Thread: GUC variables invisible to contrib/ modules under Win32

Hi everyone,

I've been experimenting with compiling PostGIS (a contrib module) under
Win32 and I've noticed a difference in accessing GUC variables between
Linux and Win32 using PostgreSQL 8.0 beta 1. In part of the PostGIS
statistics routines we have a check like this:


    #include "commands/vacuum.h"
    ...

    /* If the attstattarget column is negative, use the default
value */
    /* NB: it is okay to scribble on stats->attr since it's a copy
*/
        if (attr->attstattarget < 0)
                attr->attstattarget = default_statistics_target;


Under Linux, PostGIS compiles and links fine without any trouble.
However, when compiling under Win32 we get the following message:

Info: resolving _default_statistics_target by linking to
__imp__default_statistics_target (auto-import)
fu000024.o(.idata$3+0xc): undefined reference to `libpostgres_a_iname'
nmth000023.o(.idata$4+0x0): undefined reference to
`_nm__default_statistics_target'
c:\mingw\bin\dllwrap.exe: c:\mingw\bin\gcc exited with status 1


The current "fix" to enable this to work correctly is to change the
relevant line in /src/include/commands/vacuum.h from "extern int
default_statistics_target;" to "extern DLLIMPORT int
default_statistics_target;". However, I'm not convinced this is the
correct fix given that other contrib modules may require access to other
GUC variables.

I've looked at guc.c and found GetConfigOption() which I think may be a
better option since I don't think accessing the variable directly will
work when using custom variables. Would this be the "correct" way to
access GUC variable values from within other modules that would work
correctly under both Win32 and UNIX-type OSs?


Cheers,

Mark.

---

Mark Cave-Ayland
Webbased Ltd.
Tamar Science Park
Derriford
Plymouth
PL6 8BX
England

Tel: +44 (0)1752 764445
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Mark Cave-Ayland wrote:
> Under Linux, PostGIS compiles and links fine without any trouble.
> However, when compiling under Win32 we get the following message:
>
> Info: resolving _default_statistics_target by linking to
> __imp__default_statistics_target (auto-import)
> fu000024.o(.idata$3+0xc): undefined reference to `libpostgres_a_iname'
> nmth000023.o(.idata$4+0x0): undefined reference to
> `_nm__default_statistics_target'
> c:\mingw\bin\dllwrap.exe: c:\mingw\bin\gcc exited with status 1
>
>
> The current "fix" to enable this to work correctly is to change the
> relevant line in /src/include/commands/vacuum.h from "extern int
> default_statistics_target;" to "extern DLLIMPORT int
> default_statistics_target;". However, I'm not convinced this is the
> correct fix given that other contrib modules may require access to other
> GUC variables.


It would be fine if all non-static variables would be defined
extern DLLIMPORT, but since non-backend modules don't regularly access
those variables most won't have that keyword. Additionally, this is yet
another M$ issue, non-existent anywhere else.

A work around is to redefine the prototype when you use it, i.e. after
all pgsql includes you define
extern DLLIMPORT int default_statistics_target;
in your sources. Apparently, the compiler will respect the latest
definition and resolve the reference cleanly.

Regards,
Andreas