Thread Flags

Thread flags.

This API can be used to notify threads of conditions in a race-free and allocation-less way.

Each thread can handle up to 16 boolean flags, stored as a bitmask in the flags field of its thread. Those flags can be set or unset, using thread_flags.h::thread_flags_set(), from ISR’s, other threads or even by the thread itself.

A thread can wait for any combination of its flags to become set, using thread_flags.h::thread_flags_wait_any() or thread_flags.h::thread_flags_wait_all(). Those functions clear flags that caused them to return. It is not possible to wait for flags to become unset.

Thread flags have certain properties that make them the preferred choice over messages or mutexes in some circumstances:

  • setting thread flags cannot fail If messages are used to notify a thread of a condition from within an ISR, and the receiving thread is not waiting, has no queue or the queue is full, the ISR cannot deliver the message. A thread flag can always be set.
  • thread flags are very flexible With thread flags it is possible to wait for multiple conditions and messages at the same time. When mutexes are used to notify about events, only one event can be waited for.

Usually, if it is only of interest that an event occurred, but not how many of them, thread flags should be considered.

Note that some flags (currently the three most significant bits) are used by core functions and should not be set by the user. They can be waited for.

This API is optional and must be enabled by adding “core_thread_flags” to USEMODULE.

THREAD_FLAG_MSG_WAITING

Set when a message becomes available. 

1
(1u << 15)

This flag is set for a thread if a message becomes available either by being queued in the thread’s message queue or by another thread becoming send-blocked.

It is only reset through thread_flags_wait_*(), not by msg.h::msg_receive().

One thread flag event might point to one, many or no messages ready to be received. It is advisable to use the non-blocking msg.h::msg_try_receive() in a loop to retrieve the messages. Use e.g., the following pattern when waiting for both thread flags and messages:

THREAD_FLAG_TIMEOUT

Set by xtimer.h::xtimer_set_timeout_flag() when the timer expires. 

1
(1u << 14)

See also

xtimer.h::xtimer_set_timeout_flag()

uint16_t thread_flags_t

Type definition of thread_flags_t.

void thread_flags_set(sched.h::thread_t * thread, thread_flags.h::thread_flags_t mask)

Set thread flags, possibly waking it up.

Parameters

thread:thread to work on
mask:additional flags to be set for the current thread, represented as a bitmask

thread_flags.h::thread_flags_t thread_flags_clear(thread_flags.h::thread_flags_t mask)

Clear current thread’s flags.

Parameters

mask:unset flags for the current thread, represented as a bitmask

Return values

  • flags that have actually been cleared (mask & thread->flags before clear)
thread_flags.h::thread_flags_t thread_flags_wait_any(thread_flags.h::thread_flags_t mask)

Wait for any flag in mask to become set (blocking)

If any of the flags in mask are already set, this function will return immediately, otherwise, it will suspend the thread (as THREAD_STATUS_WAIT_ANY) until any of the flags in mask get set.

Both ways, it will clear and return (sched_active_thread-flags & mask).

Parameters

mask:mask of flags to wait for

Return values

  • flags that caused return/wakeup ((sched_active_thread-flags & mask).
thread_flags.h::thread_flags_t thread_flags_wait_all(thread_flags.h::thread_flags_t mask)

Wait for all flags in mask to become set (blocking)

If all the flags in mask are already set, this function will return immediately, otherwise, it will suspend the thread (as THREAD_STATUS_WAIT_ALL) until all of the flags in mask have been set.

Both ways, it will clear and return (sched_active_thread-flags & mask).

Parameters

mask:mask of flags to wait for

Return values

  • mask
thread_flags.h::thread_flags_t thread_flags_wait_one(thread_flags.h::thread_flags_t mask)

Wait for any flags in mask to become set (blocking), one at a time.

This function is like thread_flags.h::thread_flags_wait_any(), but will only clear and return one flag at a time, least significant set bit first.

Parameters

mask:mask of flags to wait for

Return values

  • flag that triggered the return / wait
int thread_flags_wake(sched.h::thread_t * thread)

Possibly Wake up thread waiting for flags.

Wakes up a thread if it is thread flag blocked and its condition is met. Has to be called with interrupts disabled. Does not trigger yield.