Wireless sensor networks and software for tiny systems

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Wireless sensor networks andsoftware for tiny
systems
Adam Dunkels Ph Lic, Researcher Swedish
Institute of Computer Science Computer and
Network Architectures laboratory 22 November
2005 Ericsson Mobile Platform AB, Lund
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Protothreads a replacement for explicit state
machines for flow control
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Background event-drivenprogramming

• Often used in memory constrained systems
• Uses less memory than threads
• Backwards to normal programming
• Code is executed only when events occur
• Incoming packets, I/O input, mouse clicks,
  time-out
• Nothing happens without events
• Programs have no idle loop

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Event-driven programs

• Programs are not threads
• Programs are not sequential
• Programs are event-handlers
• An event-handler is a C function
• An event-handler must always return
• An event-handler cannot perform blocking waits

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Threaded vs event-driven
f1()
f() if() BLOCK() else BLOCK()
while() BLOCK()
f2()
f3()
f4()
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The problem flow control with an explicit state
machine

• State machine keeps track of flow of control
• Control flow difficult to follow
• Not evident from reading the code
• Code hard to understand, maintain, debug, write

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Protothreads no state machine needed
PT_THREAD(f()) if() PT_WAIT_UNTIL()
else PT_WAIT_UNTIL() while()
PT_WAIT_UNTIL()

• With protothreads, we can write code like this
• Flow of control apparent
• C statements used
• Code easier to understand, maintain, debug, write

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Protothreads

• PT_WAIT_UNTIL() conditional blocking
• Protothreads are a mixture of the event-driven
  and the threaded model
• Inherits most of the good sides from both, but
  some of the bad sides too
• A protothread can be driven by an event-handler

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Protothreads can be implementedwith 100 ANSI
C!By using the C switch statement
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Can be implemented using the C switch statement!

• Unconventional use of the C switch statement
• But fully standards compatible
• Similar to Duff's device
• 100 portable no assembly language or stack
  fiddling
• Very, very low memory overhead 2 bytes
• Implemented as a set of C macros

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C switch statement example
PT_THREAD(example(void)) PT_BEGIN() while(1)
PT_WAIT_UNTIL(timer_expired()) call_a_function(
) PT_WAIT_UNTIL(timer_expired()) PT_END()
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C switch statement example
int example(void) switch(pt-gtlc) case
0 while(1) case 4 pt-gtlc 4 if(!timer_expire
d()) return 1 call_a_function() case 6 pt-gtlc
6 if(!timer_expired()) return 1 return
0
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The relation between protothreads and the C
switch statement
Dan Henry (Boulder, Co USA), in adiscussion
about protothreads on a 8052message board Many
of us use the switch/case construct to explicitly
implement concurrent state machines in our code.
The protothread macros merely provide a level
ofabstraction above that so that the code
appears more linear and the overall logic more
visible.
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Limitations

• Automatic variables (stack variables) not saved
  across a blocking wait
• Limitation inherited from the event-driven model
• Programmer must manually save automatic variables
• However, static local variables still work
  asexpected
• Standard C compiler issues a warning
• Switches cannot be (freely) mixed with
  protothreads

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How well does it work?

• We reimplemented 7 different event-driven
  programs with protothreads
• Lines of code 31 reduction
• Explicit states, state transitions almost
  entirely removed
• Much easier to write, read, debug code!
• Others are successfully using it

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Download!
http//www.sics.se/adam/pt/