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FW: openldap on AIX 4

From: Howard Chu
Subject: FW: openldap on AIX 4
Date: Wed, 11 Sep 2002 05:28:34 -0700

I'm sure I saw this discussed before, but I don't recall seeing a final
resolution. Both OpenLDAP 2.1 and Berkeley DB 4.1 are using libtool 1.4.2.
libtool explicitly adds "-lc" when linking a shared library with cc on AIX
4.2. It shouldn't; cc already adds whatever default libraries it needs and
this explicit spec breaks linking for the reentrant variants of the AIX

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun     
  Symas: Premier OpenSource Development and Support

-----Original Message-----
From: Howard Chu [mailto:address@hidden
Sent: Wednesday, September 11, 2002 5:18 AM
To: address@hidden
Subject: openldap on AIX 4

AIX 4.2 has some dynamic linking issues... AIX dynamic libraries must be
fully resolved at build time, and they store both the library/symbolname pair
in their linkage table. This is different from Linux, Solaris, and other
SVR4-derived systems that store a list of library names and a separate list
of symbol names. With the SVR4 approach, since symbols aren't bound to
library names, the runtime linker will happily resolve a symbol from the
first object file that it sees that provides the right symbol name.

AIX's DLL scheme causes problems because their pthreads implementation relies
on a different C library (-lc_r) from the non-threaded library (-lc). This
causes trouble when mixing non-threaded libraries with threaded executables.
E.g., is built without thread support, and is linked with -lc.
slapd is threaded, linked with libldap_r and liblber. Because all of slapd
uses ber_memalloc, and liblber is linked with -lc, that means all of slapd
uses the non-threaded version of malloc. This situation rapidly causes
crashes in malloc/free. The obvious fix is to link liblber with -lc_r, but
then single-threaded apps that use libldap (as opposed to libldap_r) will get
the wrong thing.

I guess we took a stab at working around this in the past; the configure
script on AIX always looks for cc_r (the reentrant C compiler) and uses it to
compile the whole source tree. This should have been good enough to insure
that all of our object code was linked with -lc_r automatically, because
that's cc_r's default behavior. Unfortunately, libtool gets in the way and
adds explicit "-lc" arguments when building (AIX?) shared libraries. (libtool
should not be doing this at all, it should let the chosen $(CC) command take
care of everything; that's its job...)

A final glitch turns out to be in the Berkeley DB library; even though we
tell BDB to use our malloc routines, there are still several instances in the
BDB library that call libc's malloc directly. So, if you didn't use cc_r to
build BDB, you get the wrong malloc. And, since BDB is also linked by
libtool, and libtool inserts -lc when it shouldn't, even if you build BDB
with cc_r you'll still get the wrong malloc.

You can outmaneuver libtool by adding "-lc_r" to LDFLAGS. Since this flag
will appear very early in the link command, it will be in front of libtool's
own "-lc" and the linker will resolve all its symbols against -lc_r and never
try to use -lc. Just using static libraries would avoid the problem as well.

I hear that newer pthreads implementations no longer depend on the existence
of -lc_r so this may not be an issue on AIX 4.3 or 5.x. I don't know offhand.

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun     
  Symas: Premier OpenSource Development and Support

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