I am trying to make some code thread safe for use with pthreads. This code is written in C++, but is linked to using SWIG. g++ compiles this correctly, but when swig tries to create a wrapper, I get: fast_alloc.hh:109: Error: Syntax error in input(3) The original, unsafe code that compiles correctly is: static void *freeLists[Num_Buckets]; and the error occurs when I change it to: static __thread void *freeLists[Num_Buckets]; I have made other parts thread safe adding "__thread", for example, this works: static __thread unsigned newCount[Num_Buckets];