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Semaphores

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Sleeping Barber

Hypothetical barber shop

1 barber

1 barber chair

A number of chairs for waiting customers

When the barber has no customers, he sleeps

When a customer arrives, he either

Sits in the barber chair and wakes the barber Sits in a vacant waiting chair

Leaves if the barber shop is full

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What does this have to do with

anything?

Analogy for a classic CS problem

Barber and customers are processes or

Tasks

Inter-process communication and

synchronization

Most common solution is to use

semaphores

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Sleeping Barber Semaphores

Customer arrives and looks for an empty seat.

He either

Sits in an empty seat and signals the barber with a

semaphore Leaves if there are no empty seats

If the barber was asleep, a waiting customers

semaphore wakes him up

If the barber has multiple customers waiting, he

uses a semaphore to signal them when hes

finished with a previous customer

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Semaphores - Definition

A semaphore is a protected variable (or

abstract data type) and constitutes the

classic method for restricting access to

shared resources (e.g. storage) in amultiprogramming environment.

Basically, simple inter-process or inter-

task communication

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How will we use them?

Semaphores are known to Salvo as

events

salvocfg.h

#define OSEVENTS (# of semaphores youll use)

#define OSTASKS (# of tasks youll use)

Semaphores are defined in main.h

main.h #define (semaphore name) OSECBP(# of semaphore)

#define BINSEM_CMD_CHAR_P OSECBP(1)

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How will we use them? (2)

In the main loop, we must create the event (just likecreating tasks) main.c

OSCreateBinSem(semaphore name, number);

OSCreateBinSem(BINSEM_CMD_CHAR_P, 0);

Now we can have Tasks wait for this semaphore In a task (or anywhere you want to wait)

OS_WaitBinSem(semaphore name, how long to wait, label);

OS_WaitBinSem(BINSEM_CMD_CHAR_P, OSNO_TIMEOUT,label);

We can also have Tasks or other parts of the programsend out this semaphore Anywhere you want to send the signal

OSSignalBinSem(semaphore name);

OSSignalBinSem(BINSEM_CMD_CHAR_P);

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Sleeping Barber

main.h has the lines

#define CUSTOMER_WAITING OSECBP(1)

#define BARBER_READY OSECBP(2)

Barber task

for(;;){

OS_WaitBinSem(CUSTOMER_WAITING,

OSNO_TIMEOUT, label);

wakeup();cuthair(customer);

OSSignalBinSem(BARBER_READY);

sleep();

}

Customer task

for(;;){

If (chair available){

OSSignalBinSem(CUSTOMER_WAITING);

sit(chair);}

If (sitting && next){

OS_WaitBinSem(BARBER_READY,

OSNO_TIMEOUT, label);

sit(barber chair);

OSSignalBinSem(CUSTOMER_WAITING);

}}

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Todays Lab

Want the Cubesat kit to spit out over the debug channelany characters it receives over the radio. I received blah

When this is working over debug, try transmitting it

If the character is a t, transmit the onboard temperaturereading

Well be passing out sensors and data sheets Figure out how to use them

Design a circuit to use them

Write the code to use them

Can also begin pseudo-coding your rovers TaskDoCmds

If (cmd==w) { move_forward(); }