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Add shared linking information to AIX FAQ.

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Laurenz Albe
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Bruce Momjian 2006-09-20 02:10:11 +00:00
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From: Zeugswetter Andreas <ZeugswetterA@spardat.at>
$Date: 2006/05/26 19:48:32 $
$Date: 2006/09/20 02:10:11 $
On AIX 4.3.2 PostgreSQL compiled with the native IBM compiler xlc
(vac.C 5.0.1) passes all regression tests. Other versions of OS and
@ -396,4 +396,81 @@ Replace this with:
hosts=local4,bind4
to deactivate searching for IPv6 addresses.
to deactivate searching for IPv6 addresses.
Shared Linking
--------------
Shared libraries in AIX are different from shared libraries in Linux.
A shared library on AIX is an 'ar' archive containing shared objects. A
shared object is produced by the linker when invoked appropriately (e.g.
with -G), it is what we call a shared library on Linux.
-> On AIX, you can do a static as well as a dynamic
-> link against a shared library, it depends on how you
-> invoke the linker.
When you link statically, the shared objects from the library are added
to your executable as required; when you link dynamically, only
references to the shared objects are included in the executable.
Consequently you do not need a separate static library on AIX if you
have a dynamic library.
However, you CAN have static libraries (ar archives containing *.o
files), and the linker will link against them. This will of course
always be a static link.
When the AIX linker searches for libraries to link, it will look for a
library libxy.a as well as for a single shared object libxy.so when you
tell it to -lyx. When it finds both in the same directory, it will
prefer libpq.a unless invoked with -brtl.
This is where the problem occurs:
By default, PostgreSQL will (in the Linux way) create a shared object
libpq.so and a static library libpq.a in the same directory.
Up to now, since the linker was invoked without the -brtl flag, linking
on AIX was always static, as the linker preferred libpq.a over libpq.so.
We could have solved the problem by linking with -brtl on AIX, but we
chose to go a more AIX-conforming way so that third party programs
linking against PostgreSQL libraries will not be fooled into linking
statically by default.
The 'new way' on AIX is:
- Create libxy.so.n as before from the static library
libxy.a with the linker.
- Remove libxy.a
- Recreate libxy.a as a dynamic library with
ar -cr libxy.a libxy.so.n
- Only install libxy.a, do not install libxy.so
Since linking is dynamic on AIX now, we have a new problem:
We must make sure that the executable finds its library even if the
library is not installed in one of the standard library paths (/usr/lib
or /lib).
On Linux this is done with an RPATH, on AIX the equivalent is LIBPATH
that can be specified at link time with -blibpath:<colon separated path>
. If you do not specify the LIBPATH, it is automatically computed from
the -L arguments given to the linker. The LIBPATH, when set, must
contain ALL directories where shared libraries should be searched,
including the standard library directories.
Makefile.aix has been changed to link executables with a LIBPATH that
contains --libdir when PostgreSQL is configured with --enable-rpath (the
default).
The AIX equivalent for the Linux environment variable LD_LIBRARY_PATH is
LIBPATH.
The regression tests rely on LD_LIBRARY_PATH and have to be changed to
set LIBPATH as well.
Laurenz Albe