Coding Standard, Testing and Style Guide
On this page:
The core FreeRTOS source files (those that are common to all ports, but not the
port layer) conform to
coding standard guidelines. Compliance is checked using
pc-lint with the
linked lint configuration files.
As the standard is many pages long, and is available for purchase from MISRA for
a very small fee, we have not replicated all the rules here.
Deviations from the MISRA standard are listed below:
FreeRTOS builds with many different compilers, some of which are more advanced
than others. For that reason FreeRTOS does not use any of the features or
syntax that have been introduced to the C language by or since the C99 standard.
The one exception to this is the use of the stdint.h header file.
The FreeRTOS/Source/include directory contains a file called
stdint.readme that can be renamed stdint.h to provide the minimum
stdint type definitions necessary to build FreeRTOS - should your compiler not
provide its own.
Two API functions have more than one exit point. A deviation was permitted
in these two cases for reasons of critical efficiency.
When creating tasks, the source code manipulates memory addresses to
locate the start and end addresses of the stack allocated to the created
task. The code has to work for all the architectures to which FreeRTOS
has been ported - which includes architectures with 8, 16, 20, 24 and 32-bit
buses. This inevitably requires some pointer arithmetic. When pointer
arithmetic is used, the arithmetic result is programatically checked for
The trace macros are, by default, empty, so do not generate any code.
Therefore, MISRA compliance checking is performed with dummy macro
MISRA rules are turned off on a line by line basis, as deemed appropriate
(that is, complying with the rule is deemed to create less appropriate
code for a deaply embedded system than carefully not complying). Each
such occurrence is accompanied by a justification using the special
pc-lint MISRA comment mark-up syntax.
This section describes the tests performed on common code (the code
the FreeRTOS/Source directory, that is built by all FreeRTOS kernel
ports), and the tests performed on the portable layer code (the code located in
subdirectories of the FreeRTOS/Source/portable directory).
It is worth noting that the thoroughness of these tests have been responsible
for finding bugs in silicon on multiple occasions.
The standard demo/test files attempt to provide 'branch' test coverage
(in most cases this actually achieves
because the kernel's coding style deliberately keeps conditions simple
specifically for this purpose),
whereby tests ensure both 'true' and 'false' paths through each decision
are exercised. 'branch' coverage is measured using GCOV by defining
the mtCOVERAGE_TEST_MARKER() macro to a NOP (no operation) instruction
in the 'else' path of each 'if()' condition if the 'else' path would
otherwise be empty. mtCOVERAGE_TEST_MARKER() is only defined while
measuring test coverage - normally it is an empty macro that does not
generate any code.
Port layer code is tested using 'reg test' tasks, and, for ports that
support interrupt nesting, the 'interrupt queue' tasks.
The 'reg test' tasks create multiple (normally two) tasks that first fill
all the CPU registers with known values, then continuously check that every
register maintains its expected known value as the other tests execute
continuously (soak test). Each reg test task uses unique values.
The 'interrupt queue' tasks perform tests on interrupts of different
priorities that nest at least three deep. Macros are used to insert
artificial delays into pertinent points within the code to ensure the
desired test coverage is achieved.
The RTOS kernel and demo application source code use the following conventions:
Variables of type uint32_t are prefixed ul,
where the 'u' denotes 'unsigned' and the 'l' denotes 'long'.
Variables of type uint16_t are prefixed us,
where the 'u' denotes 'unsigned' and the 's' denotes 'short'.
Variables of type uint8_t are prefixed uc,
where the 'u' denotes 'unsigned' and the 'c' denotes 'char'.
Variables of non stdint types are prefixed x. Examples
include BaseType_t and TickType_t, which are portable layer
defined typedefs for the natural or most efficient type for
the architecture and the type used to hold the RTOS tick
Unsigned variables of non stdint types have an additional
prefix u. For example variables of type UBaseType_t
(unsigned BaseType_t) are prefixed ux.
Variables of type size_t are also prefixed x.
Enumerated variables are prefixed e
Pointers have an additional prefixed p, for example a
pointer to a uint16_t will have prefix pus.
In line with MISRA guides, unqualified standard char
types are only permitted to hold ASCII characters and are
In line with MISRA guides, variables of type char *
are only permitted to hold pointers to ASCII strings and are
File scope static (private) functions are prefixed with prv.
API functions are prefixed with their return type, as per the
convention defined for variables, with the addition of the
prefix v for void.
API function names start with the name of the file in which
they are defined. For example vTaskDelete is defined in
tasks.c, and has a void return type.
Macros are pre-fixed with the file in which they are defined.
The pre-fix is lower case. For example, configUSE_PREEMPTION
is defined in FreeRTOSConfig.h.
Other than the pre-fix, macros are written in all upper case,
and use an underscore to separate words.
Only stdint.h types and the RTOS's own typedefs are used, with the following exceptions:
In line with MISRA guides, unqualified char types are permitted, but only
when they are used to hold ASCII characters.
In line with MISRA guides, unqualified character pointers are permitted,
but only when they are used to point to ASCII strings.
This removes the need to
suppress benign compiler warnings when standard library functions that
expect char * parameters are used, especially considering some compilers
default unqualified char types to be signed while other compilers default
unqualified char types to be unsigned.
There are four types that are defined for each port. These are:
If configUSE_16_BIT_TICKS is set to non-zero (true), then TickType_t is
defined to be an unsigned 16-bit type. If configUSE_16_BIT_TICKS is set
to zero (false), then TickType_t is defined to be an unsigned 32-bit type.
See the customisation section of
the API documentation for full information.
32-bit architectures should always set configUSE_16_BIT_TICKS to 0.
This is defined to be the most efficient, natural, type for the architecture.
For example, on a 32-bit architecture BaseType_t will be defined to be
a 32-bit type. On a 16-bit architecture BaseType_t will be defined to
be a 16-bit type.
If BaseType_t is define to char then particular care must be taken
to ensure signed chars are used for function return values that can be
negative to indicate an error.
This is an unsigned BaseType_t.
Defined to the type used by the architecture for items stored on the
stack. Normally this would be a 16-bit type on 16-bit architectures
and a 32-bit type on 32-bit architectures, although there are some
exceptions. Used internally be FreeRTOS.
Tab characters are used to indent. One tab equals four spaces.
Comments never pass column 80, unless they follow, and describe, a
C++ style double slash (//) comments are not used.
The FreeRTOS source code lay out is designed to be as easy to view and read as
possible. The code snippets below show first the file layout, then
the C code formatting.
/* Library includes come first... */
/* ...followed by FreeRTOS includes... */
/* ...followed by other includes. */
/* #defines comes next, bracketed where possible. */
#define A_DEFINITION ( 1 )
* Static (file private) function prototypes appear next, with comments
* in this style - with each line starting with a '*'.
static void prvAFunction( uint32_t ulParameter );
/* File scope variables are the last thing before the function definitions.
Comments for variables are in this style (without each line starting with
a '*'). */
static BaseType_t xMyVariable.
/* The following separate is used after the closing bracket of each function,
with a blank line following before the start of the next function definition. */
void vAFunction( void )
/* Function definition goes here - note the separator after the closing
curly bracket. */
static UBaseType_t prvNextFunction( void )
/* Function definition goes here. */
/* Function names are always written on a single line, including the return
type. As always, there is no space before the opening parenthesis. There
is a space after an opening parenthesis. There is a space before a closing
parenthesis. There is a space after each comma. Parameters are given
verbose, descriptive names (unlike this example!). The opening and closing
curly brackets appear on their own lines, lined up underneath each other. */
void vAnExampleFunction( long lParameter1, unsigned short usParameter2 )
/* Variable declarations are not indented. */
/* Code is indented. Curly brackets are always on their own lines
and lined up underneath each other. */
for( ucByte = 0U; ucByte < fileBUFFER_LENGTH; ucByte++ )
/* Indent again. */
/* For, while, do and if constructs follow a similar pattern. There is no
space before the opening parenthesis. There is a space after an opening
parenthesis. There is a space before a closing parenthesis. There is a
space after each semicolon (if there are any). There are spaces before and
after each operator. No reliance is placed on operator precedence -
parenthesis are always used to make precedence explicit. Magic numbers,
other than zero, are always replaced with a constant or #defined constant.
The opening and closing curly brackets appear on their own lines. */
for( ucByte = 0U; ucByte < fileBUFFER_LENGTH; ucByte++ )
while( ucByte < fileBUFFER_LENGTH )
/* There must be no reliance on operator precedence - every condition in a
multi-condition decision must uniquely be bracketed, as must all
if( ( ucByte < fileBUFFER_LENGTH ) && ( ucByte != 0U ) )
/* Example of no reliance on operator precedence! */
ulResult = ( ( ulValue1 + ulValue2 ) - ulValue3 ) * ulValue4;
/* Conditional compilations are laid out and indented as per any
other code. */
#if( configUSE_TRACE_FACILITY == 1 )
/* Add a counter into the TCB for tracing only. */
pxNewTCB->uxTCBNumber = uxTaskNumber;
A space is placed after an opening square bracket, and before a closing
ucBuffer[ 0 ] = 0U;
ucBuffer[ fileBUFFER_LENGTH - 1U ] = 0U;
Formatting of C Constructs
Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.
FreeRTOS v10.2.0 is available for
immediate download. MIT
licensed, and including
RISC-V and ARMv8-M
NXP tweet showing LPC5500 (ARMv8-M Cortex-M33) running FreeRTOS.
View a recording
of the "OTA Update Security and Reliability" webinar,
presented by TI and AWS.
FreeRTOS and other embedded software careers at AWS.