Q38714: Shared and Instance Segments in a DLL

Article: Q38714
Product(s): See article
Version(s): 5.10
Operating System(s): OS/2
Keyword(s): ENDUSER | SR# G881019-4906 -ND SINGLE S_LINK | mspl13_c
Last Modified: 24-JAN-1990

Question:

How can I get both shared and instance data in a DLL? I'd like to have
the default data segment (DGROUP) and most of the far segments to be
instance data and a single far data segment to be shared data. I've
tried to do this by doing the following:

1. Isolating the data for the far segment in fardata.c and compiling
   it with -NDSEG1

2. Linking with a .DEF file that contains the following:

    DATA    MULTIPLE NONSHARED
        SEGMENTS
                SEG1 CLASS 'FAR_DATA' SHARED

When I do this, ALL data segments (including SEG1) are instance; I
determined this from both my test program and the output of the exehdr
utility.

How can I make the SEG1 segment shared?

Response:

Every time you use /NDSEG1 option compiler will generate the
following segment definitions:

        SEG1        class   'FAR_DATA'
        SEG1_CONST  class   'FAR_DATA'
        SEG1_BSS    class   'FAR_DATA'
        GROUP SEG1_GROUP SEG1, SEG1_CONST, SEG1_BSS

SEG1, SEG1_CONST, and SEG1_BSS are all members of the same group
(physical segment). (SEG1_CONST AND SEG1_BSS are created automatically
by the compiler, as is the group SEG1_GROUP.) The linker overrides the
SEGMENTS specification if ANY of the segments in the group are left at
the default. The solution is to ask the linker to share ALL of the
segments in the group. In your case, insert the following into the
.DEF file:

DATA MULTIPLE NONSHARED
SEGMENTS
        SEG1         CLASS 'FAR_DATA' SHARED
        SEG1_CONST   CLASS 'FAR_DATA' SHARED
        SEG1_BSS     CLASS 'FAR_DATA' SHARED

Note: if the segment(s) you want to share are part of a group, you
must set ALL of the segments in the group to be shared or NONE will
be.

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