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.

2 files changed, 11 insertions, 18 deletions

diff --git a/include/features.h b/include/features.h
index dc8ae794a5..7de4089ed3 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 30e737efc2..17b5a05076 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)