Q50759: Calling C Run-Time from a Single-Threaded DLL Init Routine

Article: Q50759
Product(s): See article
Version(s): 5.10
Operating System(s): OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 17-JUL-1990

You can call the C run-time routines from within your initialization
code for a single threaded dynamic link library (DLL).

To accomplish this, you must ensure that the C run time itself is
initialized prior to any run-time function call. This is easiest to do
if you write your base initialization in assembly (as required), but
then call a routine written in C that will perform the majority of
your init code. This C routine must be declared as a extrn, far call
in your assembly code, and defined/prototyped as such in your C init
code.

The following is a code example:

extrn   MYINIT: far                 in the assembly init module.
int far pascal MYINIT (...) ;       in the C init module.

Once you have satisfied these items, you must then call C_INIT() from
within your C init module. This must be done before any C run-time
calls have been made. The C_INIT routine must also be prototyped as
follows:

   int far pascal C_INIT (void) ;

Note: This information applies to single-threaded DLLs only. You
cannot call the C run time from the init code of a multi-threaded DLL.

The example below details the steps necessary to call any C run-time
routine from within your single-threaded DLL initialization code.

This example involves the following files:

   PROJ.C          Source to the EXE that calls the DLL routine
   PROJ.LIB        Library created by IMPLIB (based on DLL.DEF)
                   [resolves the exported routine(s) from DLL.DLL]
   DLL.C           Source to the DLL, and the function FOOPER()
   DLL.DEF         Definitions file for the DLL
   ASMINIT.ASM     Assembly Init code
   CINIT.C         C Init code

The source files to the .EXE and .DLL are standard for this type of
programming.

The initialization routines and their requirements are discussed
below:

ASMINIT.ASM
-----------

     .model large
     extrn  MYINIT:far        ; Declare the C init routine as far.
     .code
     INIT    proc    far      ; Declare init proc....

             call    MYINIT   ; Call my C init code..
             ret              ; MYINIT exit code is in AX, so return.

     INIT    endp
             end     INIT     ; <--Specifies Entry Point for Init
                              ;    routine (reason why this must be
                              ;    in assembly).

CINIT.C
-------

#include <stdio.h>

     // Prototype C Run-Time init function, and my C init code...
     int far pascal C_INIT (void) ;
     int far pascal MYINIT (void) ;

     int far pascal MYINIT (void)
     {
       int RetVal ;

       // Call the C Run-Time Initialization code..
       RetVal = C_INIT () ;

       // Calls to the C Run-Time are now valid...
       printf ("Hello from the DLL init code...\n") ;

       // other C Run-Time calls can be made here...

       // exit with return value from this initialization code...
       return (RetVal) ;
     }

The source to the remaining files are listed below:

PROJ.C
------

#include <stdio.h>

void far pascal FOOPER (int, int) ;
void main (void)
{
  printf ("Hello from Main().  Calling DLL routine...\n") ;
  FOOPER (1, 2) ;
  printf ("\nDone.\n") ;
}

DLL.C
-----

#include <stdio.h>

void far pascal FOOPER (int, int) ;
void far pascal FOOPER (int x, int y)
{
  printf ("Hello from DLL routine 'Fooper'.\n") ;
}

DLL.DEF
-------

LIBRARY     DLL  INITINSTANCE
DESCRIPTION 'single threaded DLL showing C Run-Time in Init Code'
DATA        MULTIPLE
EXPORTS     FOOPER

These files must be compiled and linked together as follows:

  cl   /Alfu /G2s /c dll.c
  masm /mx asminit.asm;
  cl   /Od /AL /G2 /c cinit.c

  link /CO dll asminit cinit,dll.dll/NOI,, \
           llibcdll.lib doscalls.lib, dll.def;
  copy dll.dll  c:\os2\dll

  cl      /AS /G2 /c proj.c
  implib  proj.lib  dll.def

  link /CO proj.obj, /NOI,, slibcep.lib proj.lib doscalls.lib/NOD ;

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