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.. _thread_local_storage:
Thread Local Storage (TLS)
##########################
Thread Local Storage (TLS) allows variables to be allocated on a per-thread
basis. These variables are stored in the thread stack which means every
thread has its own copy of these variables.
Zephyr currently requires toolchain support for TLS.
Configuration
*************
To enable thread local storage in Zephyr, :option:`CONFIG_THREAD_LOCAL_STORAGE`
needs to be enabled. Note that this option may not be available if
the architecture or the SoC does not have the hidden option
:option:`CONFIG_ARCH_HAS_THREAD_LOCAL_STORAGE` enabled, which means
the architecture or the SoC does not have the necessary code to support
thread local storage and/or the toolchain does not support TLS.
:option:`CONFIG_ERRNO_IN_TLS` can be enabled together with
:option:`CONFIG_ERRNO` to let the variable ``errno`` be a thread local
variable. This allows user threads to access the value of ``errno`` without
making a system call.
Declaring and Using Thread Local Variables
******************************************
The keyword ``__thread`` can be used to declare thread local variables.
For example, to declare a thread local variable in header files:
.. code-block:: c
extern __thread int i;
And to declare the actual variable in source files:
.. code-block:: c
__thread int i;
Keyword ``static`` can also be used to limit the variable within a source file:
.. code-block:: c
static __thread int j;
Using the thread local variable is the same as using other variable, for example:
.. code-block:: c
void testing(void) {
i = 10;
}