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State Oriented Programming

Hierarchical State Machines in C/C++

Miro Samek and Paul Y. Montgomery

May 13, 2000

"State-Oriented Programming" was a cover story article published originally in Embedded Systems Programming magazine in August 2000. This repository contains the article PDF and the updated code.

The code accompanying the "State-Oriented Programming" ESP article is licensed under the open source MIT license (see the top-level comments in the hsm.h/c files).

The code is organized into two subdirectories: C and Cpp. Each subdirectory contains the make.bat batch file to build the code with the GCC compiler. This should work both on Windows (requires MinGW or similar on GCC for Windows) and Linux/MacOS. The examples include the digital watch discussed in the article as well as the "qhsmtst" test for the hierarchical state machine implementation (see below).

Alternatively to using the make.bat file, you can simply type the following command at the command prompt:

In the C directory:

gcc watch.c hsm.c -o watch

In the Cpp directory:

g++ watch.cpp hsm.cpp -o watch

For your convenience we have included the Windows executable file (watch.exe), which you can try directly. You inject events into the watch state machine by typing numbers on your keyboard: (0=MODE_EVT, 1=SET_EVT, 2=TICK_EVT, 3+ to exit).

Here is an example run for the C version of the watch code:

C:\tmp\State-Oriented_Programming\c>gcc watch.c hsm.c -o watch

C:\tmp\State-Oriented_Programming\c>watch
Enter:
0 for MODE_EVT
1 for SET_EVT
2 for TICK_EVT
3+ to exit

Watch::setting-START->hour;
Event<-0
time:  1:00:00
Event<-0
time:  2:00:00
Event<-1
Watch::hour-SET;
Event<-0
time:  2:01:00
Event<-0
time:  2:02:00
Event<-1
Watch::minute-SET;
Event<-0
date: 01-02
Event<-0
date: 01-03
Event<-1
Watch::day-SET;
Event<-0
date: 02-03
Event<-0
date: 03-03
Event<-1
Watch::month-SET;time:  2:02:00
Event<-2
Watch::time-TICK;time:  2:02:01
Event<-2
Watch::time-TICK;time:  2:02:02
Event<-0
Watch::time-MODE;date: 03-03
Event<-2
Watch::date-TICK;date: 03-03
Event<-2
Watch::date-TICK;date: 03-03
Event<-1
Watch::timekeeping-SET;Watch::setting-START->hour;
Event<-1
Watch::hour-SET;
Event<-1
Watch::minute-SET;
Event<-1
Watch::day-SET;
Event<-1
Watch::month-SET;date: 03-03
Event<-0
Watch::date-MODE;time:  2:02:04
Event<-4

C:\tmp\State-Oriented_Programming\c>

Embedded Projects

In order to use the code in your embedded projects you would need to extract files hsm.h and hsm.c from the C subdirectory, or files hsm.hpp and hsm.cpp form the Cpp subdirectory. We have compiled the code with a variety of C and C++ compilers, including: VC++ and Borland compilers for Windows, GCC compiler for Linux, ARM Software Development Toolkit v. 2.0 and 2.5 C compiler, and Green Hills MULTI 2000 C and EC++ ARM/THUMB compilers.

Notes About C++ Implementation

We have noticed one potential problem with one aspect of the C++ implementation. Depending on the compiler you would use you may encounter compilation errors in casting (upcasting) event handlers to a Hsm member function pointer (EvtHndlr). This upcasting is necessary to configure the state machine in the constructor. In our code we use the most commonly accepted by different compilers cast:

(EvtHndlr)&<class>::<func>

newer C++ compilers (but not EC++ compilers) may accept construct:

reinterpret_cast<EvtHndlr>(&<class>::<func>)

Your compiler may allow you to use a simpler form:

(EvtHndlr)<func>

since specifying class with the scope operator :: should not be necessary inside the class method (the constructor).

The QHsmTst Example

Since the publication of the original article, we've added a more exhaustive example of a state machine called QHsmTst. The state machine is completely artificial, but it contains all possible combinations of transitions up to 3 levels of state nesting.

NOTE: This example is borrowed from the book "Practical Statecharts in C/C++" by Miro Samek, published in 2002.

The C version of the example is located in the C directory. You build it exactly like the watch example:

gcc hsmtst.c hsm.c -o hsmtst

The C++ version of the example is located in the C directory. You build it exactly like the watch example:

g++ hsmtst.cpp hsm.cpp -o hsmtst

Here is an example run for the C++ version of the HsmTst code:

C:\tmp\State-Oriented_Programming\cpp>g++ hsmtst.cpp hsm.cpp -o hsmtst

C:\tmp\State-Oriented_Programming\cpp>hsmtst
Events:
a-h for triggering events
x to exit

top-ENTRY;top-INIT;s1-ENTRY;s1-INIT;s11-ENTRY;
Event<-a
s1-A;s11-EXIT;s1-EXIT;s1-ENTRY;s1-INIT;s11-ENTRY;
Event<-e
top-E;s11-EXIT;s1-EXIT;s2-ENTRY;s21-ENTRY;s211-ENTRY;
Event<-e
top-E;s211-EXIT;s21-EXIT;s2-EXIT;s2-ENTRY;s21-ENTRY;s211-ENTRY;
Event<-a

Event<-h
s21-H;s211-EXIT;s21-EXIT;s21-ENTRY;s21-INIT;s211-ENTRY;
Event<-h

Event<-x

C:\tmp\State-Oriented_Programming\cpp>

Updates

Since the publication of the "State-Oriented Programming" article, the presented concepts and implementations have been completely revised, improved and expanded in many direcctions. Among others, the Real-Time Framework suggested in the article has been created and the vastly improved hierarchical state machine implementation is now part of that framework. Please see the QP Real-Time Embedded Frameworks (RTEFs) at:

https://www.state-machine.com/