Skip site navigation (1) Skip section navigation (2)

Should libedit be preferred to libreadline?

From: Seneca Cunningham <scunning(at)ca(dot)afilias(dot)info>
To: pgsql-hackers(at)postgresql(dot)org
Cc: pgsql-ports(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Should libedit be preferred to libreadline?
Date: 2005-11-21 22:38:17
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackerspgsql-patchespgsql-ports
It would certainly seem so on AIX.

In tracking down why postgres 8.x would segfault on AIX 5.3, it became
apparent to me that libreadline.a is a problematic library to link
against and that libedit.a is preferable (and for reasons other than
that readline is GPL while postgres is BSD-licensed).

With AIX 5, the easiest way to get a shared object is to pass "-bexpall"
to the linker.  This results in all symbols being exported.  The problem
with this is that the linker will export all of libreadline's and
libhistory's symbols.  In the case of (and .5) on AIX 5
this includes symbols like strncpy and memmove, but on .4, not memcpy.
This is likely because libc.a does not export them.

What results from this is that when postgres is linked against readline
on AIX, it gets these memory functions through readline instead of its
own code.  When readline 4.3 is used (what IBM provides in their "AIX
Toolbox for Linux"), postgres is known to crash.  These segfaults (if
postgres was compiled with gcc) have occurred on AIX 5.3ML3, AIX 5.3ML1,
and AIX 5.2ML7.  With readline 5.0, postgres merely gets these functions
through the shared library memory segments instead of the user memory

While it is possible to build libreadline in a manner that doesn't
export strncpy, neither of the prebuilt readlines for AIX 5 that I
checked were both shared and did not export strncpy.  IBM's readline[5]
exports strncpy, UCLA's readline[4] is static.  Building a shared
readline that doesn't export strncpy requires creating export files for
libreadline and libhistory that only list the symbols that they are
supposed to export and editing the shared library Makefile to add the
exports flags to the appropriate linker calls.

Whatever strategy we might take, using readline on AIX requires
considerable trickery and hacking around with the build environments.
Simply put, it's ghastly.

On the other hand, the port of NetBSD's editline that I tried[1] works
without build-hackery to the library and has reasonable exports.  The
only changes to postgres that I needed to make were confined to telling
the configure script to check for libedit before libreadline and adding
a test for histedit.h.  The attached patch contains my modifications.

It is also possible to use a wrapper like rlwrap[2] instead of linking
postgres against libreadline or libedit.

[1] port of NetBSD's editline
[2] rlwrap
[3] IBM Redbook "AIX 5L Porting Guide", section 9.2
[4] UCLA's readline package
[5] IBM's readline package
[6] IBM Redbook "Developing and Porting C and C++ Applications on AIX",
      page 110

Seneca Cunningham

Attachment: libedit-pgsql810.patch
Description: text/x-patch (7.1 KB)


pgsql-ports by date

Next:From: Andrew DunstanDate: 2005-11-22 00:50:48
Subject: Re: [HACKERS] Should libedit be preferred to libreadline?
Previous:From: Seneca CunninghamDate: 2005-11-18 20:40:01
Subject: Re: Postgresql Installation on IBM aix 5.2

pgsql-hackers by date

Next:From: Simon RiggsDate: 2005-11-21 22:41:19
Subject: Practical error logging for very large COPY statements
Previous:From: Jim C. NasbyDate: 2005-11-21 22:35:15
Subject: Re: Heading to Mexico

pgsql-patches by date

Next:From: Andrew DunstanDate: 2005-11-22 00:50:48
Subject: Re: [HACKERS] Should libedit be preferred to libreadline?
Previous:From: Alvaro HerreraDate: 2005-11-21 21:37:52
Subject: Re: Install pg_regress by default

Privacy Policy | About PostgreSQL
Copyright © 1996-2017 The PostgreSQL Global Development Group