Using Threads

Rationale

After reading the Pthreads Primer, and attempting to apply it to eCS, I found a need for an introduction to the API and explanations on how to use it. This is an attempt to fill that void. --Myrkraverk 03:19, 4 November 2007 (CET)

Use of C++ in the Examples

I am a C++ programmer and frequently run into "small differences" when attempting to use a plain C compiler. These are mostly differences in automatic variable declaration and initialization which are leveraged somewhat with C99. However, I do not want to spend my time battling "plain C" syntax peculiarities or C99 feature command line switches, so I simply compile all my examples as C++.

The only "especially C++" feature I use is std::cout instead of printf() which any reasonably proficient C programmer can translate on the spot.

Creating Threads

Thread creation can be done with the DosCreateThread() system call, or the C runtime library _beginthread() function.

The DosCreateThread() system call is inherently low level and does not initialize the C/C++ runtime environment. In particular:

• You can not assume C++ Exceptions will work. VisualAge C++ has #pragma handler for this but it can not be relied upon with OpenWatcom or GCC.
• You can not rely on the state of the floating point unit. You can however, reset it with _fpreset().
• You can not rely on DosExit() to clear up thread specific data and/or the runtime environment; use _endthread() instead.

That is, the C/C++ runtime environment may be initialized on first use (this is documented for VisualAge C++ but apparently not for OpenWatcom or GCC). Also:

From the OpenWatcom C/C++ Programmer's Guide:

WARNING! If any thread calls a library function, you must use the _beginthread function to create the thread. Do not use the DosCreateThread API function.

From the OS/2 Toolkit 4.5:

Note: If you use DosCreateThread, you must explicitly call _endthread to terminate the thread.

None of this is an issue when using _beginthread(); it takes care of setting up the runtime environment and tearing it down when the thread function ends.

Hello, Threaded World!

Here is a short example of _beginthread():

// file: hello_thread.c++
#include <iostream>
#include <process.h>

#define INCL_DOSPROCESS
#include <os2.h>

void hello( void * )
{
  std::cout << "Hello from thread." << std::endl;
}

int main( int argc, char *argv[] )
{
  _beginthread( hello, 0, 4096 * 10, 0 );
  DosSleep( 10 );                       // Allow the thread to finish.
}

This can be compiled with OpenWatcom like so:

>wcl386 -cc++ -bm "hello_thread.c++"

or with GCC like so:

>g++ -Zmt "hello_thread.c++"

Here is the same program, using DosCreateThread(). Note that it uses a library call, despite the warnings above.

// file: hello_doscreate.c++
#include <iostream>

#define INCL_DOSPROCESS
#include <os2.h>

void _System hello( long unsigned int )
{
  std::cout << "Hello from thread." << std::endl;
}

int main( int argc, char *argv[] )
{
  TID tid = 0;
  DosCreateThread( &tid, hello, 0, CREATE_READY | STACK_SPARSE, 4096 * 10 );
  DosSleep( 10 );   // Allow the thread to finish
}

References

• Synchronization Timings
• OpenWatcom C/C++ Programmer's Guide (pdf) - Links to version 1.5 of the manual.
• OpenWatcom C Library Reference (pdf) - Links to version 1.5 of the manual.
• OS/2 Toolkit 4.5 (available on eComStation CD 2)
  • Control Program Programming Guide and Reference
  • C Library Reference
• EMX_0-9d.zip. EMX 0.9d Documentation on /book (inf).