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Q61664: Changes in OS/2 Multithreaded and DLL Support in C 6.00

Article: Q61664
Product(s): See article
Version(s): 6.00
Operating System(s): OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 9-MAY-1990

Described below are several changes that have been made in Microsoft C
version 6.00 to further support the writing of multithread (MT)
applications and dynamic link libraries (DLLs) for OS/2:

1. There is now only one set of include files with C 6.00 (as opposed
   to the separate standard and multithreaded include files in C
   5.10). The multithreaded versions have been merged with the
   standard includes and are differentiated internally via "ifdef"
   statements. If you define _MT in your program (#define _MT) or on
   the the compile command-line (/D_MT), then the multithreaded
   includes will be used, otherwise the standard includes are
   utilized.

2. All of the DLL libraries are multithreaded in C 6.00. Previously, a
   DLL statically linked with the C DLL run-time library LLIBCDLL.LIB
   could only be single-threaded; multithreaded DLLs required that
   they be linked with a DLL version of the C run-time library
   (CRTLIB.DLL). In C 6.00, LLIBCDLL.LIB is multithreaded, which
   allows statically linked multithreaded DLLs to contain C run-time
   code.

3. The C 6.00 compiler now provides three new switches, /MT, /MD, and
   /ML, to simplify the building of multithreaded programs and dynamic
   link libraries. These switches lessen the number of other
   command-line options that must be used and automatically specify
   the correct library to be used.

   The list below shows the new switches, followed by the options the
   new switches are roughly equivalent to and the default library
   names that the switches specify must be written into the object
   modules. These new switches MUST be used in order to use the
   specified libraries because the switches are NOT specifically equal
   to the expanded option lists shown.

      Switches                         Equivalent Options
      --------                         ------------------

      /MT - /ALw /FPi /G2 /D_MT        Library name - LLIBCMT.LIB
      /ML - /ALw /FPa /G2 /D_MT        Library name - LLIBCDLL.LIB
      /MD - /ALw /FPi /G2 /DDLL /D_MT  No default library name - uses
                                       DLL version of C run-time
                                       library (for example,
                                       CRTLIB.DLL).

These switches are documented further on Pages 355-356 of "Microsoft C
Advanced Programming Techniques" and in the online help. Note that in
most of the C 6.00 documentation, the symbolic constant _MT is
improperly referred to as MT (missing the leading underscore).

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