Point to a struct member? - c++

I have the following typedefined struct:
typedef struct
{
uint8_t u8Byte1; // This byte takes the needed values sometimes
uint8_t u8Byte2; // Not used
uint8_t u8Byte3; // This byte takes the needed values the other times
uint8_t u8Byte4; // Not used
} tstrMapMetadata;
And I have a thread that fills (with data from a sensor) this struct and uses one of its values:
while(true)
{
tstrMapMetadata * pstrMapMetadata = something();
if(pstrMapMetadata->u8Byte1 == SOMETHING) //<---- this line
{
//Do something special
}
}
But now I have a condition (constant during the thread) in which I want the comparison of the marked line done with u8Byte3 instead of u8Byte1.
Of course I could
if(((condition) ? pstrMapMetadata->u8Byte1 : pstrMapMetadata->u8Byte3) == SOMETHING) //<---- this line
But the I would be doing the same comparison all the time.
Is there a way to point to one of the members (are they called like this?) of the struct before its declaration?
Something like (this code of course doesn't work but gives an idea of what I'm looking for):
uint8_t * coolPointer;
if(condition)
{
coolPointer = (someOperation)tstrMapMetadata(u8Byte1);
}
else
{
coolPointer = (someOperation)tstrMapMetadata(u8Byte3);
}
while(true)
{
tstrMapMetadata * pstrMapMetadata = something();
if(coolPointer == SOMETHING) //<---- this line
{
//Do something special
}
}
Thanks!


There is pointer to member:
uint8_t tstrMapMetadata::*member = condition ?
&tstrMapMetadata::u8Byte1 :
&tstrMapMetadata::u8Byte3;
while (true)
{
tstrMapMetadata* pstrMapMetadata = something();
if (pstrMapMetadata->*member == SOMETHING) // usage with ->* ( .* for objet )
{
//Do something special
}
}

Related

is there a better way to make this software flow

I have several functions that try and evaluate some data. Each function returns a 1 if it can successfully evaluate the data or 0 if it can not. The functions are called one after the other but execution should stop if one returns a value of 1.
Example functions look like so:
int function1(std::string &data)
{
// do something
if (success)
{
return 1;
}
return 0;
}
int function2(std::string &data)
{
// do something
if (success)
{
return 1;
}
return 0;
}
... more functions ...
How would be the clearest way to organise this flow? I know I can use if statements as such:
void doSomething(void)
{
if (function1(data))
{
return;
}
if (function2(data))
{
return;
}
... more if's ...
}
But this seems long winded and has a huge number of if's that need typing. Another choice I thought of is to call the next function from the return 0 of the function like so
int function1(std::string &data)
{
// do something
if (success)
{
return 1;
}
return function2(data);
}
int function2(std::string &data)
{
// do something
if (success)
{
return 1;
}
return function3(data);
}
... more functions ...
Making calling cleaner because you only need to call function1() to evaluate as far as you need to but seems to make the code harder to maintain. If another check need to be inserted into the middle of the flow, or the order of the calls changes, then all of the functions after the new one will need to be changed to account for it.
Am I missing some smart clear c++ way of achieving this kind of program flow or is one of these methods best. I am leaning towards the if method at the moment but I feel like I am missing something.

void doSomething() {
function1(data) || function2(data) /* || ... more function calls ... */;
}
Logical-or || operator happens to have the properties you need - evaluated left to right and stops as soon as one operand is true.

I think you can make a vector of lambdas where each lambdas contains specific process on how you evaluate your data. Something like this.
std::vector<std::function<bool(std::string&)> listCheckers;
listCheckers.push_back([](std::string& p_data) -> bool { return function1(p_data); });
listCheckers.push_back([](std::string& p_data) -> bool { return function2(p_data); });
listCheckers.push_back([](std::string& p_data) -> bool { return function3(p_data); });
//...and so on...
//-----------------------------
std::string theData = "Hello I'm a Data";
//evaluate all data
bool bSuccess = false;
for(fnChecker : listCheckers){
if(fnChecker(theData)) {
bSuccess = true;
break;
}
}
if(bSuccess ) { cout << "A function has evaluated the data successfully." << endl; }
You can modify the list however you like at runtime by: external objects, config settings from file, etc...

Function overloaded by bool and enum type is not differentiated while called using multiple ternary operator in C++

Got into an interesting problem while tried to call the overloaded function using conditional operator (just to avoid multiple if else condition)
class VirtualGpio
{
typedef enum
{
OUTPUT = 0xC7,
INPUT ,
DIRINVALID
}GpioDirection;
struct pinconfig
{
struct pinmap pin;
GpioPolarity plrty;
bool IsPullupCfgValid;
bool IsTriStCfgValid;
bool IsInputFilterValid;
GpioDirection dic;
gpiolistner fptr; // Callback function pointer on event change
};
};
class factory
{
public:
VirtualGpio *GetGpiofactory(VirtualGpio::pinconfig *cfg,VirtualGpio::GpioAccessTyp acc=VirtualGpio::Pin);
private:
int setCfgSetting(VirtualGpio::pinmap * const getpin, VirtualGpio::GpioDirection const data);
int setCfgSetting(VirtualGpio::pinmap * const getpin, bool const data);
};
int factory::setCfgSetting(VirtualGpio::pinmap * const getpin, VirtualGpio::GpioDirection const data)
{
cout << "It is a Direction overloaded" << endl;
}
int factory::setCfgSetting(VirtualGpio::pinmap * const getpin, bool const data)
{
cout << "It is a bool overloaded" << endl;
}
VirtualGpio* factory::GetGpiofactory(VirtualGpio::pinconfig *cfg,VirtualGpio::GpioAccessTyp acc)
{
VirtualGpio * io = new VirtualGpio();
printf("acc : 0x%X, pin : 0x%x, port : 0x%x\n",acc, cfg->pin.pinno, cfg->pin.portno);
printf("value of expression : 0x%x\n",((acc == VirtualGpio::Pin)? cfg->dic : ((cfg->dic == VirtualGpio::INPUT)?true :false))); <= this prints the right value
if(acc == VirtualGpio::Pin)
setCfgSetting(&cfg->pin,cfg->dic);
else if(cfg->dic == VirtualGpio::INPUT)
setCfgSetting(&cfg->pin,true);
else
setCfgSetting(&cfg->pin,false);
#if 0
if(setCfgSetting(&cfg->pin, ((acc == VirtualGpio::Pin)? cfg->dic : ((cfg->dic == VirtualGpio::INPUT)?true :false))) == ERROR)
{
printf("Error Setting the IO configuration for XRA\n");
}
else
printf("Set IO config successfully\n");
#endif
return io;
}
The commented part #if 0 in GetGpiofactory() is same as the above
multiple if-else-if-else block, but if I uncomment the #if0 part to #if
1, for all the possible inputs only bool version of the overloaded
function i.e setCfgSetting(VirtualGpio::pinmap * const getpin, bool
const data) is invoked.
below is my main code.
main()
{
static struct VirtualGpio::pinconfig cfg = {
.pin = {
.location = VirtualGpio::GPIO_ON_GPIOEXP1_TCI,
.pinno = 0,
.portno = -1
},
.plrty = VirtualGpio::active_high,
.IsPullupCfgValid = true,
.IsTriStCfgValid = true,
.IsInputFilterValid = true,
.dic = VirtualGpio::OUTPUT,
.fptr = NULL
};
factory fac;
fac.GetGpiofactory(&cfg);
}
Surprised, the overloaded function works well if I don't use the ternary operator instead use multiple if-else if-else blocks. curious to understand the reason.

That is because the ternary operator always evaluates to a single type. You can't "return" different types with this operator.
When the compiler encounters such an expression he tries to figure out whether he can reduce the whole thing to one type. If that's not possible you get a compile error.
In your case there is a valid option using bool as a type. Because cfg->dic is an enum type which is implicitly convertible to bool. If you would use and enum class your code would not compile anymore showing you what your actual problem is (example).
Also I don't really see what the advantage of this kind of code is. In my opinion it makes the code much harder to read. You could reduce your ifs to just one, if you're concerned about too many of them:
if(acc == VirtualGpio::Pin)
setCfgSetting(&cfg->pin,cfg->dic);
else
setCfgSetting(&cfg->pin, cfg->dic == VirtualGpio::INPUT);

C++ How to use less conditional statements?

For my assignment, I'm storing user login infos. I'm taking in a string which is the command. The command can be create, login, remove, etc. There are 10 total options, i.e 10 different strings possible. Can anyone explain a more efficient way to write this instead of 10 if and else if statements? Basically how should I format/structure things besides using a bunch of if (string == "one"), else if (string == "two"). Thank you

I expect that your lecturer would like you to extract function to another re-usable function:
string action;
command = CreateAction(action);
command.Do(...);
Ofcourse, inside you CreateAction class you still need to have the conditionals that determine which commands need to be created.
AbstractCommand CreateAction(action)
{
if (action == "login")
return LoginCommand();
else if (action == "remove")
return RemoveCommand();
..... etc etc
}
And if you really want to get rid of all the conditionals than you can create some self-registering commands but that involves a lot more code and classes......
You should look up things like Command Pattern and Factory Pattern

You can use function pointers and a lookup table.
typedef void (*Function_Pointer)(void);
void Create(void);
void Login(void);
void Remove(void);
struct Function_Option_Entry
{
const char * option_text;
Function_Pointer p_function;
};
Function_Option_Entry option_table[] =
{
{"one", Create},
{"two", Login},
{"three", Remove},
};
const unsigned int option_table_size =
sizeof(option_table) / sizeof(option_table[0]);
//...
std::string option_text;
//...
for (i = 0; i < option_table_size; ++i)
{
if (option_text == option_table[i].option_text)
{
option_table[i].p_function();
break;
}
}

Use a switch, and a simple hash-function.
You need to use a hash-function, because C and C++ only allow switching on integral values.
template<size_t N> constexpr char myhash(const char &x[N]) { return x[0] ^ (x[1]+63); }
char myhash(const string& x) { return x.size() ? x[0] ^ (x[1]+63) : 0; }
switch(myhash(s)) {
case myhash("one"):
if(s != "one") goto nomatch;
// do things
break;
case myhash("two"):
if(s != "two") goto nomatch;
// do things
break;
default:
nomatch:
// No match
}
Slight adjustments are needed if you are not using std::string.

I would recommend you to create a function for every specific string. For example, if you receive a string "create" you will call function doCreate(), if you receive a string "login" then you call function doLogin()
The only restriction on these function is that all of them must have the same signature. In an example above it was smh like this:
typedef void (*func_t) ();
The idea is to create a std::map from strings to these functions. So you wouldn't have to write 10 if's or so because you will be able to simple choose the right function from the map by the name of a specific string name. Let me explain it by the means of a small example:
typedef void (*func_t) ();
void doCreate()
{
std::cout << "Create function called!\n";
}
void doLogin()
{
std::cout << "Login function called!\n";
}
std::map<std::string, func_t> functionMap;
void initMap()
{
functionMap["create"] = doCreate;
functionMap["login"] = doLogin;
}
int main()
{
initMap();
std::string str = "login";
functionMap[str](); // will call doLogin()
str = "create";
functionMap[str](); // will call doCreate()
std::string userStr;
// let's now assume that we also can receive a string not from our set of functions
std::cin >> userStr;
if (functionMap.count(userStr))
{
functionMap[str](); // now we call doCreate() or doLogin()
}
else
{
std::cout << "Unknown command\n";
}
return 0;
}
I hope it will help you in someway=)

You can use a map which does the comparison for you.
Something like this:
Initialise map:
std::map<std::string, std::function<void(std::string&)>> map;
map["login"] = std::bind(&Class::DoLogin, this, std::placeholders::_1);
map["create"] = std::bind(&Class::DoCreate, this, std::placeholders::_1);
Receive message:
map.at(rx.msg_type)(rx.msg_data);
Handler:
void Class::DoLogin(const std::string& data)
{
// do login
}

Maybe you can create a std::map<std::string, int> and use map lookups to get the code of the command that was passed - you can later switch on that number. Or create an enum Command and have a std::map<std::string, Command> and use the switch.
Example:
enum Command
{
CREATE,
LOGIN,
...
};
std::map<std::string, Command> commandNameToCode;
// fill the map with appropriate values
commandNameToCode["create"] = Command::CREATE;
// somehow get command name from user and store in the below variable (not shown)
std::string input;
// check if the command is in the map and if so, act accordingly
if(commandNameToCode.find(input) != commandNameToCode.end())
{
switch(commandNameToCode[input])
{
case CREATE:
// handle create
break;
...
}
}

Inherited variables are not reading correctly when using bitwise comparisons

I have a few classes set up for a game, with XMapObject as the base, and XEntity, XEnviron, and XItem inheriting it.
MapObjects have a number of flags, one of them being MAPOBJECT_SOLID. My problem is that XEntity is the only class that correctly detects MAPOBJECT_SOLID. Both Items are Environs are always considered solid by the game, regardless of the flag's state. What is important is that Environs and Item should almost never be solid.
Each class has a very basic preliminary constructor, just initializing all varibles to zero or NULL. During the CreateX() phase, Objects are linked into the map, set into a linked linked list.
Both XItem and XEnviron are a tad sloppy. They are both new, and in the middle or my debugging attempts.
Here are the relevent code samples:
XMapObject:
#define MAPOBJECT_ACTIVE 1
#define MAPOBJECT_RENDER 2
#define MAPOBJECT_SOLID 4
class XMapObject : public XObject
{
public:
Uint8 MapObjectType,Location[2],MapObjectFlags;
XMapObject *NextMapObject,*PrevMapObject;
XMapObject();
void CreateMapObject(Uint8 MapObjectType);
void SpawnMapObject(Uint8 MapObjectLocation[2]);
void RemoveMapObject();
void DeleteMapObject();
void MapObjectSetLocation(Uint8 Y,Uint8 X);
void MapObjectMapLink();
void MapObjectMapUnlink();
};
XMapObject::XMapObject()
{
MapObjectType = 0;
Location[0] = 0;
Location[1] = 1;
NextMapObject = NULL;
PrevMapObject = NULL;
}
void XMapObject::CreateMapObject(Uint8 Type)
{
MapObjectType = Type;
}
void XMapObject::SpawnMapObject(Uint8 MapObjectLocation[2])
{
if(!(MapObjectFlags & MAPOBJECT_ACTIVE)) { MapObjectFlags += MAPOBJECT_ACTIVE; }
Location[0] = MapObjectLocation[0];
Location[1] = MapObjectLocation[1];
MapObjectMapLink();
}
XEntity:
XEntity *StartEntity = NULL,*EndEntity = NULL;
class XEntity : public XMapObject
{
public:
Uint8 Health,EntityFlags;
float Speed,Time;
XEntity *NextEntity,*PrevEntity;
XItem *IventoryList;
XEntity();
void CreateEntity(Uint8 EntityType,Uint8 EntityLocation[2]);
void DeleteEntity();
void EntityLink();
void EntityUnlink();
Uint8 MoveEntity(Uint8 YOffset,Uint8 XOffset);
};
XEntity::XEntity()
{
Health = 0;
Speed = 0;
Time = 1.0;
EntityFlags = 0;
NextEntity = NULL;
PrevEntity = NULL;
IventoryList = NULL;
}
void XEntity::CreateEntity(Uint8 EntityType,Uint8 EntityLocation[2])
{
CreateMapObject(EntityType);
SpawnMapObject(EntityLocation);
if(!(MapObjectFlags & MAPOBJECT_SOLID) { MapObjectFlags += MAPOBJECT_SOLID; }
EntityFlags = ENTITY_CLIPPING;
Time = 1.0;
Speed = 1.0;
EntityLink();
}
void XEntity::EntityLink()
{
if(StartEntity == NULL)
{
StartEntity = this;
PrevEntity = NULL;
NextEntity = NULL;
}
else
{
EndEntity->NextEntity = this;
}
EndEntity = this;
}
XEnviron:
class XEnviron : public XMapObject
{
public:
Uint8 Effect,TimeOut;
void CreateEnviron(Uint8 Type,Uint8 Y,Uint8 X,Uint8 TimeOut);
};
void XEnviron::CreateEnviron(Uint8 EnvironType,Uint8 Y,Uint8 X,Uint8 TimeOut)
{
CreateMapObject(EnvironType);
Location[0] = Y;
Location[1] = X;
SpawnMapObject(Location);
XTile *Tile = GetTile(Y,X);
Tile->Environ = this;
MapObjectFlags = MAPOBJECT_ACTIVE + MAPOBJECT_SOLID;
printf("%i\n",MapObjectFlags);
}
XItem:
class XItem : public XMapObject
{
public:
void CreateItem(Uint8 Type,Uint8 Y,Uint8 X);
};
void XItem::CreateItem(Uint8 Type,Uint8 Y,Uint8 X)
{
CreateMapObject(Type);
Location[0] = Y;
Location[1] = X;
SpawnMapObject(Location);
}
And lastly, the entity move code. Only entities are capable of moving themselves.
Uint8 XEntity::MoveEntity(Uint8 YOffset,Uint8 XOffset)
{
Uint8
NewY = Location[0] + YOffset,
NewX = Location[1] + XOffset;
if((NewY >= 0 && NewY < MAPY) && (NewX >= 0 && NewX < MAPX))
{
XTile *Tile = GetTile(NewY,NewX);
if(Tile->MapList != NULL)
{
XMapObject *MapObject = Tile->MapList;
while(MapObject != NULL)
{
if(MapObject->MapObjectFlags & MAPOBJECT_SOLID)
{
printf("solid\n");
return 0;
}
MapObject = MapObject->NextMapObject;
}
}
if(Tile->Flags & TILE_SOLID && EntityFlags & ENTITY_CLIPPING)
{
return 0;
}
this->MapObjectSetLocation(NewY,NewX);
return 1;
}
return 0;
}
What is wierd, is that the bitwise operator always returns true when the MapObject is an Environ or an Item, but it works correctly for Entities. For debug I am using the printf "Solid", and also a printf containing the value of the flag for both Environs and Items.
Any help is greatly appreciated, as this is a major bug for the small game I am working on. I am also very new at Object Oriented programming, anything tips, suggestions and/or criticism are also welcome.

Your problem appears to be that you never initialize MapObjectFlags in any classes other than XEnviron so, as a basic type, it will have an unspecified value in XItem, XEntity and other XMapObject derived objects. I suggest that, as a member of XMapObject you explicitly initialize it to a known value.
As a rule, it is generally a good idea to ensure that all members of basic type are explicitly initialized in the initializer list of every constructor that you define.
e.g.
XMapObject()
: MapObjectFlags(0)
, // ... other initializers
{
// Other initializations
}

You can't (legally) be calling XEntity::MoveEntity on a MapObject or Environ because they don't have such a method. If you're using static_cast to change your object pointer into an XEntity so you can call MoveEntity on it, then you really have no guarantees about how the bit operation will work. In some implementations, things may appear to work in MoveEntity, but what's actually happening is it's interpreting the other object's memory as an XEntity. When it tries to access the offset where it believes MapObjectFlags exists, it's not actually there and always has that bit set to 1.

I figured out the problem earlier today - It didn't have any relation to OO programming, inheritance, or bitwise; it was a simple scope error.
The problem was in the fact that during my quick test to get an Environ in game, I declared the new variable inside of the control switch sequence, so the next time any control was used, the Environ would act in unpredictable ways.
switch(Event.key.keysym.sym)
{
...
case SDLK_c: { XEnviron Environ; Environ.InitEnviron(...); }
...
}

C state-machine design [closed]

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I am crafting a small project in mixed C and C++. I am building one small-ish state-machine at the heart of one of my worker thread.
I was wondering if you gurus on SO would share your state-machine design techniques.
NOTE: I am primarily after tried & tested implementation techniques.
UPDATED: Based on all the great input gathered on SO, I've settled on this architecture:

State machines that I've designed before (C, not C++) have all come down to a struct array and a loop. The structure basically consists of a state and event (for look-up) and a function that returns the new state, something like:
typedef struct {
int st;
int ev;
int (*fn)(void);
} tTransition;
Then you define your states and events with simple defines (the ANY ones are special markers, see below):
#define ST_ANY -1
#define ST_INIT 0
#define ST_ERROR 1
#define ST_TERM 2
: :
#define EV_ANY -1
#define EV_KEYPRESS 5000
#define EV_MOUSEMOVE 5001
Then you define all the functions that are called by the transitions:
static int GotKey (void) { ... };
static int FsmError (void) { ... };
All these function are written to take no variables and return the new state for the state machine. In this example global variables are used for passing any information into the state functions where necessary.
Using globals isn't as bad as it sounds since the FSM is usually locked up inside a single compilation unit and all variables are static to that unit (which is why I used quotes around "global" above - they're more shared within the FSM, than truly global). As with all globals, it requires care.
The transitions array then defines all possible transitions and the functions that get called for those transitions (including the catch-all last one):
tTransition trans[] = {
{ ST_INIT, EV_KEYPRESS, &GotKey},
: :
{ ST_ANY, EV_ANY, &FsmError}
};
#define TRANS_COUNT (sizeof(trans)/sizeof(*trans))
What that means is: if you're in the ST_INIT state and you receive the EV_KEYPRESS event, make a call to GotKey.
The workings of the FSM then become a relatively simple loop:
state = ST_INIT;
while (state != ST_TERM) {
event = GetNextEvent();
for (i = 0; i < TRANS_COUNT; i++) {
if ((state == trans[i].st) || (ST_ANY == trans[i].st)) {
if ((event == trans[i].ev) || (EV_ANY == trans[i].ev)) {
state = (trans[i].fn)();
break;
}
}
}
}
As alluded to above, note the use of ST_ANY as wild-cards, allowing an event to call a function no matter the current state. EV_ANY also works similarly, allowing any event at a specific state to call a function.
It can also guarantee that, if you reach the end of the transitions array, you get an error stating your FSM hasn't been built correctly (by using the ST_ANY/EV_ANY combination.
I've used code similar for this on a great many communications projects, such as an early implementation of communications stacks and protocols for embedded systems. The big advantage was its simplicity and relative ease in changing the transitions array.
I've no doubt there will be higher-level abstractions which may be more suitable nowadays but I suspect they'll all boil down to this same sort of structure.
And, as ldog states in a comment, you can avoid the globals altogether by passing a structure pointer to all functions (and using that in the event loop). This will allow multiple state machines to run side-by-side without interference.
Just create a structure type which holds the machine-specific data (state at a bare minimum) and use that instead of the globals.
The reason I've rarely done that is simply because most of the state machines I've written have been singleton types (one-off, at-process-start, configuration file reading for example), not needing to run more than one instance. But it has value if you need to run more than one.

The other answers are good, but a very "lightweight" implementation I've used when the state machine is very simple looks like:
enum state { ST_NEW, ST_OPEN, ST_SHIFT, ST_END };
enum state current_state = ST_NEW;
while (current_state != ST_END)
{
input = get_input();
switch (current_state)
{
case ST_NEW:
/* Do something with input and set current_state */
break;
case ST_OPEN:
/* Do something different and set current_state */
break;
/* ... etc ... */
}
}
I would use this when the state machine is simple enough that the function pointer & state transition table approach is overkill. This is often useful for character-by-character or word-by-word parsing.

Pardon me for breaking every rule in computer science, but a state machine is one of the few (I can count only two off hand) places where a goto statement is not only more efficient, but also makes your code cleaner and easier to read. Because goto statements are based on labels, you can name your states instead of having to keep track of a mess of numbers or use an enum. It also makes for much cleaner code since you don't need all the extra cruft of function pointers or huge switch statements and while loops. Did I mention it's more efficient too?
Here's what a state machine might look like:
void state_machine() {
first_state:
// Do some stuff here
switch(some_var) {
case 0:
goto first_state;
case 1:
goto second_state;
default:
return;
}
second_state:
// Do some stuff here
switch(some_var) {
case 0:
goto first_state;
case 1:
goto second_state;
default:
return;
}
}
You get the general idea. The point is that you can implement the state machine in an efficient way and one that is relatively easy to read and screams at the reader that they are looking at a state machine. Note that if you are using goto statements, you must still be careful as it is very easy to shoot yourself in the foot while doing so.

You might consider the State Machine Compiler http://smc.sourceforge.net/
This splendid open source utility accepts a description of a state machine in a simple language and compiles it to any one of a dozen or so languages - including C and C++. The utility itself is written in Java, and can be included as part of a build.
The reason to do this, rather than hand coding using GoF State pattern or any other approach, is that once your state machine is expressed as code, the underlying structure tends to disappear under the weight of boilerplate that needs to be generated to support it. Using this approach gives you an excellent separation of concerns, and you keep the structure of your state machine 'visible'. The auto-generated code goes into modules that you don't need to touch, so that you can go back and fiddle with the state machine's structure without impacting the supporting code that you have written.
Sorry, I am being over-enthusiastic, and doubtless putting everyone off. But it is a top notch utility, and well-documented too.

Be sure to check the work of Miro Samek (blog State Space, website State Machines & Tools), whose articles at the C/C++ Users Journal were great.
The website contains a complete (C/C++) implementation in both open source and commercial license of a state machine framework (QP Framework), an event handler (QEP), a basic modeling tool (QM) and a tracing tool (QSpy) which allow to draw state machines, create code and debug them.
The book contains an extensive explanation on the what/why of the implementation and how to use it and is also great material to gain understanding of the fundamentals of hierachical and finite state machines.
The website also contains links to several board support packages for use of the software with embedded platforms.

I've done something similar to what paxdiablo describes, only instead of an array of state/event transitions, I set up a 2-dimensional array of function pointers, with the event value as the index of one axis and the current state value as the other. Then I just call state = state_table[event][state](params) and the right thing happens. Cells representing invalid state/event combinations get a pointer to a function that says so, of course.
Obviously, this only works if the state and event values are both contiguous ranges and start at 0 or close enough.

A very nice template-based C++ state machine "framework" is given by Stefan Heinzmann in his article.
Since there's no link to a complete code download in the article, I've taken the liberty to paste the code into a project and check it out. The stuff below is tested and includes the few minor but pretty much obvious missing pieces.
The major innovation here is that the compiler is generating very efficient code. Empty entry/exit actions have no cost. Non-empty entry/exit actions are inlined. The compiler is also verifying the completeness of the statechart. Missing actions generate linking errors. The only thing that is not caught is the missing Top::init.
This is a very nice alternative to Miro Samek's implementation, if you can live without what's missing -- this is far from a complete UML Statechart implementation, although it correctly implements the UML semantics, whereas Samek's code by design doesn't handle exit/transition/entry actions in correct order.
If this code works for what you need to do, and you have a decent C++ compiler for your system, it will probably perform better than Miro's C/C++ implementation. The compiler generates a flattened, O(1) transition state machine implementation for you. If the audit of assembly output confirms that the optimizations work as desired, you get close to theoretical performance. Best part: it's relatively tiny, easy to understand code.
#ifndef HSM_HPP
#define HSM_HPP
// This code is from:
// Yet Another Hierarchical State Machine
// by Stefan Heinzmann
// Overload issue 64 december 2004
// http://accu.org/index.php/journals/252
/* This is a basic implementation of UML Statecharts.
* The key observation is that the machine can only
* be in a leaf state at any given time. The composite
* states are only traversed, never final.
* Only the leaf states are ever instantiated. The composite
* states are only mechanisms used to generate code. They are
* never instantiated.
*/
// Helpers
// A gadget from Herb Sutter's GotW #71 -- depends on SFINAE
template<class D, class B>
class IsDerivedFrom {
class Yes { char a[1]; };
class No { char a[10]; };
static Yes Test(B*); // undefined
static No Test(...); // undefined
public:
enum { Res = sizeof(Test(static_cast<D*>(0))) == sizeof(Yes) ? 1 : 0 };
};
template<bool> class Bool {};
// Top State, Composite State and Leaf State
template <typename H>
struct TopState {
typedef H Host;
typedef void Base;
virtual void handler(Host&) const = 0;
virtual unsigned getId() const = 0;
};
template <typename H, unsigned id, typename B>
struct CompState;
template <typename H, unsigned id, typename B = CompState<H, 0, TopState<H> > >
struct CompState : B {
typedef B Base;
typedef CompState<H, id, Base> This;
template <typename X> void handle(H& h, const X& x) const { Base::handle(h, x); }
static void init(H&); // no implementation
static void entry(H&) {}
static void exit(H&) {}
};
template <typename H>
struct CompState<H, 0, TopState<H> > : TopState<H> {
typedef TopState<H> Base;
typedef CompState<H, 0, Base> This;
template <typename X> void handle(H&, const X&) const {}
static void init(H&); // no implementation
static void entry(H&) {}
static void exit(H&) {}
};
template <typename H, unsigned id, typename B = CompState<H, 0, TopState<H> > >
struct LeafState : B {
typedef H Host;
typedef B Base;
typedef LeafState<H, id, Base> This;
template <typename X> void handle(H& h, const X& x) const { Base::handle(h, x); }
virtual void handler(H& h) const { handle(h, *this); }
virtual unsigned getId() const { return id; }
static void init(H& h) { h.next(obj); } // don't specialize this
static void entry(H&) {}
static void exit(H&) {}
static const LeafState obj; // only the leaf states have instances
};
template <typename H, unsigned id, typename B>
const LeafState<H, id, B> LeafState<H, id, B>::obj;
// Transition Object
template <typename C, typename S, typename T>
// Current, Source, Target
struct Tran {
typedef typename C::Host Host;
typedef typename C::Base CurrentBase;
typedef typename S::Base SourceBase;
typedef typename T::Base TargetBase;
enum { // work out when to terminate template recursion
eTB_CB = IsDerivedFrom<TargetBase, CurrentBase>::Res,
eS_CB = IsDerivedFrom<S, CurrentBase>::Res,
eS_C = IsDerivedFrom<S, C>::Res,
eC_S = IsDerivedFrom<C, S>::Res,
exitStop = eTB_CB && eS_C,
entryStop = eS_C || eS_CB && !eC_S
};
// We use overloading to stop recursion.
// The more natural template specialization
// method would require to specialize the inner
// template without specializing the outer one,
// which is forbidden.
static void exitActions(Host&, Bool<true>) {}
static void exitActions(Host&h, Bool<false>) {
C::exit(h);
Tran<CurrentBase, S, T>::exitActions(h, Bool<exitStop>());
}
static void entryActions(Host&, Bool<true>) {}
static void entryActions(Host& h, Bool<false>) {
Tran<CurrentBase, S, T>::entryActions(h, Bool<entryStop>());
C::entry(h);
}
Tran(Host & h) : host_(h) {
exitActions(host_, Bool<false>());
}
~Tran() {
Tran<T, S, T>::entryActions(host_, Bool<false>());
T::init(host_);
}
Host& host_;
};
// Initializer for Compound States
template <typename T>
struct Init {
typedef typename T::Host Host;
Init(Host& h) : host_(h) {}
~Init() {
T::entry(host_);
T::init(host_);
}
Host& host_;
};
#endif // HSM_HPP
Test code follows.
#include <cstdio>
#include "hsm.hpp"
#include "hsmtest.hpp"
/* Implements the following state machine from Miro Samek's
* Practical Statecharts in C/C++
*
* |-init-----------------------------------------------------|
* | s0 |
* |----------------------------------------------------------|
* | |
* | |-init-----------| |-------------------------| |
* | | s1 |---c--->| s2 | |
* | |----------------|<--c----|-------------------------| |
* | | | | | |
* |<-d-| |-init-------| | | |-init----------------| | |
* | | | s11 |<----f----| | s21 | | |
* | /--| |------------| | | |---------------------| | |
* | a | | | | | | | | |
* | \->| | |------g--------->|-init------| | | |
* | | |____________| | | |-b->| s211 |---g--->|
* | |----b---^ |------f------->| | | | |
* | |________________| | |<-d-|___________|<--e----|
* | | |_____________________| | |
* | |_________________________| |
* |__________________________________________________________|
*/
class TestHSM;
typedef CompState<TestHSM,0> Top;
typedef CompState<TestHSM,1,Top> S0;
typedef CompState<TestHSM,2,S0> S1;
typedef LeafState<TestHSM,3,S1> S11;
typedef CompState<TestHSM,4,S0> S2;
typedef CompState<TestHSM,5,S2> S21;
typedef LeafState<TestHSM,6,S21> S211;
enum Signal { A_SIG, B_SIG, C_SIG, D_SIG, E_SIG, F_SIG, G_SIG, H_SIG };
class TestHSM {
public:
TestHSM() { Top::init(*this); }
~TestHSM() {}
void next(const TopState<TestHSM>& state) {
state_ = &state;
}
Signal getSig() const { return sig_; }
void dispatch(Signal sig) {
sig_ = sig;
state_->handler(*this);
}
void foo(int i) {
foo_ = i;
}
int foo() const {
return foo_;
}
private:
const TopState<TestHSM>* state_;
Signal sig_;
int foo_;
};
bool testDispatch(char c) {
static TestHSM test;
if (c<'a' || 'h'<c) {
return false;
}
printf("Signal<-%c", c);
test.dispatch((Signal)(c-'a'));
printf("\n");
return true;
}
int main(int, char**) {
testDispatch('a');
testDispatch('e');
testDispatch('e');
testDispatch('a');
testDispatch('h');
testDispatch('h');
return 0;
}
#define HSMHANDLER(State) \
template<> template<typename X> inline void State::handle(TestHSM& h, const X& x) const
HSMHANDLER(S0) {
switch (h.getSig()) {
case E_SIG: { Tran<X, This, S211> t(h);
printf("s0-E;");
return; }
default:
break;
}
return Base::handle(h, x);
}
HSMHANDLER(S1) {
switch (h.getSig()) {
case A_SIG: { Tran<X, This, S1> t(h);
printf("s1-A;"); return; }
case B_SIG: { Tran<X, This, S11> t(h);
printf("s1-B;"); return; }
case C_SIG: { Tran<X, This, S2> t(h);
printf("s1-C;"); return; }
case D_SIG: { Tran<X, This, S0> t(h);
printf("s1-D;"); return; }
case F_SIG: { Tran<X, This, S211> t(h);
printf("s1-F;"); return; }
default: break;
}
return Base::handle(h, x);
}
HSMHANDLER(S11) {
switch (h.getSig()) {
case G_SIG: { Tran<X, This, S211> t(h);
printf("s11-G;"); return; }
case H_SIG: if (h.foo()) {
printf("s11-H");
h.foo(0); return;
} break;
default: break;
}
return Base::handle(h, x);
}
HSMHANDLER(S2) {
switch (h.getSig()) {
case C_SIG: { Tran<X, This, S1> t(h);
printf("s2-C"); return; }
case F_SIG: { Tran<X, This, S11> t(h);
printf("s2-F"); return; }
default: break;
}
return Base::handle(h, x);
}
HSMHANDLER(S21) {
switch (h.getSig()) {
case B_SIG: { Tran<X, This, S211> t(h);
printf("s21-B;"); return; }
case H_SIG: if (!h.foo()) {
Tran<X, This, S21> t(h);
printf("s21-H;"); h.foo(1);
return;
} break;
default: break;
}
return Base::handle(h, x);
}
HSMHANDLER(S211) {
switch (h.getSig()) {
case D_SIG: { Tran<X, This, S21> t(h);
printf("s211-D;"); return; }
case G_SIG: { Tran<X, This, S0> t(h);
printf("s211-G;"); return; }
}
return Base::handle(h, x);
}
#define HSMENTRY(State) \
template<> inline void State::entry(TestHSM&) { \
printf(#State "-ENTRY;"); \
}
HSMENTRY(S0)
HSMENTRY(S1)
HSMENTRY(S11)
HSMENTRY(S2)
HSMENTRY(S21)
HSMENTRY(S211)
#define HSMEXIT(State) \
template<> inline void State::exit(TestHSM&) { \
printf(#State "-EXIT;"); \
}
HSMEXIT(S0)
HSMEXIT(S1)
HSMEXIT(S11)
HSMEXIT(S2)
HSMEXIT(S21)
HSMEXIT(S211)
#define HSMINIT(State, InitState) \
template<> inline void State::init(TestHSM& h) { \
Init<InitState> i(h); \
printf(#State "-INIT;"); \
}
HSMINIT(Top, S0)
HSMINIT(S0, S1)
HSMINIT(S1, S11)
HSMINIT(S2, S21)
HSMINIT(S21, S211)

The technique I like for state machines (at least ones for program control) is to use function pointers. Each state is represented by a different function. The function takes an input symbol and returns the function pointer for the next state. The central dispatch loop monitors takes the next input, feeds it to the current state, and processes the result.
The typing on it gets a little odd, since C doesn't have a way to indicate types of function pointers returning themselves, so the state functions return void*. But you can do something like this:
typedef void* (*state_handler)(input_symbol_t);
void dispatch_fsm()
{
state_handler current = initial_handler;
/* Let's assume returning null indicates end-of-machine */
while (current) {
current = current(get_input);
}
}
Then your individual state functions can switch on their input to process and return the appropriate value.

Simplest case
enum event_type { ET_THIS, ET_THAT };
union event_parm { uint8_t this; uint16_t that; }
static void handle_event(enum event_type event, union event_parm parm)
{
static enum { THIS, THAT } state;
switch (state)
{
case THIS:
switch (event)
{
case ET_THIS:
// Handle event.
break;
default:
// Unhandled events in this state.
break;
}
break;
case THAT:
// Handle state.
break;
}
}
Points:
State is private, not only to the compilation unit but also to the event_handler.
Special cases may be handled separately from the main switch using whatever construct deemed necessary.
More complex case
When the switch gets bigger than a couple of screens full, split it into functions that handle each state, using a state table to look up the function directly. The state is still private to the event handler. The state handler functions return the next state. If needed some events can still receive special treatment in the main event handler. I like to throw in pseudo-events for state entry and exit and perhaps state machine start:
enum state_type { THIS, THAT, FOO, NA };
enum event_type { ET_START, ET_ENTER, ET_EXIT, ET_THIS, ET_THAT, ET_WHATEVER, ET_TIMEOUT };
union event_parm { uint8_t this; uint16_t that; };
static void handle_event(enum event_type event, union event_parm parm)
{
static enum state_type state;
static void (* const state_handler[])(enum event_type event, union event_parm parm) = { handle_this, handle_that };
enum state_type next_state = state_handler[state](event, parm);
if (NA != next_state && state != next_state)
{
(void)state_handler[state](ET_EXIT, 0);
state = next_state;
(void)state_handler[state](ET_ENTER, 0);
}
}
I am not sure if I nailed the syntax, especially regarding the array of function pointers. I have not run any of this through a compiler. Upon review, I noticed that I forgot to explicitly discard the next state when handling the pseudo events (the (void) parenthesis before the call to state_handler()). This is something that I like to do even if compilers accept the omission silently. It tells readers of the code that "yes, I did indeed mean to call the function without using the return value", and it may stop static analysis tools from warning about it. It may be idiosyncratic because I do not recall having seen anybody else doing this.
Points: adding a tiny bit of complexity (checking if the next state is different from the current), can avoid duplicated code elsewhere, because the state handler functions can enjoy the pseudo events that occur when a state is entered and left. Remember that state cannot change when handling the pseudo events, because the result of the state handler is discarded after these events. You may of course choose to modify the behaviour.
A state handler would look like so:
static enum state_type handle_this(enum event_type event, union event_parm parm)
{
enum state_type next_state = NA;
switch (event)
{
case ET_ENTER:
// Start a timer to do whatever.
// Do other stuff necessary when entering this state.
break;
case ET_WHATEVER:
// Switch state.
next_state = THAT;
break;
case ET_TIMEOUT:
// Switch state.
next_state = FOO;
break;
case ET_EXIT:
// Stop the timer.
// Generally clean up this state.
break;
}
return next_state;
}
More complexity
When the compilation unit becomes too large (whatever you feel that is, I should say around 1000 lines), put each state handler in a separate file. When each state handler becomes longer than a couple of screens, split each event out in a separate function, similar to the way that the state switch was split. You may do this in a number of ways, separately from the state or by using a common table, or combining various schemes. Some of them have been covered here by others. Sort your tables and use binary search if speed is a requirement.
Generic programming
I should like the preprocessor to deal with issues such as sorting tables or even generating state machines from descriptions, allowing you to "write programs about programs". I believe this is what the Boost people are exploiting C++ templates for, but I find the syntax cryptic.
Two-dimensional tables
I have used state/event tables in the past but I have to say that for the simplest cases I do not find them necessary and I prefer the clarity and readability of the switch statement even if it does extend past one screen full. For more complex cases the tables quickly get out of hand as others have noted. The idioms I present here allow you to add a slew of events and states when you feel like it, without having to maintain a memory consuming table (even if it may be program memory).
Disclaimer
Special needs may render these idioms less useful, but I have found them to be very clear and maintainable.

Saw this somewhere
#define FSM
#define STATE(x) s_##x :
#define NEXTSTATE(x) goto s_##x
FSM {
STATE(x) {
...
NEXTSTATE(y);
}
STATE(y) {
...
if (x == 0)
NEXTSTATE(y);
else
NEXTSTATE(x);
}
}

Extremely untested, but fun to code, now in a more refined version than my original answer; up-to-date versions can be found at mercurial.intuxication.org:
sm.h
#ifndef SM_ARGS
#error "SM_ARGS undefined: " \
"use '#define SM_ARGS (void)' to get an empty argument list"
#endif
#ifndef SM_STATES
#error "SM_STATES undefined: " \
"you must provide a list of comma-separated states"
#endif
typedef void (*sm_state) SM_ARGS;
static const sm_state SM_STATES;
#define sm_transit(STATE) ((sm_state (*) SM_ARGS)STATE)
#define sm_def(NAME) \
static sm_state NAME ## _fn SM_ARGS; \
static const sm_state NAME = (sm_state)NAME ## _fn; \
static sm_state NAME ## _fn SM_ARGS
example.c
#include <stdio.h>
#define SM_ARGS (int i)
#define SM_STATES EVEN, ODD
#include "sm.h"
sm_def(EVEN)
{
printf("even %i\n", i);
return ODD;
}
sm_def(ODD)
{
printf("odd %i\n", i);
return EVEN;
}
int main(void)
{
int i = 0;
sm_state state = EVEN;
for(; i < 10; ++i)
state = sm_transit(state)(i);
return 0;
}

I really liked paxdiable's answer and decided to implement all the missing features for my application like guard variables and state machine specific data.
I uploaded my implementation to this site to share with the community. It has been tested using IAR Embedded Workbench for ARM.
https://sourceforge.net/projects/compactfsm/

Another interesting open source tool is Yakindu Statechart Tools on statecharts.org. It makes use of Harel statecharts and thus provides hierarchical and parallel states and generates C and C++ (as well as Java) code. It does not make use of libraries but follows a 'plain code' approach. The code basically applies switch-case structures. The code generators can also be customized. Additionally the tool provides many other features.

Here is an example of a Finite State Machine for Linux that uses message queues as the events. The events are put on the queue and handled in order. The state changes depending on what happens for each event.
This is an example for a data connection with states like:
Uninitialized
Initialized
Connected
MTU Negotiated
Authenticated
One little extra feature I added was a timestamp for each message/event. The event handler will ignore events that are too old (they have expired). This can happen a lot in the real world where you might get stuck in a state unexpectedly.
This example runs on Linux, use the Makefile below to compile it and play around with it.
state_machine.c
#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <unistd.h> // sysconf()
#include <errno.h> // errno
#include <string.h> // strerror()
#include <sys/time.h> // gettimeofday()
#include <fcntl.h> // For O_* constants
#include <sys/stat.h> // For mode constants
#include <mqueue.h>
#include <poll.h>
//------------------------------------------------
// States
//------------------------------------------------
typedef enum
{
ST_UNKNOWN = 0,
ST_UNINIT,
ST_INIT,
ST_CONNECTED,
ST_MTU_NEGOTIATED,
ST_AUTHENTICATED,
ST_ERROR,
ST_DONT_CHANGE,
ST_TERM,
} fsmState_t;
//------------------------------------------------
// Events
//------------------------------------------------
typedef enum
{
EV_UNKNOWN = 0,
EV_INIT_SUCCESS,
EV_INIT_FAIL,
EV_MASTER_CMD_MSG,
EV_CONNECT_SUCCESS,
EV_CONNECT_FAIL,
EV_MTU_SUCCESS,
EV_MTU_FAIL,
EV_AUTH_SUCCESS,
EV_AUTH_FAIL,
EV_TX_SUCCESS,
EV_TX_FAIL,
EV_DISCONNECTED,
EV_DISCON_FAILED,
EV_LAST_ENTRY,
} fsmEvName_t;
typedef struct fsmEvent_type
{
fsmEvName_t name;
struct timeval genTime; // Time the event was generated.
// This allows us to see how old the event is.
} fsmEvent_t;
// Finite State Machine Data Members
typedef struct fsmData_type
{
int connectTries;
int MTUtries;
int authTries;
int txTries;
} fsmData_t;
// Each row of the state table
typedef struct stateTable_type {
fsmState_t st; // Current state
fsmEvName_t evName; // Got this event
int (*conditionfn)(void *); // If this condition func returns TRUE
fsmState_t nextState; // Change to this state and
void (*fn)(void *); // Run this function
} stateTable_t;
// Finite State Machine state structure
typedef struct fsm_type
{
const stateTable_t *pStateTable; // Pointer to state table
int numStates; // Number of entries in the table
fsmState_t currentState; // Current state
fsmEvent_t currentEvent; // Current event
fsmData_t *fsmData; // Pointer to the data attributes
mqd_t mqdes; // Message Queue descriptor
mqd_t master_cmd_mqdes; // Master command message queue
} fsm_t;
// Wildcard events and wildcard state
#define EV_ANY -1
#define ST_ANY -1
#define TRUE (1)
#define FALSE (0)
// Maximum priority for message queues (see "man mq_overview")
#define FSM_PRIO (sysconf(_SC_MQ_PRIO_MAX) - 1)
static void addev (fsm_t *fsm, fsmEvName_t ev);
static void doNothing (void *fsm) {addev(fsm, EV_MASTER_CMD_MSG);}
static void doInit (void *fsm) {addev(fsm, EV_INIT_SUCCESS);}
static void doConnect (void *fsm) {addev(fsm, EV_CONNECT_SUCCESS);}
static void doMTU (void *fsm) {addev(fsm, EV_MTU_SUCCESS);}
static void reportFailConnect (void *fsm) {addev(fsm, EV_ANY);}
static void doAuth (void *fsm) {addev(fsm, EV_AUTH_SUCCESS);}
static void reportDisConnect (void *fsm) {addev(fsm, EV_ANY);}
static void doDisconnect (void *fsm) {addev(fsm, EV_ANY);}
static void doTransaction (void *fsm) {addev(fsm, EV_TX_FAIL);}
static void fsmError (void *fsm) {addev(fsm, EV_ANY);}
static int currentlyLessThanMaxConnectTries (void *fsm) {
fsm_t *l = (fsm_t *)fsm;
return (l->fsmData->connectTries < 5 ? TRUE : FALSE);
}
static int isMoreThanMaxConnectTries (void *fsm) {return TRUE;}
static int currentlyLessThanMaxMTUtries (void *fsm) {return TRUE;}
static int isMoreThanMaxMTUtries (void *fsm) {return TRUE;}
static int currentyLessThanMaxAuthTries (void *fsm) {return TRUE;}
static int isMoreThanMaxAuthTries (void *fsm) {return TRUE;}
static int currentlyLessThanMaxTXtries (void *fsm) {return FALSE;}
static int isMoreThanMaxTXtries (void *fsm) {return TRUE;}
static int didNotSelfDisconnect (void *fsm) {return TRUE;}
static int waitForEvent (fsm_t *fsm);
static void runEvent (fsm_t *fsm);
static void runStateMachine(fsm_t *fsm);
static int newEventIsValid(fsmEvent_t *event);
static void getTime(struct timeval *time);
void printState(fsmState_t st);
void printEvent(fsmEvName_t ev);
// Global State Table
const stateTable_t GST[] = {
// Current state Got this event If this condition func returns TRUE Change to this state and Run this function
{ ST_UNINIT, EV_INIT_SUCCESS, NULL, ST_INIT, &doNothing },
{ ST_UNINIT, EV_INIT_FAIL, NULL, ST_UNINIT, &doInit },
{ ST_INIT, EV_MASTER_CMD_MSG, NULL, ST_INIT, &doConnect },
{ ST_INIT, EV_CONNECT_SUCCESS, NULL, ST_CONNECTED, &doMTU },
{ ST_INIT, EV_CONNECT_FAIL, &currentlyLessThanMaxConnectTries, ST_INIT, &doConnect },
{ ST_INIT, EV_CONNECT_FAIL, &isMoreThanMaxConnectTries, ST_INIT, &reportFailConnect },
{ ST_CONNECTED, EV_MTU_SUCCESS, NULL, ST_MTU_NEGOTIATED, &doAuth },
{ ST_CONNECTED, EV_MTU_FAIL, &currentlyLessThanMaxMTUtries, ST_CONNECTED, &doMTU },
{ ST_CONNECTED, EV_MTU_FAIL, &isMoreThanMaxMTUtries, ST_CONNECTED, &doDisconnect },
{ ST_CONNECTED, EV_DISCONNECTED, &didNotSelfDisconnect, ST_INIT, &reportDisConnect },
{ ST_MTU_NEGOTIATED, EV_AUTH_SUCCESS, NULL, ST_AUTHENTICATED, &doTransaction },
{ ST_MTU_NEGOTIATED, EV_AUTH_FAIL, &currentyLessThanMaxAuthTries, ST_MTU_NEGOTIATED, &doAuth },
{ ST_MTU_NEGOTIATED, EV_AUTH_FAIL, &isMoreThanMaxAuthTries, ST_MTU_NEGOTIATED, &doDisconnect },
{ ST_MTU_NEGOTIATED, EV_DISCONNECTED, &didNotSelfDisconnect, ST_INIT, &reportDisConnect },
{ ST_AUTHENTICATED, EV_TX_SUCCESS, NULL, ST_AUTHENTICATED, &doDisconnect },
{ ST_AUTHENTICATED, EV_TX_FAIL, &currentlyLessThanMaxTXtries, ST_AUTHENTICATED, &doTransaction },
{ ST_AUTHENTICATED, EV_TX_FAIL, &isMoreThanMaxTXtries, ST_AUTHENTICATED, &doDisconnect },
{ ST_AUTHENTICATED, EV_DISCONNECTED, &didNotSelfDisconnect, ST_INIT, &reportDisConnect },
{ ST_ANY, EV_DISCON_FAILED, NULL, ST_DONT_CHANGE, &doDisconnect },
{ ST_ANY, EV_ANY, NULL, ST_UNINIT, &fsmError } // Wildcard state for errors
};
#define GST_COUNT (sizeof(GST)/sizeof(stateTable_t))
int main()
{
int ret = 0;
fsmData_t dataAttr;
dataAttr.connectTries = 0;
dataAttr.MTUtries = 0;
dataAttr.authTries = 0;
dataAttr.txTries = 0;
fsm_t lfsm;
memset(&lfsm, 0, sizeof(fsm_t));
lfsm.pStateTable = GST;
lfsm.numStates = GST_COUNT;
lfsm.currentState = ST_UNINIT;
lfsm.currentEvent.name = EV_ANY;
lfsm.fsmData = &dataAttr;
struct mq_attr attr;
attr.mq_maxmsg = 30;
attr.mq_msgsize = sizeof(fsmEvent_t);
// Dev info
//printf("Size of fsmEvent_t [%ld]\n", sizeof(fsmEvent_t));
ret = mq_unlink("/abcmq");
if (ret == -1) {
fprintf(stderr, "Error on mq_unlink(), errno[%d] strerror[%s]\n",
errno, strerror(errno));
}
lfsm.mqdes = mq_open("/abcmq", O_CREAT | O_RDWR, S_IWUSR | S_IRUSR, &attr);
if (lfsm.mqdes == (mqd_t)-1) {
fprintf(stderr, "Error on mq_open(), errno[%d] strerror[%s]\n",
errno, strerror(errno));
return -1;
}
doInit(&lfsm); // This will generate the first event
runStateMachine(&lfsm);
return 0;
}
static void runStateMachine(fsm_t *fsm)
{
int ret = 0;
if (fsm == NULL) {
fprintf(stderr, "[%s] NULL argument\n", __func__);
return;
}
// Cycle through the state machine
while (fsm->currentState != ST_TERM) {
printf("current state [");
printState(fsm->currentState);
printf("]\n");
ret = waitForEvent(fsm);
if (ret == 0) {
printf("got event [");
printEvent(fsm->currentEvent.name);
printf("]\n");
runEvent(fsm);
}
sleep(2);
}
}
static int waitForEvent(fsm_t *fsm)
{
//const int numFds = 2;
const int numFds = 1;
struct pollfd fds[numFds];
int timeout_msecs = -1; // -1 is forever
int ret = 0;
int i = 0;
ssize_t num = 0;
fsmEvent_t newEv;
if (fsm == NULL) {
fprintf(stderr, "[%s] NULL argument\n", __func__);
return -1;
}
fsm->currentEvent.name = EV_ANY;
fds[0].fd = fsm->mqdes;
fds[0].events = POLLIN;
//fds[1].fd = fsm->master_cmd_mqdes;
//fds[1].events = POLLIN;
ret = poll(fds, numFds, timeout_msecs);
if (ret > 0) {
// An event on one of the fds has occurred
for (i = 0; i < numFds; i++) {
if (fds[i].revents & POLLIN) {
// Data may be read on device number i
num = mq_receive(fds[i].fd, (void *)(&newEv),
sizeof(fsmEvent_t), NULL);
if (num == -1) {
fprintf(stderr, "Error on mq_receive(), errno[%d] "
"strerror[%s]\n", errno, strerror(errno));
return -1;
}
if (newEventIsValid(&newEv)) {
fsm->currentEvent = newEv;
} else {
return -1;
}
}
}
} else {
fprintf(stderr, "Error on poll(), ret[%d] errno[%d] strerror[%s]\n",
ret, errno, strerror(errno));
return -1;
}
return 0;
}
static int newEventIsValid(fsmEvent_t *event)
{
if (event == NULL) {
fprintf(stderr, "[%s] NULL argument\n", __func__);
return FALSE;
}
printf("[%s]\n", __func__);
struct timeval now;
getTime(&now);
if ( (event->name < EV_LAST_ENTRY) &&
((now.tv_sec - event->genTime.tv_sec) < (60*5))
)
{
return TRUE;
} else {
return FALSE;
}
}
//------------------------------------------------
// Performs event handling on the FSM (finite state machine).
// Make sure there is a wildcard state at the end of
// your table, otherwise; the event will be ignored.
//------------------------------------------------
static void runEvent(fsm_t *fsm)
{
int i;
int condRet = 0;
if (fsm == NULL) {
fprintf(stderr, "[%s] NULL argument\n", __func__);
return;
}
printf("[%s]\n", __func__);
// Find a relevant entry for this state and event
for (i = 0; i < fsm->numStates; i++) {
// Look in the table for our current state or ST_ANY
if ( (fsm->pStateTable[i].st == fsm->currentState) ||
(fsm->pStateTable[i].st == ST_ANY)
)
{
// Is this the event we are looking for?
if ( (fsm->pStateTable[i].evName == fsm->currentEvent.name) ||
(fsm->pStateTable[i].evName == EV_ANY)
)
{
if (fsm->pStateTable[i].conditionfn != NULL) {
condRet = fsm->pStateTable[i].conditionfn(fsm->fsmData);
}
// See if there is a condition associated
// or we are not looking for any condition
//
if ( (condRet != 0) || (fsm->pStateTable[i].conditionfn == NULL))
{
// Set the next state (if applicable)
if (fsm->pStateTable[i].nextState != ST_DONT_CHANGE) {
fsm->currentState = fsm->pStateTable[i].nextState;
printf("new state [");
printState(fsm->currentState);
printf("]\n");
}
// Call the state callback function
fsm->pStateTable[i].fn(fsm);
break;
}
}
}
}
}
//------------------------------------------------
// EVENT HANDLERS
//------------------------------------------------
static void getTime(struct timeval *time)
{
if (time == NULL) {
fprintf(stderr, "[%s] NULL argument\n", __func__);
return;
}
printf("[%s]\n", __func__);
int ret = gettimeofday(time, NULL);
if (ret != 0) {
fprintf(stderr, "gettimeofday() failed: errno [%d], strerror [%s]\n",
errno, strerror(errno));
memset(time, 0, sizeof(struct timeval));
}
}
static void addev (fsm_t *fsm, fsmEvName_t ev)
{
int ret = 0;
if (fsm == NULL) {
fprintf(stderr, "[%s] NULL argument\n", __func__);
return;
}
printf("[%s] ev[%d]\n", __func__, ev);
if (ev == EV_ANY) {
// Don't generate a new event, just return...
return;
}
fsmEvent_t newev;
getTime(&(newev.genTime));
newev.name = ev;
ret = mq_send(fsm->mqdes, (void *)(&newev), sizeof(fsmEvent_t), FSM_PRIO);
if (ret == -1) {
fprintf(stderr, "[%s] mq_send() failed: errno [%d], strerror [%s]\n",
__func__, errno, strerror(errno));
}
}
//------------------------------------------------
// end EVENT HANDLERS
//------------------------------------------------
void printState(fsmState_t st)
{
switch(st) {
case ST_UNKNOWN:
printf("ST_UNKNOWN");
break;
case ST_UNINIT:
printf("ST_UNINIT");
break;
case ST_INIT:
printf("ST_INIT");
break;
case ST_CONNECTED:
printf("ST_CONNECTED");
break;
case ST_MTU_NEGOTIATED:
printf("ST_MTU_NEGOTIATED");
break;
case ST_AUTHENTICATED:
printf("ST_AUTHENTICATED");
break;
case ST_ERROR:
printf("ST_ERROR");
break;
case ST_TERM:
printf("ST_TERM");
break;
default:
printf("unknown state");
break;
}
}
void printEvent(fsmEvName_t ev)
{
switch (ev) {
case EV_UNKNOWN:
printf("EV_UNKNOWN");
break;
case EV_INIT_SUCCESS:
printf("EV_INIT_SUCCESS");
break;
case EV_INIT_FAIL:
printf("EV_INIT_FAIL");
break;
case EV_MASTER_CMD_MSG:
printf("EV_MASTER_CMD_MSG");
break;
case EV_CONNECT_SUCCESS:
printf("EV_CONNECT_SUCCESS");
break;
case EV_CONNECT_FAIL:
printf("EV_CONNECT_FAIL");
break;
case EV_MTU_SUCCESS:
printf("EV_MTU_SUCCESS");
break;
case EV_MTU_FAIL:
printf("EV_MTU_FAIL");
break;
case EV_AUTH_SUCCESS:
printf("EV_AUTH_SUCCESS");
break;
case EV_AUTH_FAIL:
printf("EV_AUTH_FAIL");
break;
case EV_TX_SUCCESS:
printf("EV_TX_SUCCESS");
break;
case EV_TX_FAIL:
printf("EV_TX_FAIL");
break;
case EV_DISCONNECTED:
printf("EV_DISCONNECTED");
break;
case EV_LAST_ENTRY:
printf("EV_LAST_ENTRY");
break;
default:
printf("unknown event");
break;
}
}
Makefile
CXX = gcc
COMPFLAGS = -c -Wall -g
state_machine: state_machine.o
$(CXX) -lrt state_machine.o -o state_machine
state_machine.o: state_machine.c
$(CXX) $(COMPFLAGS) state_machine.c
clean:
rm state_machine state_machine.o

Coming to this late (as usual) but scanning the answers to date I thinks something important is missing;
I have found in my own projects that it can be very helpful to not have a function for every valid state/event combination. I do like the idea of effectively having a 2D table of states/events. But I like the table elements to be more than a simple function pointer. Instead I try to organize my design so at it's heart it comprises a bunch of simple atomic elements or actions. That way I can list those simple atomic elements at each intersection of my state/event table. The idea is that you don't have to define a mass of N squared (typically very simple) functions. Why have something so error-prone, time consuming, hard to write, hard to read, you name it ?
I also include an optional new state, and an optional function pointer for each cell in the table. The function pointer is there for those exceptional cases where you don't want to just fire off a list of atomic actions.
You know you are doing it right when you can express a lot of different functionality, just by editing your table, with no new code to write.

Alrght, I think mine's just a little different from everybody else's. A little more separation of code and data than I see in the other answers. I really read up on the theory to write this, which implements a full Regular-language (without regular expressions, sadly). Ullman, Minsky, Chomsky. Can't say I understood it all, but I've drawn from the old masters as directly as possible: through their words.
I use a function pointer to a predicate that determines the transition to a 'yes' state or a 'no' state. This facilitates the creation of a finite state acceptor for a regular language that you program in a more assembly-language-like manner.
Please don't be put-off by my silly name choices. 'czek' == 'check'. 'grok' == [go look it up in the Hacker Dictionary].
So for each iteration, czek calls a predicate function with the current character as argument. If the predicate returns true, the character is consumed (the pointer advanced) and we follow the 'y' transition to select the next state. If the predicate returns false, the character is NOT consumed and we follow the 'n' transition. So every instruction is a two-way branch! I must have been reading The Story of Mel at the time.
This code comes straight from my postscript interpreter, and evolved into its current form with much guidance from the fellows on comp.lang.c. Since postscript basically has no syntax (only requiring balanced brackets), a Regular Language Accepter like this functions as the parser as well.
/* currentstr is set to the start of string by czek
and used by setrad (called by israd) to set currentrad
which is used by israddig to determine if the character
in question is valid for the specified radix
--
a little semantic checking in the syntax!
*/
char *currentstr;
int currentrad;
void setrad(void) {
char *end;
currentrad = strtol(currentstr, &end, 10);
if (*end != '#' /* just a sanity check,
the automaton should already have determined this */
|| currentrad > 36
|| currentrad < 2)
fatal("bad radix"); /* should probably be a simple syntaxerror */
}
/*
character classes
used as tests by automatons under control of czek
*/
char *alpha = "0123456789" "ABCDE" "FGHIJ" "KLMNO" "PQRST" "UVWXYZ";
#define EQ(a,b) a==b
#define WITHIN(a,b) strchr(a,b)!=NULL
int israd (int c) {
if (EQ('#',c)) { setrad(); return true; }
return false;
}
int israddig(int c) {
return strchrnul(alpha,toupper(c))-alpha <= currentrad;
}
int isdot (int c) {return EQ('.',c);}
int ise (int c) {return WITHIN("eE",c);}
int issign (int c) {return WITHIN("+-",c);}
int isdel (int c) {return WITHIN("()<>[]{}/%",c);}
int isreg (int c) {return c!=EOF && !isspace(c) && !isdel(c);}
#undef WITHIN
#undef EQ
/*
the automaton type
*/
typedef struct { int (*pred)(int); int y, n; } test;
/*
automaton to match a simple decimal number
*/
/* /^[+-]?[0-9]+$/ */
test fsm_dec[] = {
/* 0*/ { issign, 1, 1 },
/* 1*/ { isdigit, 2, -1 },
/* 2*/ { isdigit, 2, -1 },
};
int acc_dec(int i) { return i==2; }
/*
automaton to match a radix number
*/
/* /^[0-9]+[#][a-Z0-9]+$/ */
test fsm_rad[] = {
/* 0*/ { isdigit, 1, -1 },
/* 1*/ { isdigit, 1, 2 },
/* 2*/ { israd, 3, -1 },
/* 3*/ { israddig, 4, -1 },
/* 4*/ { israddig, 4, -1 },
};
int acc_rad(int i) { return i==4; }
/*
automaton to match a real number
*/
/* /^[+-]?(d+(.d*)?)|(d*.d+)([eE][+-]?d+)?$/ */
/* represents the merge of these (simpler) expressions
[+-]?[0-9]+\.[0-9]*([eE][+-]?[0-9]+)?
[+-]?[0-9]*\.[0-9]+([eE][+-]?[0-9]+)?
The complexity comes from ensuring at least one
digit in the integer or the fraction with optional
sign and optional optionally-signed exponent.
So passing isdot in state 3 means at least one integer digit has been found
but passing isdot in state 4 means we must find at least one fraction digit
via state 5 or the whole thing is a bust.
*/
test fsm_real[] = {
/* 0*/ { issign, 1, 1 },
/* 1*/ { isdigit, 2, 4 },
/* 2*/ { isdigit, 2, 3 },
/* 3*/ { isdot, 6, 7 },
/* 4*/ { isdot, 5, -1 },
/* 5*/ { isdigit, 6, -1 },
/* 6*/ { isdigit, 6, 7 },
/* 7*/ { ise, 8, -1 },
/* 8*/ { issign, 9, 9 },
/* 9*/ { isdigit, 10, -1 },
/*10*/ { isdigit, 10, -1 },
};
int acc_real(int i) {
switch(i) {
case 2: /* integer */
case 6: /* real */
case 10: /* real with exponent */
return true;
}
return false;
}
/*
Helper function for grok.
Execute automaton against the buffer,
applying test to each character:
on success, consume character and follow 'y' transition.
on failure, do not consume but follow 'n' transition.
Call yes function to determine if the ending state
is considered an acceptable final state.
A transition to -1 represents rejection by the automaton
*/
int czek (char *s, test *fsm, int (*yes)(int)) {
int sta = 0;
currentstr = s;
while (sta!=-1 && *s) {
if (fsm[sta].pred((int)*s)) {
sta=fsm[sta].y;
s++;
} else {
sta=fsm[sta].n;
}
}
return yes(sta);
}
/*
Helper function for toke.
Interpret the contents of the buffer,
trying automatons to match number formats;
and falling through to a switch for special characters.
Any token consisting of all regular characters
that cannot be interpreted as a number is an executable name
*/
object grok (state *st, char *s, int ns,
object *src,
int (*next)(state *,object *),
void (*back)(state *,int, object *)) {
if (czek(s, fsm_dec, acc_dec)) {
long num;
num = strtol(s,NULL,10);
if ((num==LONG_MAX || num==LONG_MIN) && errno==ERANGE) {
error(st,limitcheck);
/* } else if (num > INT_MAX || num < INT_MIN) { */
/* error(limitcheck, OP_token); */
} else {
return consint(num);
}
}
else if (czek(s, fsm_rad, acc_rad)) {
long ra,num;
ra = (int)strtol(s,NULL,10);
if (ra > 36 || ra < 2) {
error(st,limitcheck);
}
num = strtol(strchr(s,'#')+1, NULL, (int)ra);
if ((num==LONG_MAX || num==LONG_MIN) && errno==ERANGE) {
error(st,limitcheck);
/* } else if (num > INT_MAX || num < INT_MAX) { */
/* error(limitcheck, OP_token); */
} else {
return consint(num);
}
}
else if (czek(s, fsm_real, acc_real)) {
double num;
num = strtod(s,NULL);
if ((num==HUGE_VAL || num==-HUGE_VAL) && errno==ERANGE) {
error(st,limitcheck);
} else {
return consreal(num);
}
}
else switch(*s) {
case '(': {
int c, defer=1;
char *sp = s;
while (defer && (c=next(st,src)) != EOF ) {
switch(c) {
case '(': defer++; break;
case ')': defer--;
if (!defer) goto endstring;
break;
case '\\': c=next(st,src);
switch(c) {
case '\n': continue;
case 'a': c = '\a'; break;
case 'b': c = '\b'; break;
case 'f': c = '\f'; break;
case 'n': c = '\n'; break;
case 'r': c = '\r'; break;
case 't': c = '\t'; break;
case 'v': c = '\v'; break;
case '\'': case '\"':
case '(': case ')':
default: break;
}
}
if (sp-s>ns) error(st,limitcheck);
else *sp++ = c;
}
endstring: *sp=0;
return cvlit(consstring(st,s,sp-s));
}
case '<': {
int c;
char d, *x = "0123456789abcdef", *sp = s;
while (c=next(st,src), c!='>' && c!=EOF) {
if (isspace(c)) continue;
if (isxdigit(c)) c = strchr(x,tolower(c)) - x;
else error(st,syntaxerror);
d = (char)c << 4;
while (isspace(c=next(st,src))) /*loop*/;
if (isxdigit(c)) c = strchr(x,tolower(c)) - x;
else error(st,syntaxerror);
d |= (char)c;
if (sp-s>ns) error(st,limitcheck);
*sp++ = d;
}
*sp = 0;
return cvlit(consstring(st,s,sp-s));
}
case '{': {
object *a;
size_t na = 100;
size_t i;
object proc;
object fin;
fin = consname(st,"}");
(a = malloc(na * sizeof(object))) || (fatal("failure to malloc"),0);
for (i=0 ; objcmp(st,a[i]=toke(st,src,next,back),fin) != 0; i++) {
if (i == na-1)
(a = realloc(a, (na+=100) * sizeof(object))) || (fatal("failure to malloc"),0);
}
proc = consarray(st,i);
{ size_t j;
for (j=0; j<i; j++) {
a_put(st, proc, j, a[j]);
}
}
free(a);
return proc;
}
case '/': {
s[1] = (char)next(st,src);
puff(st, s+2, ns-2, src, next, back);
if (s[1] == '/') {
push(consname(st,s+2));
opexec(st, op_cuts.load);
return pop();
}
return cvlit(consname(st,s+1));
}
default: return consname(st,s);
}
return null; /* should be unreachable */
}
/*
Helper function for toke.
Read into buffer any regular characters.
If we read one too many characters, put it back
unless it's whitespace.
*/
int puff (state *st, char *buf, int nbuf,
object *src,
int (*next)(state *,object *),
void (*back)(state *,int, object *)) {
int c;
char *s = buf;
while (isreg(c=next(st,src))) {
if (s-buf >= nbuf-1) return false;
*s++ = c;
}
*s = 0;
if (!isspace(c) && c != EOF) back(st,c,src); /* eat interstice */
return true;
}
/*
Helper function for Stoken Ftoken.
Read a token from src using next and back.
Loop until having read a bona-fide non-whitespace non-comment character.
Call puff to read into buffer up to next delimiter or space.
Call grok to figure out what it is.
*/
#define NBUF MAXLINE
object toke (state *st, object *src,
int (*next)(state *, object *),
void (*back)(state *, int, object *)) {
char buf[NBUF] = "", *s=buf;
int c,sta = 1;
object o;
do {
c=next(st,src);
//if (c==EOF) return null;
if (c=='%') {
if (DUMPCOMMENTS) fputc(c, stdout);
do {
c=next(st,src);
if (DUMPCOMMENTS) fputc(c, stdout);
} while (c!='\n' && c!='\f' && c!=EOF);
}
} while (c!=EOF && isspace(c));
if (c==EOF) return null;
*s++ = c;
*s = 0;
if (!isdel(c)) sta=puff(st, s,NBUF-1,src,next,back);
if (sta) {
o=grok(st,buf,NBUF-1,src,next,back);
return o;
} else {
return null;
}
}

boost.org comes with 2 different state chart implementations:
Meta State Machine
Statechart
As always, boost will beam you into template hell.
The first library is for more performance-critical state machines. The second library gives you a direct transition path from a UML Statechart to code.
Here's the SO question asking for a comparison between the two where both of the authors respond.

You can consider UML-state-machine-in-c, a "lightweight" state machine framework in C. I have written this framework to support both Finite state machine and Hierarchical state machine. Compare to state tables or simple switch cases, a framework approach is more scalable. It can be used for simple finite state machines to complex hierarchical state machines.
State machine is represented by state_machine_t structure. It contains only two members "Event" and a pointer to the "state_t".
struct state_machine_t
{
uint32_t Event; //!< Pending Event for state machine
const state_t* State; //!< State of state machine.
};
state_machine_t must be the first member of your state machine structure. e.g.
struct user_state_machine
{
state_machine_t Machine; // Base state machine. Must be the first member of user derived state machine.
// User specific state machine members
uint32_t param1;
uint32_t param2;
...
};
state_t contains a handler for the state and also optional handlers for entry and exit action.
//! finite state structure
struct finite_state{
state_handler Handler; //!< State handler to handle event of the state
state_handler Entry; //!< Entry action for state
state_handler Exit; //!< Exit action for state.
};
If the framework is configured for a hierarchical state machine then the state_t contains a pointer to parent and child state.
Framework provides an API dispatch_event to dispatch the event to the state machine and switch_state to trigger state transition.
For further details on how to implement a hierarchical state machine refer to the GitHub repository.
code examples,
https://github.com/kiishor/UML-State-Machine-in-C/blob/master/demo/simple_state_machine/readme.md
https://github.com/kiishor/UML-State-Machine-in-C/blob/master/demo/simple_state_machine_enhanced/readme.md

This series of Ars OpenForum posts about a somewhat complicated bit of control logic includes a very easy-to-follow implementation as a state machine in C.

Your question is quite generic,
Here are two reference articles that might be useful,
Embedded State Machine Implementation
This article describes a simple approach to implementing a state machine for an embedded system. For purposes of this article, a state machine is defined as an algorithm that can be in one of a small number of states. A state is a condition that causes a prescribed relationship of inputs to outputs, and of inputs to next states.
A savvy reader will quickly note that the state machines described in this article are Mealy machines. A Mealy machine is a state machine where the outputs are a function of both present state and input, as opposed to a Moore machine, in which the outputs are a function only of state.
Coding State Machines in C and C++
My preoccupation in this article is with state-machine fundamentals and some straightforward programming guidelines for coding state machines in C or C++. I hope that these simple techniques can become more common, so that you (and others) can readily see the state-machine structure right from the source code.

I have used State Machine Compiler in Java and Python projects to with success.

Given that you imply you can use C++ and hence OO code, I would suggest evaluating the 'GoF'state pattern (GoF = Gang of Four, the guys who wrote the design patterns book which brought design patterns into the limelight).
It is not particularly complex and it is widely used and discussed so it is easy to see examples and explanations on line.
It will also quite likely be recognizable by anyone else maintaining your code at a later date.
If efficiency is the worry, it would be worth actually benchmarking to make sure that a non OO approach is more efficient as lots of factors affect performance and it is not always simply OO bad, functional code good. Similarly, if memory usage is a constraint for you it is again worth doing some tests or calculations to see if this will actually be an issue for your particular application if you use the state pattern.
The following are some links to the 'Gof' state pattern, as Craig suggests:
http://en.wikipedia.org/wiki/State_pattern
http://www.vincehuston.org/dp/state.html

This is an old post with lots of answers, but I thought I'd add my own approach to the finite state machine in C. I made a Python script to produce the skeleton C code for any number of states. That script is documented on GituHub at FsmTemplateC
This example is based on other approaches I've read about. It doesn't use goto or switch statements but instead has transition functions in a pointer matrix (look-up table). The code relies on a big multi-line initializer macro and C99 features (designated initializers and compound literals) so if you don't like these things, you might not like this approach.
Here is a Python script of a turnstile example which generates skeleton C-code using FsmTemplateC:
# dict parameter for generating FSM
fsm_param = {
# main FSM struct type string
'type': 'FsmTurnstile',
# struct type and name for passing data to state machine functions
# by pointer (these custom names are optional)
'fopts': {
'type': 'FsmTurnstileFopts',
'name': 'fopts'
},
# list of states
'states': ['locked', 'unlocked'],
# list of inputs (can be any length > 0)
'inputs': ['coin', 'push'],
# map inputs to commands (next desired state) using a transition table
# index of array corresponds to 'inputs' array
# for this example, index 0 is 'coin', index 1 is 'push'
'transitiontable': {
# current state | 'coin' | 'push' |
'locked': ['unlocked', ''],
'unlocked': [ '', 'locked']
}
}
# folder to contain generated code
folder = 'turnstile_example'
# function prefix
prefix = 'fsm_turnstile'
# generate FSM code
code = fsm.Fsm(fsm_param).genccode(folder, prefix)
The generated output header contains the typedefs:
/* function options (EDIT) */
typedef struct FsmTurnstileFopts {
/* define your options struct here */
} FsmTurnstileFopts;
/* transition check */
typedef enum eFsmTurnstileCheck {
EFSM_TURNSTILE_TR_RETREAT,
EFSM_TURNSTILE_TR_ADVANCE,
EFSM_TURNSTILE_TR_CONTINUE,
EFSM_TURNSTILE_TR_BADINPUT
} eFsmTurnstileCheck;
/* states (enum) */
typedef enum eFsmTurnstileState {
EFSM_TURNSTILE_ST_LOCKED,
EFSM_TURNSTILE_ST_UNLOCKED,
EFSM_TURNSTILE_NUM_STATES
} eFsmTurnstileState;
/* inputs (enum) */
typedef enum eFsmTurnstileInput {
EFSM_TURNSTILE_IN_COIN,
EFSM_TURNSTILE_IN_PUSH,
EFSM_TURNSTILE_NUM_INPUTS,
EFSM_TURNSTILE_NOINPUT
} eFsmTurnstileInput;
/* finite state machine struct */
typedef struct FsmTurnstile {
eFsmTurnstileInput input;
eFsmTurnstileCheck check;
eFsmTurnstileState cur;
eFsmTurnstileState cmd;
eFsmTurnstileState **transition_table;
void (***state_transitions)(struct FsmTurnstile *, FsmTurnstileFopts *);
void (*run)(struct FsmTurnstile *, FsmTurnstileFopts *, const eFsmTurnstileInput);
} FsmTurnstile;
/* transition functions */
typedef void (*pFsmTurnstileStateTransitions)(struct FsmTurnstile *, FsmTurnstileFopts *);
enum eFsmTurnstileCheck is used to determine whether a transition was blocked with EFSM_TURNSTILE_TR_RETREAT, allowed to progress with EFSM_TURNSTILE_TR_ADVANCE, or the function call was not preceded by a transition with EFSM_TURNSTILE_TR_CONTINUE.
enum eFsmTurnstileState is simply the list of states.
enum eFsmTurnstileInput is simply the list of inputs.
The FsmTurnstile struct is the heart of the state machine with the transition check, function lookup table, current state, commanded state, and an alias to the primary function that runs the machine.
Every function pointer (alias) in FsmTurnstile should only be called from the struct and has to have its first input as a pointer to itself so as to maintain a persistent state, object-oriented style.
Now for the function declarations in the header:
/* fsm declarations */
void fsm_turnstile_locked_locked (FsmTurnstile *fsm, FsmTurnstileFopts *fopts);
void fsm_turnstile_locked_unlocked (FsmTurnstile *fsm, FsmTurnstileFopts *fopts);
void fsm_turnstile_unlocked_locked (FsmTurnstile *fsm, FsmTurnstileFopts *fopts);
void fsm_turnstile_unlocked_unlocked (FsmTurnstile *fsm, FsmTurnstileFopts *fopts);
void fsm_turnstile_run (FsmTurnstile *fsm, FsmTurnstileFopts *fopts, const eFsmTurnstileInput input);
Function names are in the format {prefix}_{from}_{to}, where {from} is the previous (current) state and {to} is the next state. Note that if the transition table does not allow for certain transitions, a NULL pointer instead of a function pointer will be set. Finally, the magic happens with a macro. Here we build the transition table (matrix of state enums) and the state transition functions look up table (a matrix of function pointers):
/* creation macro */
#define FSM_TURNSTILE_CREATE() \
{ \
.input = EFSM_TURNSTILE_NOINPUT, \
.check = EFSM_TURNSTILE_TR_CONTINUE, \
.cur = EFSM_TURNSTILE_ST_LOCKED, \
.cmd = EFSM_TURNSTILE_ST_LOCKED, \
.transition_table = (eFsmTurnstileState * [EFSM_TURNSTILE_NUM_STATES]) { \
(eFsmTurnstileState [EFSM_TURNSTILE_NUM_INPUTS]) { \
EFSM_TURNSTILE_ST_UNLOCKED, \
EFSM_TURNSTILE_ST_LOCKED \
}, \
(eFsmTurnstileState [EFSM_TURNSTILE_NUM_INPUTS]) { \
EFSM_TURNSTILE_ST_UNLOCKED, \
EFSM_TURNSTILE_ST_LOCKED \
} \
}, \
.state_transitions = (pFsmTurnstileStateTransitions * [EFSM_TURNSTILE_NUM_STATES]) { \
(pFsmTurnstileStateTransitions [EFSM_TURNSTILE_NUM_STATES]) { \
fsm_turnstile_locked_locked, \
fsm_turnstile_locked_unlocked \
}, \
(pFsmTurnstileStateTransitions [EFSM_TURNSTILE_NUM_STATES]) { \
fsm_turnstile_unlocked_locked, \
fsm_turnstile_unlocked_unlocked \
} \
}, \
.run = fsm_turnstile_run \
}
When creating the FSM, the macro FSM_EXAMPLE_CREATE() has to be used.
Now, in the source code every state transition function declared above should be populated. The FsmTurnstileFopts struct can be used to pass data to/from the state machine. Every transition must set fsm->check to be equal to either EFSM_EXAMPLE_TR_RETREAT to block it from transitioning or EFSM_EXAMPLE_TR_ADVANCE to allow it to transition to the commanded state.
A working example can be found at (FsmTemplateC)[https://github.com/ChisholmKyle/FsmTemplateC].
Here is the very simple actual usage in your code:
/* create fsm */
FsmTurnstile fsm = FSM_TURNSTILE_CREATE();
/* create fopts */
FsmTurnstileFopts fopts = {
.msg = ""
};
/* initialize input */
eFsmTurnstileInput input = EFSM_TURNSTILE_NOINPUT;
/* main loop */
for (;;) {
/* wait for timer signal, inputs, interrupts, whatever */
/* optionally set the input (my_input = EFSM_TURNSTILE_IN_PUSH for example) */
/* run state machine */
my_fsm.run(&my_fsm, &my_fopts, my_input);
}
All that header business and all those functions just to have a simple and fast interface is worth it in my mind.

You could use the open source library OpenFST.
OpenFst is a library for constructing, combining, optimizing, and searching weighted finite-state transducers (FSTs). Weighted finite-state transducers are automata where each transition has an input label, an output label, and a weight. The more familiar finite-state acceptor is represented as a transducer with each transition's input and output label equal. Finite-state acceptors are used to represent sets of strings (specifically, regular or rational sets); finite-state transducers are used to represent binary relations between pairs of strings (specifically, rational transductions). The weights can be used to represent the cost of taking a particular transition.

void (* StateController)(void);
void state1(void);
void state2(void);
void main()
{
StateController=&state1;
while(1)
{
(* StateController)();
}
}
void state1(void)
{
//do something in state1
StateController=&state2;
}
void state2(void)
{
//do something in state2
//Keep changing function direction based on state transition
StateController=&state1;
}

I personally use self referencing structs in combination with pointer arrays.
I uploaded a tutorial on github a while back, link:
https://github.com/mmelchger/polling_state_machine_c
Note: I do realise that this thread is quite old, but I hope to get input and thoughts on the design of the state-machine as well as being able to provide an example for a possible state-machine design in C.

Here is a method for a state machine that uses macros such that each function can have its own set of states: https://www.codeproject.com/Articles/37037/Macros-to-simulate-multi-tasking-blocking-code-at
It is titled "simulate multi tasking" but that is not the only use.
This method uses callbacks to pickup in each function where it left off. Each function contains a list of states unique to each function. A central "idle loop" is used to run the state machines. The "idle loop" has no idea how the state machines work, it is the individual functions that "know what to do". In order to write code for the functions, one just creates a list of states and uses the macros to "suspend" and "resume". I used these macros at Cisco when I wrote the Transceiver Library for the Nexus 7000 switch.