uFSM is a statechart library written in C. uFSM is designed without any external dependencies and uses no dynamic memory allocation or recursion.

uFSM is designed with embedded applications in mind but can also be used in other environments.

Supported UML statechart features:

Feature	Implemented	Test case
Simple state	Yes	test_simple
Composite states	Yes	test_simple_substate, test_xmi_machine
Sub machines	Yes	test_xmi_machine
Compound transition	Yes	test_compound_transition
Fork	Yes	test_fork
Join	Yes	test_join
Guards/Actions	Yes	test_guards_actions + various
Shallow/Deep history	Yes	test_xmi_machine, test_deephistory
Exit/Entry points	Yes	test_compound_transition
Init/Final	Yes	all
Event deferral	Yes	test_defer
Terminate	Yes	test_terminate
Choice	Yes	test_choice
Junction	Yes	test_junction
Do activity	Yes	test_do
Connection point ref	Yes	test_conref
Protocol Machines	No

Planned additions:

• More examples
• Simulation tool
• Analysis tool
• Test XMI files from other tools
• Potentially support SCXML data

Examples

All examples are located in 'src/examples'. Some of the examples have specific platform dependencies. For example, 'dhcpclient' will only work with linux.

Simple

This video gives an introduction to ufsm, drawing statecharts and how to import them.

dhcpclient

This is a work in progress.

Design

uFSM can be used as it is, without any XMI files or graphical editors by writing the various ufsm - structs manually, This works for trivial state machines. This, however, defeats the purpose of uFSM, and quickly becomes difficult to work with. The real use case for this library is with applications that have complex logic.

See 'test_simple' or 'test_simple_substate' for examples on how to create state machines manually.

uFSM comes with an XMI import tool 'ufsmimport'. The import tool can be called by an application makefile and used to generate the data structures needed by uFSM.

See examples/dhcpclient for a more detailed example on how this can be done.

uFSM can be part of an application by including it as a sub repo and add ufsm.c, ufsm.h ufsm_stack.c and ufsm_queue.c to the applications makefile. Alternatively, just copy these files into the target application.

Calling the topmost makefile in this repository builds the library with gcov and code coverage flags which is not something that should be done for an application. Furthermore, calling the top makefile will build the import tool and run through all of the test cases.

Build parameters

Parameter	Default	Description
UFSM_STACK_SIZE	128	uFSM stack size
UFSM_QUEUE_SIZE	16	Number of events that can be queued
UFSM_DEFER_QUEUE_SIZE	16	Number of events that can be deferred

These are all highly dependant on the complexity of the state machine and must be manually tuned for each application.

Transitions

The UML specification does not enforce how transitions are owned but suggests that the transition should be owned by the least common region. This, however, comes at a much greater computational cost in the transition algorithm. uFSM stores the transition in the region where the source state is located.

Event deferral

uFSM implements event deferral by using an internal transition on the state where a event should be deferred. The local transition should have an action with the name 'ufsm_defer'. This is a special keyword which is detected by the transition algorithm. Whenever an 'ufsm_defer' action is found, the event will be stored on a deferred event queue.

Code complexity and memory usage

uFSM is designed with embedded and safety critical applications in mind. uFSM does not use any dynamic memory allocation and uses no recursion. Instead uFSM uses a statically allocated stack; 'ufsm_stack'. The stack depth can be adjusted by setting 'UFSM_STACK_SIZE' build variable.

Functions with highest cyclomatic complexity:

CCN	LoC	Function
15	51	ufsm_process
13	124	ufsm_make_transition
13	49	ufsm_process_final_state
12	53	ufsm_enter_parent_states
10	41	ufsm_enter_state

Description of test cases

All of the state charts shown below were drawn in StarUML and the XMI files generated with the 'XMI' plugin.

XMI machine

This is a mixture of different tests, most notably:

• The actual state machines are generated by importing an XMI file, using ufsmimport
• Use of sub state machines
• Shallow history
• Orthogonal regions

Top level state machine

Sub statemachine

Deep history

This example illustrates the deep history pseudostate. In this example, the machine is initialised and events are sent to enter the D substate. At this point, the B event is sent and consequently state A is left but history is stored before leaving to state B.

At a later point, A is re-entered and history is recovered which means that the machine will be in state A, C and D.

Compound transitions

This statechart is taken from the UML standard - see chapter 14.2.3.9.6 for a complete description.

'test_compound_transition' tests entries and exits of parent states up until a least common ancestor. When event 'sig' is dispatched, the transition algorithm executes the following: xS11; t1; xS1; t2; eT1; eT11; t3; eT111

Fork

This example demonstrates the use of a fork pseudostate to enter more than one state in different, orthogonal regions.

Connection references

Using connection references, it is possible to transition into a specific state of a sub statemachine.

Event deferral

Junction