Coding Style and Conventions
Naming Conventions
Unlike desktop operating systems, where applications are written in user-space and drivers are used to cross the boundary between kernel and user space, all applications in the Zephyr Kernel are written in kernel space. Applications are linked with the kernel, creating a shared and common namespace.
To ensure proper execution of both kernel and applications, it makes sense to divide the namespace into kernel and application subspaces. This is achieved by restricting the kernel’s global symbols and macros to a well-defined set of name prefixes. These prefixes apply both to public symbols, which applications can reference, and to private symbols, which only the kernel itself is permitted to reference. Symbols that do not begin with a kernel namespace prefix are available to applications with a few exceptions. See Exceptions to the Namespace for details.
Prefix | Description | Example |
---|---|---|
_ | Denotes a private symbol. | _k_signal_event |
atomic_ | Denotes an atomic operation. | atomic_inc |
device_ | Denotes an API relating to devices and their initialization. | device_get_binding |
fiber_ | Denotes an operation invoked by a fiber; typically a microkernel operation. | fiber_event_send |
irq_ | Denotes an IRQ management operation. | irq_disable |
isr_ | Denotes an operation called by an Interrupt Service Routine; typically a microkernel operation. | isr_event_send |
k_ | Microkernel-specific function. | k_memcpy |
k_do_ | Microkernel-specific functions indicating essential operation within the kernel space. Do not use these functions unless absolutely necessary. | k_do_event_signal |
nano_ | Denotes an operation provided by the nanokernel; typically used in a microkernel system, not just a nanokernel system. | nano_fifo_put |
sys_ | Catch-all for APIs that do not fit into the other namespaces. | sys_write32 |
task_ | Denotes an operation invoked by a task; typically a microkernel operation. | task_send_event |
If your additional symbol does not fall into the above classification, consider renaming it.
Exceptions to the Namespace
Some kernel APIs use well-known names that lack prefixes. A few examples are:
ntohl
open
close
read
write
ioctl
- In rare cases a few global symbols do not use the normal kernel prefixes;
kernel_version_get()
is one such example.
Subsystem Naming Conventions
Generally, any sub-system can define its own naming conventions for symbols.
However, these should be implemented with their own namespace prefix (for
example, bt\_
for BlueTooth, or net\_
for IP). This limits possible
clashes with applications. Following this prefix convention with subsystems
keeps a consistent interface for all users.
Minimize Include Paths
The current build system uses a series of defs.objs
files to define the
common pieces for a given subsystem. For example, common defines for x86
architecture are located under $ROOT/arch/x86
, with platform-specific
defines underneath it, like $ROOT/arch/x86/platform/ia32
.
Be careful to not add all possible include
paths to the
defs.obj
files. Too many default paths can cause problems when more than
one file has the same name. The only include paths
into
$vBASE/include
should be $vBASE/include
itself, and the header
files should be included with:
#include <subdirectory/header.h>.
For example, if you have two files, include/pci.h
and
include/drvers/pci.h
, and have set both -Iinclude/drivers
and -Iinclude
for your compile, then any code using
#include <pci.h> becomes ambiguous, while
#include <drivers/pci.h>
is not. Not having -Iinclude/drivers
forces users to use the second
form which is more explicit.
Return Codes
Zephyr uses the standard codes in errno.h
for all APIs.
As a general rule, 0
indicates success; a negative errno.h code indicates
an error condition. The table below shows the error code conventions based on
device driver use cases, but they can also be applied to other kernel
components.
Code | Meaning |
---|---|
0 | Success. |
-EIO | General failure. |
-ENOTSUP | Operation is not supported or operation is invalid. |
-EINVAL | Device configuration is not valid or function argument is not valid. |
-EBUSY | Device controller is busy. |
-EACCES | Device controller is not accessible. |
-ENODEV | Device type is not supported. |
-EPERM | Device is not configured or operation is not permitted. |
-ENOSYS | Function is not implemented. |
Coding Style
Use this coding guideline to ensure that your development complies with the project’s style and naming conventions.
In general, follow the Linux kernel coding style, with the following exceptions:
- Add braces to every
if
andelse
body, even for single-line code blocks. Use the--ignore BRACES
flag to make checkpatch stop complaining. - Use hard tab stops. Set the tab width 8 spaces. Break lines at 80 characters. If you are trying to align comments after declarations, use spaces instead of tabs to align them.
- Use C89-style single line comments,
/* */
. The C99-style single line comment, //, is not allowed. - Use
/** */
for any comments that need to appear in the documentation.
Checking for Conformity Using Checkpatch
The Linux kernel GPL-licensed tool checkpatch is used to
check coding style conformity. Checkpatch is available in the
scripts directory. To invoke it when committing code, edit your
.git/hooks/pre-commit
file to contain:
#!/bin/sh
set -e exec
exec git diff --cached | ${ZEPHYR_BASE}/scripts/checkpatch.pl - || true