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Sharing data in a 32-bit dll

Posted on 1999-01-14
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Last Modified: 2013-11-20
I have converted a 16-bit dll to 32-bit. The old dll had been written to allow data to be shared between processes. I have modified the shared data to fit inside a #pragma data_seg("shared") section. However I have a question about how this works. If I write the code like:

typedef struct mesg_type
{
  UINT num_msg;
  char mesg[MAX_LENGTH];
} mesg_type_item;

#pragma data_seg("shared")
  mesg_type_item shared_list[MAX_ITEMS];
#pragma data_seg ()

the data does not become shared. But if I do this:

#pragma data_seg("shared")
  mesg_type_item shared_list[MAX_ITEMS] = {0,""};
#pragma data_seg ()

It appears to work fine.

Any explanations?
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Question by:sdj_work
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jrmcg earned 100 total points
ID: 1327679
You must initialize the data when using data_seg

Excerpt from...

HOWTO: Specify Shared and Nonshared Data in a DLL
Last reviewed: March 20, 1998
Article ID: Q100634

The system tries to load the shared memory block created by #pragma data_seg at the same address in each process. However, if the block cannot be loaded into the same memory address, it is mapped to a different address, but it is still shared.

NOTE: If the block contains pointers, this can be a problem. If the pointer holds the address of a variable not in the shared data segment then this address is valid only in one process space. If the address is in the shared data segment, it will be valid as long as the above relocation doesn’t occur. Since this is unreliable, you should not use pointers. You can use arrays in a shared data segment, but do so with caution. The array name is a pointer. Do not pass this value between processes. For example, if you have a string declared in the shared data segment as char Customer[20] = {0}, it’s okay for each process to use that variable name, as in strcpy(buf, Customer) or char FirstInitial = Customer[0]. However, do not pass the value of Customer to another process as in PostMessage(hwndNotMyWindow, WM_USER, 0, (LPARAM)Customer).

Below is a sample of how to define a named data section in your DLL. The first line directs the compiler to include all the data declared in this section in the .MYSEC data segment. This means that the iSharedVar variable would be considered part of the .MYSEC data segment. By default, data is nonshared.

Note that you must initialize all data in your named section. The data_seg pragma only applies to initialized data.

The third line below, "#pragma data_seg()", directs the compiler to reset allocation to the default data section.



Sample Code

   #pragma data_seg(".MYSEC")
   int iSharedVar = 0;
   #pragma data_seg()

You must also tell the linker that the variables in the section you defined are to be shared by modifying your .def file to include a SECTIONS section or by specifying /SECTION:.MYSEC,RWS in your link line. For example, aSECTIONS section could look like:

   SECTIONS
     .MYSEC   READ WRITE SHARED


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by:jrmcg
ID: 1327680
This was taken from MSDN.

J.R.
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