Thread control

Description

These functions provide some control over whether or not a particular thread can run. Apart from the required use of cyg_thread_resume to start a newly-created thread, application code should normally use proper synchronization primitives such as condition variables or mail boxes.

Yield

cyg_thread_yield allows a thread to relinquish control of the processor to some other runnable thread which has the same priority. This can have no effect on any higher-priority thread since, if such a thread were runnable, the current thread would have been preempted in its favour. Similarly it can have no effect on any lower-priority thread because the current thread will always be run in preference to those. As a consequence this function is only useful in configurations with a scheduler that allows multiple threads to run at the same priority, for example the mlqueue scheduler. If instead the bitmap scheduler was being used then cyg_thread_yield() would serve no purpose.

Even if a suitable scheduler such as the mlqueue scheduler has been configured, cyg_thread_yield will still rarely prove useful: instead timeslicing will be used to ensure that all threads of a given priority get a fair slice of the available processor time. However it is possible to disable timeslicing via the configuration option CYGSEM_KERNEL_SCHED_TIMESLICE, in which case cyg_thread_yield can be used to implement a form of cooperative multitasking.

Delay

cyg_thread_delay allows a thread to suspend until the specified number of clock ticks have occurred. For example, if a value of 1 is used and the system clock runs at a frequency of 100Hz then the thread will sleep for up to 10 milliseconds. This functionality depends on the presence of a real-time system clock, as controlled by the configuration option CYGVAR_KERNEL_COUNTERS_CLOCK.

If the application requires delays measured in milliseconds or similar units rather than in clock ticks, some calculations are needed to convert between these units as described in Clocks. Usually these calculations can be done by the application developer, or at compile-time. Performing such calculations prior to every call to cyg_thread_delay adds unnecessary overhead to the system.

Suspend and Resume

Associated with each thread is a suspend counter. When a thread is first created this counter is initialized to 1. cyg_thread_suspend can be used to increment the suspend counter, and cyg_thread_resume decrements it. The scheduler will never run a thread with a non-zero suspend counter. Therefore a newly created thread will not run until it has been resumed.

An occasional problem with the use of suspend and resume functionality is that a thread gets suspended more times than it is resumed and hence never becomes runnable again. This can lead to very confusing behaviour. To help with debugging such problems the kernel provides a configuration option CYGNUM_KERNEL_MAX_SUSPEND_COUNT_ASSERT which imposes an upper bound on the number of suspend calls without matching resumes, with a reasonable default value. This functionality depends on infrastructure assertions being enabled.

Releasing a Blocked Thread

When a thread is blocked on a synchronization primitive such as a semaphore or a mutex, or when it is waiting for an alarm to trigger, it can be forcibly woken up using cyg_thread_release. Typically this will call the affected synchronization primitive to return false, indicating that the operation was not completed successfully. This function has to be used with great care, and in particular it should only be used on threads that have been designed appropriately and check all return codes. If instead it were to be used on, say, an arbitrary thread that is attempting to claim a mutex then that thread might not bother to check the result of the mutex lock operation - usually there would be no reason to do so. Therefore the thread will now continue running in the false belief that it has successfully claimed a mutex lock, and the resulting behaviour is undefined. If the system has been built with assertions enabled then it is possible that an assertion will trigger when the thread tries to release the mutex it does not actually own.

The main use of cyg_thread_release is in the POSIX compatibility layer, where it is used in the implementation of per-thread signals and cancellation handlers.

Valid contexts

cyg_thread_yield can only be called from thread context, A DSR must always run to completion and cannot yield the processor to some thread. cyg_thread_suspend, cyg_thread_resume, and cyg_thread_release may be called from thread or DSR context.