what dll to link with C program using iris-callin.h

To link your C++ program with the iris-callin.h header file for accessing InterSystems IRIS, you need to include the appropriate dynamic link libraries (DLLs). Specifically:

• For Windows environments, use irisdbt.lib (or its equivalent shared library).
• For Linux environments, you would typically link with irisdbt.so or the equivalent library located in your IRIS development environment's designated directories.

These libraries ensure that the required functions from the IRIS database are accessible to your program [1]. During your development process, make sure the Development installation option is chosen, enabling access to the necessary libraries and tools [2][3].

Sources:

You may take a look here:
The Callin Interface
or the related example in OEX  https://openexchange.intersystems.com/package/iris-c-language