completely removed from the current C and C++ standards.
glibc follows suit by not declaring it in _GNU_SOURCE mode either,
but it remains present in older compatibility modes.  Internally,
two test cases need to see stdio.h make the declaration, but all our
internal code is of course compiled under _GNU_SOURCE.  This is currently
kludged by duplicating the gets declaration, fortify wrapper and all,
in include/stdio.h.  Also, the conditional in the public headers for
deciding when to declare gets is complicated and repeated in two places.

This patch adds a new macro to features.h that encapsulates the
complicated rule for when to declare gets.  stdio.h and bits/stdio2.h
then simply test __GLIBC_USE (DEPRECATED_GETS), and instead of having
a duplicate gets declaration in include/stdio.h, debug/tst-chk1.c and
stdio-common/tst-gets.c can force gets to be declared.

        * include/features.h (__GLIBC_USE_DEPRECATED_GETS): New macro.
        * libio/stdio.h, libio/bits/stdio2.h: Condition gets on
        __GLIBC_USE (DEPRECATED_GETS).  Update comments to indicate
        gets was removed from C++ in C++14.
        * include/stdio.h: Remove redundant declaration of gets.
        * debug/tst-chk1.c, stdio-common/tst-gets.c: Force gets to
        be declared, since we are testing it.
        * stdio-common/Makefile (tst-gets.c): Compile with
        -Wno-deprecated-declarations.
	* debug/Makefile (tst-chk1.c, tst-chk2.c, tst-chk3.c, tst-chk4.cc)
	(tst-chk5.cc, tst-chk6.cc, tst-lfschk1.c, tst-lfschk2.c)
	(tst-lfschk3.c, tst-lfschk4.cc, tst-lfschk5.cc, tst-lfschk6.cc):
	Compile with -Wno-deprecated-declarations.

9 files changed, 62 insertions, 43 deletions

diff --git a/ChangeLog b/ChangeLog
index 01f123c..6d41756 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,19 @@
+2017-02-25  Zack Weinberg  <zackw@panix.com>
+
+	* include/features.h (__GLIBC_USE_DEPRECATED_GETS): New macro.
+	* libio/stdio.h, libio/bits/stdio2.h: Condition gets on
+	__GLIBC_USE (DEPRECATED_GETS).  Update comments to indicate
+	gets was removed from C++ in C++14.
+	* include/stdio.h: Remove redundant declaration of gets.
+	* debug/tst-chk1.c, stdio-common/tst-gets.c: Force gets to
+	be declared, since we are testing it.
+	* stdio-common/Makefile (tst-gets.c): Compile with
+	-Wno-deprecated-declarations.
+	* debug/Makefile (tst-chk1.c, tst-chk2.c, tst-chk3.c, tst-chk4.cc)
+	(tst-chk5.cc, tst-chk6.cc, tst-lfschk1.c, tst-lfschk2.c)
+	(tst-lfschk3.c, tst-lfschk4.cc, tst-lfschk5.cc, tst-lfschk6.cc):
+	Compile with -Wno-deprecated-declarations.
+
 2017-02-24  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
 
 	* math/libm-test-support.c (check_ulp): Use LIT() around literal
diff --git a/debug/Makefile b/debug/Makefile
index 3d268f0..cd4975c 100644
--- a/debug/Makefile
+++ b/debug/Makefile
@@ -103,21 +103,22 @@ CFLAGS-tst-longjmp_chk3.c = -fexceptions -fasynchronous-unwind-tables
 CPPFLAGS-tst-longjmp_chk3.c = -D_FORTIFY_SOURCE=1
 
 # We know these tests have problems with format strings, this is what
-# we are testing.  Disable that warning.  They also generate warnings
-# from warning attributes, which cannot be disabled via pragmas, so
-# require -Wno-error to be used.
-CFLAGS-tst-chk1.c = -Wno-format -Wno-error
-CFLAGS-tst-chk2.c = -Wno-format -Wno-error
-CFLAGS-tst-chk3.c = -Wno-format -Wno-error
-CFLAGS-tst-chk4.cc = -Wno-format -Wno-error
-CFLAGS-tst-chk5.cc = -Wno-format -Wno-error
-CFLAGS-tst-chk6.cc = -Wno-format -Wno-error
-CFLAGS-tst-lfschk1.c = -Wno-format -Wno-error
-CFLAGS-tst-lfschk2.c = -Wno-format -Wno-error
-CFLAGS-tst-lfschk3.c = -Wno-format -Wno-error
-CFLAGS-tst-lfschk4.cc = -Wno-format -Wno-error
-CFLAGS-tst-lfschk5.cc = -Wno-format -Wno-error
-CFLAGS-tst-lfschk6.cc = -Wno-format -Wno-error
+# we are testing.  Disable that warning.  They are also testing
+# deprecated functions (notably gets) so disable that warning as well.
+# And they also generate warnings from warning attributes, which
+# cannot be disabled via pragmas, so require -Wno-error to be used.
+CFLAGS-tst-chk1.c = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-chk2.c = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-chk3.c = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-chk4.cc = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-chk5.cc = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-chk6.cc = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-lfschk1.c = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-lfschk2.c = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-lfschk3.c = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-lfschk4.cc = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-lfschk5.cc = -Wno-format -Wno-deprecated-declarations -Wno-error
+CFLAGS-tst-lfschk6.cc = -Wno-format -Wno-deprecated-declarations -Wno-error
 LDLIBS-tst-chk4 = -lstdc++
 LDLIBS-tst-chk5 = -lstdc++
 LDLIBS-tst-chk6 = -lstdc++
diff --git a/debug/tst-chk1.c b/debug/tst-chk1.c
index 164d0ae..60c8e1e 100644
--- a/debug/tst-chk1.c
+++ b/debug/tst-chk1.c
@@ -16,6 +16,11 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+/* This file tests gets.  Force it to be declared.  */
+#include <features.h>
+#undef __GLIBC_USE_DEPRECATED_GETS
+#define __GLIBC_USE_DEPRECATED_GETS 1
+
 #include <assert.h>
 #include <fcntl.h>
 #include <locale.h>
diff --git a/include/features.h b/include/features.h
index dc8ae79..7de4089 100644
--- a/include/features.h
+++ b/include/features.h
@@ -136,6 +136,7 @@
 #undef	__USE_GNU
 #undef	__USE_FORTIFY_LEVEL
 #undef	__KERNEL_STRICT_NAMES
+#undef	__GLIBC_USE_DEPRECATED_GETS
 
 /* Suppress kernel-name space pollution unless user expressedly asks
    for it.  */
@@ -383,6 +384,16 @@
 # define __USE_FORTIFY_LEVEL 0
 #endif
 
+/* The function 'gets' existed in C89, but is impossible to use
+   safely.  It has been removed from ISO C11 and ISO C++14.  Note: for
+   compatibility with various implementations of <cstdio>, this test
+   must consider only the value of __cplusplus when compiling C++.  */
+#if defined __cplusplus ? __cplusplus >= 201402L : defined __USE_ISOC11
+# define __GLIBC_USE_DEPRECATED_GETS 0
+#else
+# define __GLIBC_USE_DEPRECATED_GETS 1
+#endif
+
 /* Get definitions of __STDC_* predefined macros, if the compiler has
    not preincluded this header automatically.  */
 #include <stdc-predef.h>
diff --git a/include/stdio.h b/include/stdio.h
index 30e737e..17b5a05 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -181,24 +181,6 @@ libc_hidden_proto (__vasprintf_chk)
 libc_hidden_proto (__vdprintf_chk)
 libc_hidden_proto (__obstack_vprintf_chk)
 
-/* The <stdio.h> header does not include the declaration for gets
-   anymore when compiling with _GNU_SOURCE.  Provide a copy here.  */
-extern char *gets (char *__s);
-#  if __USE_FORTIFY_LEVEL > 0
-extern char *__gets_chk (char *__str, size_t) __wur;
-extern char *__REDIRECT (__gets_warn, (char *__str), gets)
-     __wur __warnattr ("please use fgets or getline instead, gets can't "
-		       "specify buffer size");
-
-__fortify_function __wur char *
-gets (char *__str)
-{
-  if (__bos (__str) != (size_t) -1)
-    return __gets_chk (__str, __bos (__str));
-  return __gets_warn (__str);
-}
-#  endif
-
 extern FILE * __fmemopen (void *buf, size_t len, const char *mode);
 libc_hidden_proto (__fmemopen)
 
diff --git a/libio/bits/stdio2.h b/libio/bits/stdio2.h
index e31386d..e9f9d69 100644
--- a/libio/bits/stdio2.h
+++ b/libio/bits/stdio2.h
@@ -222,8 +222,7 @@ __NTH (obstack_vprintf (struct obstack *__restrict __obstack,
 
 #endif
 
-#if !defined __USE_ISOC11 \
-    || (defined __cplusplus && __cplusplus <= 201103L && !defined __USE_GNU)
+#if __GLIBC_USE (DEPRECATED_GETS)
 extern char *__gets_chk (char *__str, size_t) __wur;
 extern char *__REDIRECT (__gets_warn, (char *__str), gets)
      __wur __warnattr ("please use fgets or getline instead, gets can't "
diff --git a/libio/stdio.h b/libio/stdio.h
index a589e36..c4f734c 100644
--- a/libio/stdio.h
+++ b/libio/stdio.h
@@ -625,16 +625,13 @@ __BEGIN_NAMESPACE_STD
 extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
      __wur;
 
-#if !defined __USE_ISOC11 \
-    || (defined __cplusplus && __cplusplus <= 201103L)
+#if __GLIBC_USE (DEPRECATED_GETS)
 /* Get a newline-terminated string from stdin, removing the newline.
-   DO NOT USE THIS FUNCTION!!  There is no limit on how much it will read.
 
-   The function has been officially removed in ISO C11.  This opportunity
-   is used to also remove it from the GNU feature list.  It is now only
-   available when explicitly using an old ISO C, Unix, or POSIX standard.
-   GCC defines _GNU_SOURCE when building C++ code and the function is still
-   in C++11, so it is also available for C++.
+   This function is impossible to use safely.  It has been officially
+   removed from ISO C11 and ISO C++14, and we have also removed it
+   from the _GNU_SOURCE feature list.  It remains available when
+   explicitly using an old ISO C, Unix, or POSIX standard.
 
    This function is a possible cancellation point and therefore not
    marked with __THROW.  */
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index f693fa0..01a6dd0 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -141,6 +141,9 @@ CFLAGS-scanf15.c = -I../libio -I../stdlib -I../wcsmbs -I../time -I../string \
 CFLAGS-scanf17.c = -I../libio -I../stdlib -I../wcsmbs -I../time -I../string \
 		   -I../wctype
 
+# tst-gets.c tests a deprecated function.
+CFLAGS-tst-gets.c += -Wno-deprecated-declarations
+
 CPPFLAGS += $(libio-mtsafe)
 
 $(objpfx)tst-setvbuf1.out: /dev/null $(objpfx)tst-setvbuf1
diff --git a/stdio-common/tst-gets.c b/stdio-common/tst-gets.c
index 102f235..9340c76 100644
--- a/stdio-common/tst-gets.c
+++ b/stdio-common/tst-gets.c
@@ -17,6 +17,11 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+/* This file tests gets.  Force it to be declared.  */
+#include <features.h>
+#undef __GLIBC_USE_DEPRECATED_GETS
+#define __GLIBC_USE_DEPRECATED_GETS 1
+
 #include <stdio.h>
 #include <string.h>