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API Terminology

The following terms may be used as shorthand API tags to indicate the allowed calling context (thread, ISR, pre-kernel), the effect of a call on the current thread state, and other behavioral characteristics.

reschedule: if executing the function reaches a reschedule point
sleep: if executing the function can cause the invoking thread to sleep
no-wait: if a parameter to the function can prevent the invoking thread from trying to sleep
isr-ok: if the function can be safely called and will have its specified effect whether invoked from interrupt or thread context
pre-kernel-ok: if the function can be safely called before the kernel has been fully initialized and will have its specified effect when invoked from that context.
async: if the function may return before the operation it initializes is complete (i.e. function return and operation completion are asynchronous)
supervisor: if the calling thread must have supervisor privileges to execute the function

Details on the behavioral impact of each attribute are in the following sections.

reschedule

The reschedule attribute is used on a function that can reach a reschedule point within its execution.

Details

The significance of this attribute is that when a rescheduling function is invoked by a thread it is possible for that thread to be suspended as a consequence of a higher-priority thread being made ready. Whether the suspension actually occurs depends on the operation associated with the reschedule point and the relative priorities of the invoking thread and the head of the ready queue.

Note that in the case of timeslicing, or reschedule points executed from interrupts, any thread may be suspended in any function.

Functions that are not reschedule may be invoked from either thread or interrupt context.

Functions that are reschedule may be invoked from thread context.

Functions that are reschedule but not sleep may be invoked from interrupt context.

sleep

The sleep attribute is used on a function that can cause the invoking thread to sleep.

Explanation

This attribute is of relevance specifically when considering applications that use only non-preemptible threads, because the kernel will not replace a running cooperative-only thread at a reschedule point unless that thread has explicitly invoked an operation that caused it to sleep.

This attribute does not imply the function will sleep unconditionally, but that the operation may require an invoking thread that would have to suspend, wait, or invoke k_yield() before it can complete its operation. This behavior may be mediated by no-wait.

Functions that are sleep are implicitly reschedule.

Functions that are sleep may be invoked from thread context.

Functions that are sleep may be invoked from interrupt and pre-kernel contexts if and only if invoked in no-wait mode.

no-wait

The no-wait attribute is used on a function that is also sleep to indicate that a parameter to the function can force an execution path that will not cause the invoking thread to sleep.

Explanation

The paradigmatic case of a no-wait function is a function that takes a timeout, to which K_NO_WAIT can be passed. The semantics of this special timeout value are to execute the function’s operation as long as it can be completed immediately, and to return an error code rather than sleep if it cannot.

It is use of the no-wait feature that allows functions like k_sem_take() to be invoked from ISRs, since it is not permitted to sleep in interrupt context.

A function with a no-wait path does not imply that taking that path guarantees the function is synchronous.

Functions with this attribute may be invoked from interrupt and pre-kernel contexts only when the parameter selects the no-wait path.

isr-ok

The isr-ok attribute is used on a function to indicate that it works whether it is being invoked from interrupt or thread context.

Explanation

Any function that is not sleep is inherently isr-ok. Functions that are sleep are isr-ok if the implementation ensures that the documented behavior is implemented even if called from an interrupt context. This may be achieved by having the implementation detect the calling context and transfer the operation that would sleep to a thread, or by documenting that when invoked from a non-thread context the function will return a specific error (generally -EWOULDBLOCK).

Note that a function that is no-wait is safe to call from interrupt context only when the no-wait path is selected. isr-ok functions need not provide a no-wait path.

pre-kernel-ok

The pre-kernel-ok attribute is used on a function to indicate that it works as documented even when invoked before the kernel main thread has been started.

Explanation

This attribute is similar to isr-ok in function, but is intended for use by any API that is expected to be called in DEVICE_DEFINE() or SYS_INIT() calls that may be invoked with PRE_KERNEL_1 or PRE_KERNEL_2 initialization levels.

Generally a function that is pre-kernel-ok checks k_is_pre_kernel() when determining whether it can fulfill its required behavior. In many cases it would also check k_is_in_isr() so it can be isr-ok as well.

async

A function is async (i.e. asynchronous) if it may return before the operation it initiates has completed. An asynchronous function will generally provide a mechanism by which operation completion is reported, e.g. a callback or event.

A function that is not asynchronous is synchronous, i.e. the operation will always be complete when the function returns. As most functions are synchronous this behavior does not have a distinct attribute to identify it.

Explanation

Be aware that async is orthogonal to context-switching. Some APIs may provide completion information through a callback, but may suspend while waiting for the resource necessary to initiate the operation; an example is spi_transceive_async().

If a function is both no-wait and async then selecting the no-wait path only guarantees that the function will not sleep. It does not affect whether the operation will be completed before the function returns.

supervisor

The supervisor attribute is relevant only in user-mode applications, and indicates that the function cannot be invoked from user mode.