> We run 64-bit MIDAS on RHEL4 with 64-bit ROOT and everything generally works,
> except for compatibility problems with 32-bit MIDAS.
> 
> The big problem is that 64-bit and 32-bit ODB turned out to be incompatible ...

I have now identified 3 data structures that change size when compiled with "-m64":

EVENT_REQUEST: stores a pointer to a function. Pointer size is 4 bytes with -m32 and 8 bytes with -m64.
This structure is part of an array inside BUFFER_HEADER, resulting in a sizable size mismatch between 32
bit and 64 bit shared memory data buffers.

The fix is simple: the function pointer is not used anywhere. Replace is with a "DWORD unused_filler"
makes -m32 and -m64 data buffers compatible. (But breaks compatibility with previous -m64 compiled midas).

CHN_SETTINGS and CHN_STATISTICS: apparently, -m32 and -m64 GCC has different packing rules and in -m64
mode, 4 bytes of padding are added to these data structures. Size size mismatch appears to be benign,
but will result in "size mismatch" complaints from ODB.

The fix is simple: adding "__attribute__ ((__packed__))" to the definition of the data structure makes
-m64 identical to -m32.

The "svn diff" of changes involved is attached below.

The biggest problem here is that making 32-bit ODB and 64-bit ODB compatible requires breaking one or
the other (My proposed changes break the 64-bit version. Alternatively, one could add explicit padding
to these data structures and break the 32-bit ODB).

I think it is important to make 32-bit and 64-bit code compatible: at TRIUMF we have to use a mixed
environment because out latest host computers all run 64-bit Linux while all our VME processors and all
older machines can only run 32-bit code; this incompatibility causes us weekly headaches.

Any thoughts?

K.O.

(this output of svn diff is doctored for clarity)

ladd00:midas$ svn diff
Index: include/midas.h
===================================================================
--- include/midas.h     (revision 3744)
+++ include/midas.h     (working copy)
-   void (*dispatch) (HNDLE, HNDLE, EVENT_HEADER *, void *);
+   INT unused; // was void (*dispatch) (HNDLE, HNDLE, EVENT_HEADER *, void *);
 } EVENT_REQUEST;
 
--- include/msystem.h   (revision 3744)
+++ include/msystem.h   (working copy)

+#define PACKED __attribute__ ((__packed__))  <--- this goes into midas.h inside the #ifdef "we use GCC"
 
-typedef struct {
+typedef struct PACKED { ... CHN_SETTINGS
 
-typedef struct {
+typedef struct PACKED { ... CHN_STATISTICS