state machines tutorials

Question

I am just wondering if anyone know of some good tutorials on the Internet for developing state machines  Or ebooks   I am starting working on state machines and just need something general to get me started

User · Answer

State machines are not something that inherently needs a tutorial to be explained or even used. What I suggest is that you take a look at the data and how it needs to be parsed.

For example, I had to parse the data protocol for a Near Space balloon flight computer, it stored data on the SD card in a specific format (binary) which needed to be parsed out into a comma seperated file. Using a state machine for this makes the most sense because depending on what the next bit of information is we need to change what we are parsing.

The code is written using C++, and is available as ParseFCU. As you can see, it first detects what version we are parsing, and from there it enters two different state machines.

It enters the state machine in a known-good state, at that point we start parsing and depending on what characters we encounter we either move on to the next state, or go back to a previous state. This basically allows the code to self-adapt to the way the data is stored and whether or not certain data exists at all even.

In my example, the GPS string is not a requirement for the flight computer to log, so processing of the GPS string may be skipped over if the ending bytes for that single log write is found.

State machines are simple to write, and in general I follow the rule that it should flow. Input going through the system should flow with certain ease from state to state.

User · Answer

Unfortunately  most of the articles on state machines are written for C   or other languages that have direct support for polymorphism as it s nice to model the states in an FSM implementation as classes that derive from an abstract state class   However  it s pretty easy to implement state machines in C using either switch statements to dispatch events to states  for simple FSMs  they pretty much code right up  or using tables to map events to state transitions   There are a couple of simple  but decent articles on a basic framework for state machines in C here    http   www gedan net 2008 09 08 finite-state-machine-matrix-style-c-implementation  http   www gedan net 2009 03 18 finite-state-machine-matrix-style-c-implementation-function-pointers-addon    Edit  Site  under maintenance   web archive links    http   web archive org web 20160517005245 http   www gedan net 2008 09 08 finite-state-machine-matrix-style-c-implementation http   web archive org web 20160808120758 http   www gedan net 2009 03 18 finite-state-machine-matrix-style-c-implementation-function-pointers-addon    switch statement-based state machines often use a set of macros to  hide  the mechanics of the switch statement  or use a set of if then else statements instead of a switch  and make what amounts to a  FSM language  for describing the state machine in C source   I personally prefer the table-based approach  but these certainly have merit  are widely used  and can be effective especially for simpler FSMs   One such framework is outlined by Steve Rabin in  Game Programming Gems  Chapter 3 0  Designing a General Robust AI Engine    A similar set of macros is discussed here     https   contrarymotion net 2008 02 12 some-code-snippets-for-a-simple-c-state-machine    If you re also interested in C   state machine implementations there s a lot more that can be found  I ll post pointers if you re interested

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Real-Time Object-Oriented Modeling was fantastic  published in 1994 and now selling for as little as 81 cents  plus  3 99 shipping

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I prefer using function pointers over gigantic switch statements  but in contrast to qrdl s answer I normally don t use explicit return codes or transition tables   Also  in most cases you ll want a mechanism to pass along additional data  Here s an example state machine    include  lt stdio h gt   struct state  typedef void state fn struct state      struct state       state fn   next      int i     data     state fn foo  bar   void foo struct state   state        printf   s  i n     func      state- gt i       state- gt next   bar     void bar struct state   state        printf   s  i n     func      state- gt i       state- gt next   state- gt i  lt  10   foo   0     int main void        struct state state     foo  0        while state next  state next  amp state

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There is a lot of lesson to learn handcrafting state machines in C  but let me also suggest Ragel state machine compiler   http   www complang org ragel   It has quite simple way of defining state machines and then you can generate graphs  generate code in different styles  table-driven  goto-driven   analyze that code if you want to  etc  And it s powerful  can be used in production code for various protocols

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State machines are very simple in C if you use function pointers   Basically you need 2 arrays - one for state function pointers and one for state transition rules  Every state function returns the code  you lookup state transition table by state and return code to find the next state and then just execute it   int entry state void   int foo state void   int bar state void   int exit state void       array and enum below must be in sync     int    state    void      entry state  foo state  bar state  exit state   enum state codes   entry  foo  bar  end    enum ret codes   ok  fail  repeat   struct transition       enum state codes src state      enum ret codes   ret code      enum state codes dst state        transitions from end state aren t needed    struct transition state transitions            entry  ok      foo        entry  fail    end        foo    ok      bar        foo    fail    end        foo    repeat  foo        bar    ok      end        bar    fail    end        bar    repeat  foo      define EXIT STATE end  define ENTRY STATE entry  int main int argc  char  argv          enum state codes cur state   ENTRY STATE      enum ret codes rc      int    state fun  void        for                state fun   state cur state           rc   state fun            if  EXIT STATE    cur state              break          cur state   lookup transitions cur state  rc              return EXIT SUCCESS      I don t put lookup transitions   function as it is trivial   That s the way I do state machines for years

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This is all you need to know   int state   0  while  state  lt  3        switch  state                case 0                 Do State 0 Stuff             if  should go to next state                                state                              break          case 1                 Do State 1 Stuff                 if  should go back                                 state--                                else if  should go to next state                                 state                              break          case 2                 Do State 2 Stuff                 if  should go back two                                 state -  2                                else if  should go to next state                                 state                              break          default              break

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State machines can be very complex for a complex problem  They are also subject to unexpected bugs  They can turn into a nightmare if someone runs into a bug or needs to change the logic in the future  They are also difficult to follow and debug without the state diagram  Structured programming is much better  for example you would probably not use a state machine at mainline level   You can use structured programming even in interrupt context  which is where state machines are usually used   See this article  Macros to simulate multi-tasking blocking code at interrupt level  found at codeproject com

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