Proposal: Deprecate direct access stdio internals (for future hard removal)

[Florian Weimer <fweimer at redhat dot com>]

I would like to remove quite a few libio internals in a future glibc 
version.

Note that the symbol list does not include _IO_stdin &c, __uflow, 
__underflow, and other bits which were used by old inline 
implementations of the C interface (for things like getc_unlocked).  We 
can preserve support for those by keeping the layout of the start of 
_IO_FILE unchanged, always signaling empty read/write buffers, thus 
redirecting the old inline implementations into the slow path.

The immediate goal is to remove the vtables, marker support, streamline 
save area support (for an implementation of fgetln), and no longer 
depend on the _IO_stdin_used mechanism to detect that the 
stdin/stdout/stderr variables are in use.  Old library versions had 
macros for them, not variables, and did not support reassignment.  This 
still causes problems when linking new DSOs because you have to be 
careful to include _IO_stdin_used in the linker script, otherwise 
strange things will happen.

Thanks,
Florian

commit 5127ac1e7b4276eebe06ac5d6c427c848d3d7871
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Dec 22 13:44:21 2017 +0100

    NEWS: Deprecate direct access libio internals

diff --git a/NEWS b/NEWS
index 2a1a4bfd85..0a162f6643 100644
--- a/NEWS
+++ b/NEWS
@@ -71,6 +71,33 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
+* Support for old programs which use internal stdio data structures and
+  functions is deprecated.  This includes programs which use the C++ streams
+  provided by libstdc++ in GCC 2.95.  Programs which use the internal
+  symbols _IO_adjust_wcolumn, _IO_default_doallocate, _IO_default_finish,
+  _IO_default_pbackfail, _IO_default_uflow, _IO_default_xsgetn,
+  _IO_default_xsputn, _IO_doallocbuf, _IO_do_write, _IO_file_attach,
+  _IO_file_close, _IO_file_close_it, _IO_file_doallocate, _IO_file_fopen,
+  _IO_file_init, _IO_file_jumps, _IO_fileno, _IO_file_open,
+  _IO_file_overflow, _IO_file_read, _IO_file_seek, _IO_file_seekoff,
+  _IO_file_setbuf, _IO_file_stat, _IO_file_sync, _IO_file_underflow,
+  _IO_file_write, _IO_file_xsputn, _IO_flockfile, _IO_flush_all,
+  _IO_flush_all_linebuffered, _IO_free_backup_area, _IO_free_wbackup_area,
+  _IO_init, _IO_init_marker, _IO_init_wmarker, _IO_iter_begin, _IO_iter_end,
+  _IO_iter_file, _IO_iter_next, _IO_least_wmarker, _IO_link_in,
+  _IO_list_all, _IO_list_lock, _IO_list_resetlock, _IO_list_unlock,
+  _IO_marker_delta, _IO_marker_difference, _IO_remove_marker, _IO_seekmark,
+  _IO_seekwmark, _IO_str_init_readonly, _IO_str_init_static,
+  _IO_str_overflow, _IO_str_pbackfail, _IO_str_seekoff, _IO_str_underflow,
+  _IO_switch_to_main_wget_area, _IO_switch_to_wget_mode,
+  _IO_unsave_wmarkers, _IO_wdefault_doallocate, _IO_wdefault_finish,
+  _IO_wdefault_pbackfail, _IO_wdefault_setbuf, _IO_wdefault_uflow,
+  _IO_wdefault_xsgetn, _IO_wdefault_xsputn, _IO_wdoallocbuf, _IO_wdo_write,
+  _IO_wfile_jumps, _IO_wfile_overflow, _IO_wfile_sync, _IO_wfile_underflow,
+  _IO_wfile_xsputn, _IO_wmarker_delta, or _IO_wsetb may stop working with a
+  future version of glibc.  Unlike other symbol removals, these old
+  applications will not be supported using compatibility symbols.
+
 * On GNU/Linux, the obsolete Linux constant PTRACE_SEIZE_DEVEL is no longer
   defined by <sys/ptrace.h>.