Thread Local Storage

Thread local storage (TLS) is a mechanism that allows each thread to have its own copy of a variable. This is useful for variables that are used by multiple functions in a thread, but should not be shared with other threads.

There are some approaches to using TLS in NuttX:

1. Use the pthread_key_create() and pthread_setspecific() that from the POSIX standard. This is the most portable approach, but it requires that the platform support pthreads. 2. Use the thread_local or __thread keyword from the C standard: https://gcc.gnu.org/onlinedocs/gcc/extensions-to-the-c-language-family/thread-local-storage.html

Configuration

CONFIG_SCHED_THREAD_LOCAL  /* Enable native thread local storage support */

Enable it to support native thread local storage, which is required that the compiler configured with --enable-tls option, this approach is more efficient if the compiler support it.

If your compiler support it then you still need further configuration to use it:

Enable CONFIG_SCHED_THREAD_LOCAL in menuconfig
Handle tbss and tdata sections in linker script, your can refer to the example in rv-virt

To confirm that your compiler supports TLS, you can try this command:

arm-none-eabi-gcc --verbose
COLLECT_GCC=arm-none-eabi-gcc
COLLECT_LTO_WRAPPER=/home/huang/.local/pkg/arm/bin/../libexec/gcc/arm-none-eabi/13.3.1/lto-wrapper
Target: arm-none-eabi
Configured with: /data/jenkins/workspace/GNU-toolchain/arm-13/src/gcc/configure --target=arm-none-eabi --prefix=/data/jenkins/workspace/GNU-toolchain/arm-13/build-arm-none-eabi/install --with-gmp=/data/jenkins/workspace/GNU-toolchain/arm-13/build-arm-none-eabi/host-tools --with-mpfr=/data/jenkins/workspace/GNU-toolchain/arm-13/build-arm-none-eabi/host-tools --with-mpc=/data/jenkins/workspace/GNU-toolchain/arm-13/build-arm-none-eabi/host-tools --with-isl=/data/jenkins/workspace/GNU-toolchain/arm-13/build-arm-none-eabi/host-tools --disable-shared --disable-nls --disable-threads --disable-tls --enable-checking=release --enable-languages=c,c++,fortran --with-newlib --with-gnu-as --with-headers=yes --with-gnu-ld --with-native-system-header-dir=/include --with-sysroot=/data/jenkins/workspace/GNU-toolchain/arm-13/build-arm-none-eabi/install/arm-none-eabi --with-multilib-list=aprofile,rmprofile --with-pkgversion='Arm GNU Toolchain 13.3.Rel1 (Build arm-13.24)' --with-bugurl=https://bugs.linaro.org/
Thread model: single
Supported LTO compression algorithms: zlib
gcc version 13.3.1 20240614 (Arm GNU Toolchain 13.3.Rel1 (Build arm-13.24))

Then you can see --disable-tls in the output, which means that your compiler does not support TLS.

In this case, you can still use the thread local relative keyword, but it would be implemented by libgcc’s emutls.