.. _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, :kconfig: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 :kconfig: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. :kconfig:option:`CONFIG_ERRNO_IN_TLS` can be enabled together with :kconfig: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; }