FreeRTOS V8 is Backward Compatible with FreeRTOS V7
FreeRTOS V8.x.x is a drop-in compatible replacement for FreeRTOS V7.x, although a
change to the type used to reference character strings may result in application
code generating a few (easily clearable) compiler warnings after the upgrade,
and an updated typedef naming convention means use of the old typedef names is
now discouraged (see config configENABLE_BACKWARD_COMPATIBILITY).
FreeRTOS V8 includes a brand new feature - Event Groups. The event groups
implementation is contained in a new source file named event_groups.c.
Like the existing timers.c source file, event_groups.c is
optional and need only be included in the project if your application is using
Ideally an interrupt service routine (ISR) is kept as short as possible, but
sometimes an ISR either has a lot of processing to do, or needs to perform
processing that is not deterministic. Historically in these cases it was common
to use a semaphore to unblock a high priority application task from the interrupt,
deferring the actually processing to the application task (so called deferred
interrupt processing). FreeRTOS allows the interrupt to return directly to the
unblocked task to ensure the processing happenes immediately.
FreeRTOS V8 provides a centralised deferred interrupt processing mechanism - allowing
deferred processing to be performed in the RTOS daemon task (formally called the
timer daemon task), removing the burden of implementing deferred interrupt mechanisms
from the user.
Other than for character pointers (see below) all standard C types have been
replaced with their equivalent typedef from the stdint.h header file.
For example, unsigned long has been replaced by uint32_t,
short has been replaced by int16_t, etc.
Character pointers are used to reference text strings, such as the name allocated
to a task.
Previously, and in line with good software engineering practice, the FreeRTOS
coding standard did not permit the use of char types that were not explicitly
qualified as either signed or unsigned.
As a result char pointers used to reference strings required casts, as did the use
of any standard string handling functions. The casts ensured compiler warnings
were not generated by compilers that defaulted unqualified char types to
be signed or compilers that defaulted unqualified char types to be
As it has in later MISRA standards, this rule has now been relaxed, and unqualified
char types are now permitted, but only when:
The char is used to point to a human readable text string.
The char is used to hold a single ASCII character.
To ensure consistency with the newer FreeRTOS+ software components, provide
consistency with the stdint.h defined typedefs, and to
accommodate multiple user requests, the names assigned to typedefs within the
core FreeRTOS code have changed as per the table below. All typedefed types now
end with '_t'.
NOTE: To ensure backward compatibility the old names are maintained within
the code, but the use of the old names is strongly discouraged (see config
Used to reference tasks.
Used to reference queues.
Used to reference binary, counting, recursive and mutex type semaphores.
Used to reference software timers.
Used to reference co-routines.
Used to hold tick count values
Defined to the most efficient signed type for the architecture.
Defined to the most efficient unsigned type for the architecture.