Re: need to unbreak Emacs build on AIX

[Paul Eggert <eggert@cs.ucla.edu>]

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On 1/1/21 10:47 PM, Eli Zaretskii wrote:
> On the emacs-27
> branch, please just countermand the problematic condition with
> AIX-specific condition, as I don't want to change the configure script
> there.

I doubt whether the problem occurs only on AIX. As I understand it now, 
the problem can happen on any platform that uses the curses library from 
BSD via SVR4 and X/Open.

I now see that I misspoke when I wrote that Emacs uses libncurses on 
AIX. Emacs uses libcurses, because XLC-generated executables cannot link 
to libncurses, as libncurses requires libgcc_s.a which is a GCC-ism.

I don't offhand see how to fix this with a condition in the C code, so 
on the emacs-27 branch I merely documented the issue via the attached patch.

My guess is that the issue is rare, as the original curses library has 
been obsolete for quite some time. And given that curses does not work 
in Emacs 27 and this is the first bug report about it and I found it 
only by doing some extensive portability checking, perhaps we should 
instead drop support for curses, and insist on ncurses or tinfo or 
terminfo. I wouldn't be surprised if the Emacs curses code has other 
issues that we don't know about because nobody uses it.

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From 32c960bdc61e26521507f2d629ba4da8a4db842e Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 3 Jan 2021 12:32:27 -0800
Subject: [PATCH] Mention -lcurses problem on AIX

* etc/PROBLEMS: Describe problem with Emacs 27 and -lcurses.
Do not merge to master.
---
 etc/PROBLEMS | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index 2ab76e50bd..7499726678 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -2879,6 +2879,19 @@ A solution is to link with GCC, like this:
 Since the .o object files already exist, this will not recompile Emacs
 with GCC, but just restart by trying again to link temacs.
 
+*** Building Emacs with -lcurses fails with undefined symbols like BC.
+
+The 'configure' script attempts to use several terminal libraries,
+including tinfo, ncurses, and terminfo, and curses (in that order).
+If it happens to choose the long-obsolete curses library, Emacs will
+not link correctly.  Emacs 28 is expected to work around this problem;
+in the meantime you can work around it by installing tinfo, ncurses or
+terminfo instead.
+
+This problem can happen on AIX 7.2 if you build with IBM's compiler XLC,
+as AIX's ncurses library suffers from the libgcc problem mentioned above.
+To work around this, configure and build with GCC.
+
 *** Sun with acc: Link failure when using acc on a Sun.
 
 To use acc, you need additional options just before the libraries, such as
-- 
2.27.0