It's hard to tell without seeing some code or error messages, but here goes:

function myExt
  ....
external "C" ... = myExt(...) annotation(Library = "myExt.o"); // Note the required .o, not present in Dymola... We really should fix this
end myExt;

model A
  TypeA myType = myExt(...);
end A;

myExt.cpp:

... C++ code ...

extern "C" { /* Export the function as if it was a C function */
... myExt (...) {
  ...
}
}

Thanks for your reply. I tried the simple example using external libraries according to openmodelica user guide and that was successful. Now i try to using modelica program call the external socket function, and there are many problem about it. i post the related file in attachment, and then i will describe my mind in detail.
1.i write a .c file which contains a simple socket function called startUP(). the .c file called my_start.c and it includes <winsock2.h> because it uses socket.
2.i write a .h file called my_start.h includs the definition of the function startUP.In this function i print a sentence to see if the modelica model calls the function.
3.generate a .o file by command : gcc -c -o my_start.o my_start.c
4.write a modelica model which calls the external function.
5.write a .mos file to run the modelica model.
But it didn't print the sentence which i write in the c function. Does it call the function? How can it work well? Thanks.
All the files are in the .zip file in attaachments below.
TEXT

What program are you using to call the mos-file? If you're running OMShell, I don't think we redirect stdout, so you won't see that printf statement.

Add a getErrorString() after the simulation and loadFile() commands.


model mysocket
m_startup( );
end mysocket;

Is invalid syntax. getErrorString() after loadFile() should catch this.

model mysocket
equation
when initial()
  m_startup( );
end when;
end mysocket;

Would probably be better. Or possibly encapsulating the socket in an external object.