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Kernel Services

The Zephyr kernel lies at the heart of every Zephyr application. It provides a low footprint, high performance, multi-threaded execution environment with a rich set of available features. The rest of the Zephyr ecosystem, including device drivers, networking stack, and application-specific code, uses the kernel’s features to create a complete application.

The configurable nature of the kernel allows you to incorporate only those features needed by your application, making it ideal for systems with limited amounts of memory (as little as 2 KB!) or with simple multi-threading requirements (such as a set of interrupt handlers and a single background task). Examples of such systems include: embedded sensor hubs, environmental sensors, simple LED wearable, and store inventory tags.

Applications requiring more memory (50 to 900 KB), multiple communication devices (like Wi-Fi and Bluetooth Low Energy), and complex multi-threading, can also be developed using the Zephyr kernel. Examples of such systems include: fitness wearables, smart watches, and IoT wireless gateways.

Scheduling, Interrupts, and Synchronization

These pages cover basic kernel services related to thread scheduling and synchronization.

• Threads
• Scheduling
• CPU Idling
• System Threads
• Workqueue Threads
• Operation without Threads
• Interrupts
• Polling API
• Semaphores
• Mutexes
• Condition Variables
• Events
• Symmetric Multiprocessing

Data Passing

These pages cover kernel objects which can be used to pass data between threads and ISRs.

The following table summarizes their high-level features.

Object	Bidirectional?	Data structure	Data item size	Data Alignment	ISRs can receive?	ISRs can send?	Overrun handling
FIFO	No	Queue	Arbitrary [1]	4 B [2]	Yes [3]	Yes	N/A
LIFO	No	Queue	Arbitrary [1]	4 B [2]	Yes [3]	Yes	N/A
Stack	No	Array	Word	Word	Yes [3]	Yes	Undefined behavior
Message queue	No	Ring buffer	Power of two	Power of two	Yes [3]	Yes	Pend thread or return -errno
Mailbox	Yes	Queue	Arbitrary [1]	Arbitrary	No	No	N/A
Pipe	No	Ring buffer [4]	Arbitrary	Arbitrary	No	No	Pend thread or return -errno

[1] Callers allocate space for queue overhead in the data elements themselves.

[2] Objects added with k_fifo_alloc_put() and k_lifo_alloc_put() do not have alignment constraints, but use temporary memory from the calling thread’s resource pool.

[3] ISRs can receive only when passing K_NO_WAIT as the timeout argument.

[4] Optional.

• Queues
• FIFOs
• LIFOs
• Stacks
• Message Queues
• Mailboxes
• Pipes

Memory Management

See Memory Management.

Timing

These pages cover timing related services.

• Kernel Timing
• Timers

Other

These pages cover other kernel services.

• Atomic Services
• Floating Point Services
• Version
• Fatal Errors
• Thread Local Storage (TLS)