Thread local storage GCC Compiler

Question

I declare a variable __thread int my_id; Info of my platform and compiler:

Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.1-9ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --enable-targets=all --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=i686-linux-gnu --host=i686-linux-gnu --target=i686-linux-gnu
Thread model: posix
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3) 

I use flag -lpthread to compile. But Compiler complains: error: storage class specified for 'my_id'.

Answer 1

You've declared thread local storage for a variable. This kind of storage for a variable cannot be declared as existing on the stack. Making it part of a struct/class implicitly permits it to be on the stack.

The only effective way to accomplish this is to make the variable static - the reason for this is that it causes the variable to be stored in the thread local storage pool instead of on the stack.

e.g. at the global scope:

__thread int my_id;

will work, as the variable is not in a structure. It can automatically be placed into the thread local storage pool.

for a structure, the following:

struct xx {
    __thread int my_id;
};

will not work, as the variable can end up on the stack, so you need to use:

struct xx {
    static __thread int my_id;
};

which causes it to be placed in the thread local storage pool, and thus be a valid thread variable.

Note: I'd previously described the variable as being on the heap. This is strictly speaking incorrect - the variable is taken from a per-thread block of memory that is allocated at thread creation time; I've renamed the term to the 'thread local storage' pool. This pool is of a platform specific size, and can be filled up if you have too many separate __thread variables.

The Wikipedia entry on Thread Local Storage explains this in good detail (including some gotchas).