Re: [PATCH] Enable building GDB without installed libtermcap

[Pedro Alves <palves at redhat dot com>]

On 02/26/2015 06:55 PM, Eli Zaretskii wrote:
>> From: Pedro Alves <palves@redhat.com>
>> Given that this stub file never needed these variables while it was
>> Windows-only, how about we simply not define the variables if
>> compiling for mingw/cygwin, but define them as weak everywhere else?
> 
> Works for me, thanks.

Alright.  I recalled that Cygwin doesn't really care about old gcc's,
so I filed it under not-a-problem and limited the #ifdefery to mingw.

I'd like to hear more opinions on this (whole thing) before
pushing ahead.

>From b99d25c7fc2b10d93b51743f44c08558bf06dfd6 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Thu, 26 Feb 2015 17:01:06 +0000
Subject: [PATCH] Fallback to stub-termcap.c on all hosts

Currently building gdb is impossible without an installed termcap or
curses library.  But, GDB already has a very minimal termcap in the
tree to handle this situation for Windows -- gdb/stub-termcap.c.  This
patch makes that the fallback for all hosts.

Testing this on GNU/Linux (by simply hacking away the termcap/curses
detection in gdb/configure.ac), I tripped on:

 ../readline/libreadline.a(terminal.o): In function `_rl_init_terminal_io':
 /home/pedro/gdb/mygit/src/readline/terminal.c:527: undefined reference to `PC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:528: undefined reference to `BC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:529: undefined reference to `UP'
 /home/pedro/gdb/mygit/src/readline/terminal.c:538: undefined reference to `PC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:539: undefined reference to `BC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:540: undefined reference to `UP'

These are globals that are normally defined by termcap (or ncurses'
termcap emulation).

Now, we could just define replacements in stub-termcap.c, but
readline/terminal.c (at least the copy in our tree) has this "beauty":

 #if !defined (__linux__) && !defined (NCURSES_VERSION)
 #  if defined (__EMX__) || defined (NEED_EXTERN_PC)
 extern
 #  endif /* __EMX__ || NEED_EXTERN_PC */
 char PC, *BC, *UP;
 #endif /* !__linux__ && !NCURSES_VERSION */

which can result in readline defining the globals too.  That will
usually work out in C, given that "-fcommon" is usually the default
for C compilers, but that won't work for C++, or C with -fno-common
(link fails with "multiple definition" errors)...

Mirroring those #ifdef conditions in the stub termcap screams
"brittle" to me -- I can see them changing in latter readline
versions.

My idea to work around that is to simply use __attribute__((weak)).
Of all supported hosts
(https://sourceware.org/gdb/wiki/Systems#Supported_Hosts), except Windows.

Windows/PE/COFF's do support weak, but not on gcc 3.4 based toolchains
(4.8.x does work).  Given the file never needed the variables while it
was Windows-only, just continue not defining them there.

gdb/ChangeLog:
2015-02-26  Bernd Edlinger  <bernd.edlinger@hotmail.de>
	    Pedro Alves  <palves@redhat.com>

	* configure.ac: Remove the mingw32-specific stub-termcap.o
	fallback, and instead fallback to the stub termcap on all hosts.
	* configure: Regenerate.
	* stub-termcap.c (PC, BC, UP): Define as weak symbols.
---
 gdb/configure.ac   |  7 +------
 gdb/stub-termcap.c | 21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/gdb/configure.ac b/gdb/configure.ac
index 6ac8adb..79fc115 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -610,18 +610,13 @@ case $host_os in
   go32* | *djgpp*)
     ac_cv_search_tgetent="none required"
     ;;
-  *mingw32*)
-    if test x"$curses_found" != xyes; then
-      ac_cv_search_tgetent="none required"
-      CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
-    fi ;;
 esac
 
 # These are the libraries checked by Readline.
 AC_SEARCH_LIBS(tgetent, [termcap tinfo curses ncurses])
 
 if test "$ac_cv_search_tgetent" = no; then
-  AC_MSG_ERROR([no termcap library found])
+  CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
 fi
 
 AC_ARG_WITH([system-readline],
diff --git a/gdb/stub-termcap.c b/gdb/stub-termcap.c
index cc8632c..df5f888 100644
--- a/gdb/stub-termcap.c
+++ b/gdb/stub-termcap.c
@@ -32,9 +32,28 @@ extern char* tgetstr (char *name, char **area);
 extern int tputs (char *string, int nlines, int (*outfun) ());
 extern char *tgoto (const char *cap, int col, int row);
 
+/* These globals below are global termcap variables that readline
+   references.
+
+   Actually, depending on preprocessor conditions that we don't want
+   to mirror here (as they may change depending on readline versions),
+   readline may define these globals as well, relying on the linker
+   merging them if needed (-fcommon).  That doesn't work with
+   -fno-common or C++, so instead we define the symbols as weak.
+   Don't do this on Windows though, as MinGW gcc 3.4.2 doesn't support
+   weak (later versions, e.g., 4.8, do support it).  Given this stub
+   file originally was Windows only, and we only needed this when we
+   made it work on other hosts, it should be OK.  */
+#ifndef __MINGW32__
+char PC __attribute__((weak));
+char *BC __attribute__((weak));
+char *UP __attribute__((weak));
+#endif
+
 /* Each of the files below is a minimal implementation of the standard
    termcap function with the same name, suitable for use in a Windows
-   console window.  */
+   console window, or when a real termcap/curses library isn't
+   available.  */
 
 int
 tgetent (char *buffer, char *termtype)
-- 
1.9.3