August 2, 2026

The QGIS program - Writing my own plugin -- planning

Year ago I purchased sets of TOPO! CD maps for several western states. Arizona, California, Nevada, Utah, Colorado, and New Mexico.

I copied all of these to my hard drive with some minor organization. Then I wrote a program in C called "gtopo" that runs on linux and displays maps. I use it regularly.

The files are in /u1/topo on my machine.

Now that I am learning how to use QGIS, it occurs to me that it would be fantastic to have a convenient way to pull this data into QGIS.

QGIS and plugins

You have the choice of C++ or Python. Python is generally recommended because the API is more stable. C++ is definitely possible, but will periodically require work as new QGIS versions come along. Python can be used as a wrapper around C or C++ code.

Here is what I have learned from some searching:

The core software of QGIS is built with C++ and uses the Qt framework, its internal object-oriented API requires C++ classes.

The C++ Plugin route

Pros: Direct compilation into machine code, blazing-fast processing speed, and zero overhead between QGIS and your C code.

Cons: It is highly complex. The QGIS C++ API lacks stable cross-version compatibility, meaning you must rebuild your plugin for virtually every QGIS sub-version update. You must also handle compiling separate binaries for Windows, macOS, and Linux.

The Python with C Plugin route

You compile your C code into a shared library (a .dll file on Windows or a .so file on Linux). Then, use Python libraries like ctypes or cffi inside your QGIS Python plugin to call the functions directly from that library.

Pros: Significantly easier setup. Python plugins maintain better API stability between updates, and you can quickly build user interfaces using tools like Plugin Builder.

Cons: You still have to compile and distribute the specific platform-dependent binary files alongside your python plugin.

The Python route

Here you do everything in Python. This is the recommended route. What I need to do is balance the advantages of working in Python against the fact that I already have working C code in my gtopo project. My bet is that this is the best choice. I can use my C code as a guide to writing python code to read my topo files.

A search "can I write a qgis plugin in python" gives plenty of information.

Feedback? Questions? Drop me a line!

Tom's Geology Info / tom@mmto.org