Nrk-api-signals-semaphores

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= Signals =
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A signal is a message that a task can use to wakeup one or more tasks waiting on an
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event or events. When waiting on an event, a task is suspended and does not consume
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CPU time. Nano-RK supports 32 unique signals. It is possible to wait on multiple
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signals such that any one of them can wake a task from sleep. All signals must be
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created using nrk_sig_create() before they can be used.  Any task that wishes to wakeup on a signal
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must register the signal using nrk_sig_register().  There are a few special case signals that are
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generated by the kernel used to support event timeouts and notification of special actions.  See the
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'''Special Kernel Signals''' subsection for more information.  For more information about signals, please refer
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to the basic_signals project that comes with Nano-RK: 
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http://www.nanork.org:8000/nano-RK/browser/nano-RK/projects/basic_signals/main.c
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{{{
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#!c
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// This is a macro used to convert a signal into a bitmask for nrk_event_wait()
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// See nrk_event_wait() for an example.
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#define SIG(x)  ((uint32_t)1)<<x 
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// These are typedefs used to represent signals and semaphores
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typedef int8_t nrk_sig_t;
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typedef uint32_t nrk_sig_mask_t;
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typedef int8_t nrk_sem_t;
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}}}
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'''nrk_sig_t nrk_signal_create();'''
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This function creates a signal.  Upon failure, this function returns NRK_ERROR.  Upon success a positive value
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representing the signal is returned.
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'''int8_t nrk_delete_signal(nrk_sig_t sig_id);'''
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This function deletes a signal ''sig_id'' so that it can be reused by different tasks.  This function returns NRK_OK upon success and NRK_ERROR on failure.
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'''int8_t nrk_event_signal(nrk_sig_t sig_id);'''
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This function is used to signal tasks that are waiting on events using nrk_event_wait().
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''sig_id'' is the signal that is sent.  This function returns NRK_OK upon success and NRK_ERROR upon failure.
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If a lower priority task signals a higher priority task, the high priority task will begin to execute at 
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the next context swap.  Normally this happens when the low priority task suspends, but it is also possible
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that a medium priority task could preempt the low priority task causing a context swap that would then
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schedule the waiting high priority task.  For this reason, processing that needs to be complete before the signaled task
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executes should be done before the signals are sent. 
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'''nrk_sig_mask_t nrk_event_wait(nrk_sig_mask_t event_mask);'''
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This function will wait for a set events. nano-RK supports up to 32 signals. Each signal
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represents a bit in a 32 bit number so it is possible to logically
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OR multiple signals together if you wish to wait on a combination of events. The 32
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bit number returned by nrk_event_wait() corresponds to the signal or signals that were returned.
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When waiting on multiple signals, the return value can be used to determine which signal
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triggered the wakeup. All signals need to be registered in order for a task to receive them.
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This function returns 0 upon failure if a signal is specified that does not exist, or is not
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registered.
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'''int8_t nrk_signal_register(nrk_sig_t sig_id);'''
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This function registers a signal ''sig_id'' so that a task is able to receive it.  This function returns NRK_OK upon success and NRK_ERROR upon failure if the signal does not exist.  A signal only needs to be registered for reception and not transmission.
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'''int8_t nrk_signal_unregister(nrk_sig_t sig_id);'''
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This function unregisters a signal ''sig_id'' so that the task is no longer able to be unsuspended by that event.  This function returns NRK_OK upon success and NRK_ERROR uopn failure.
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'''nrk_sig_mask_t nrk_get_active_signal_mask();'''
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This function returns the current registered signal mask for a task.
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= Semaphores =
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A semaphore is a protected variable and constitutes the classic method for restricting
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access to shared resources (e.g. storage,actuators etc) in a multiprogramming environment.
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Nano-RK implements semaphores and signals such that
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tasks that are suspended on an event or waiting for access to a semaphore will not be
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scheduled until the corresponding signal is sent or semaphore becomes available. For more information
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on using semaphores please refer to the basic_sem project that comes with the Nano-RK distribution: 
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http://www.nanork.org:8000/nano-RK/browser/nano-RK/projects/basic_sem/main.c
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''Note that the Atmel ISA does not have a test-and-set instruction, so we provide a best effort implementation by disabling interrupts.'' 
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'''nrk_sem_t* nrk_sem_create(uint8_t count, uint8_t ceiling_priority);'''
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This function creates a semaphore resource, with a priority ceiling value used by the task when accessing the resource. This facilitates the Priority Ceiling Protocol Emulation (PCPE) algorithm used in Nano-RK to avoid priority inversion. ''count'' specifies the number of entries allowed into the critical section. ''ceiling_priority'' sets the ceiling value for the task; Note if
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using PCPE this should be the highest priority task accessing the critical section.  Below is an example of declaring and creating a semaphore:
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{{{
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#!c
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   nrk_sem_t *my_semaphore;
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   ...
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   my_semaphore = nrk_sem_create(1,4);
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   if(my_semaphore==NULL) nrk_kprintf( PSTR("Error creating Semaphore\r\n" ));
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}}}
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This created a semaphore with a count of 1 (also called a mutex or binary semaphore) with a priority ceiling value of 4.
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'''int8_t nrk_sem_pend(nrk_sem_t *rsrc );''' 
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Semaphore pend takes the address of the created semaphore and attempts to access the resource.  If the resource is available,
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pend will decrement the resource counter and allow the program to continue, otherwise pend will suspend until the resource is
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posted by another task. This can be used to protect critical sections of code. Below is an example of a task pending on a semaphore:
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{{{
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#!c
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   nrk_kprintf( PSTR("Task accessing semaphore\r\n"));
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   v = nrk_sem_pend(my_semaphore);
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   if(v==NRK_ERROR) nrk_kprintf( PSTR("Error calling pend\r\n"));
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   nrk_kprintf( PSTR("Task is now holding semaphore\r\n"));
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}}}
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'''int8_t nrk_sem_post(nrk_sem_t* rsrc);'''
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Semaphore post takes the address of a created semaphore and releases access to the resource.
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This should be called after exiting a critical section that was pended.  Below is an example of a task posting a semaphore:
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{{{
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#!c
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   v = nrk_sem_post(my_semaphore);
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   if(v==NRK_ERROR) nrk_kprintf( PSTR("Error calling post\r\n"));
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   nrk_kprintf( PSTR("Task released semaphore\r\n"));
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}}}
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= Special Kernel Signals =
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'''nrk_wakeup_signal'''
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Sometimes it might be convenient to have a timeout associated with an nrk_event_wait() call.   This can be achieved using the kernel generated nrk_wakeup_signal.  This signal is sent on a per-task basis (not globally) when the task’s internal next-wakeup timer expires.  Normally, a task’s next wakeup is set for its next period, however this can be adjusted using the nrk_set_next_wakeup() function. nrk_wakeup_signal is created by the kernel during nrk_init().  In order for nrk_event_wait() to receive the nrk_wakeup_signal from a task it must simply be registered and muxed in like any other signal.  Note, each task has its own instance of the nrk_wakeup_signal.  Unlike other signals, it is not globally broadcast to all tasks.  See below for an example of how to use it as an event  timeout.
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'''int8_t nrk_set_next_wakeup(nrk_time_t timeout);'''
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This function sets the task's next wakeup timer.  It returns NRK_OK upon success and NRK_ERROR on failure.  This function does not
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suspend the task, it only changes when the next wakeup will happen if the task suspends on an event.  Calling other suspend functions like nrk_wait_until_next_period() will replace whatever wakeup value you might have previously set. 
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{{{
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#!c
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   nrk_time_t timeout;
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   timeout.secs=10;
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   timeout.nano_secs=0;
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   ...
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   nrk_set_next_wakeup(timeout);
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   my_sigs=nrk_event_wait( SIG(signal_one) | SIG(nrk_wakeup_signal) );
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   // Lets check which signal we got...
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   if(my_sigs==0)                        nrk_kprintf( PSTR( "Error calling nrk_event_wait()\r\n" ));
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   if(my_sigs & SIG(signal_one))         nrk_kprintf( PSTR( "Task got signal_one\r\n") );
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   if(my_sigs & SIG(nrk_wakeup_signal))  nrk_kprintf( PSTR( "Task got timeout signal! \r\n") );
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}}}
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