Are function static variables thread-safe in GCC?

c++ gcc static initialization thread-safety
CsTamas picture CsTamas · Aug 13, 2009 · Viewed 26.8k times · Source

In the example code

void foo()
{
  static Bar b;
  ...
}

compiled with GCC is it guaranteed that b will be created and initialized in a thread-safe manner ?

In gcc's man page, found the -fno-threadsafe-statics command line option:

Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.

  1. Does it mean, that local statics are thread-safe by default with GCC ? So no reason to put explicit guarding e.g. with pthread_mutex_lock/unlock ?

  2. How to write portable code - how to check if compiler will add its guards ? Or is it better to turn off this feature of GCC ?

Answer

CB Bailey picture CB Bailey · Aug 13, 2009

  1. No, it means that the initialization of local statics is thread-safe.

  2. You definitely want to leave this feature enabled. Thread-safe initialization of local statics is very important. If you need generally thread-safe access to local statics then you will need to add the appropriate guards yourself.

Related questions

Static variables initialisation order

C++ guarantees that variables in a compilation unit (.cpp file) are initialised in order of declaration. For number of compilation units this rule works for each one separately (I mean static variables outside of classes). But, the order of initialization …

Read More
C++: When (and how) are C++ Global Static Constructors Called?

I'm working on some C++ code and I've run into a question which has been nagging me for a while... Assuming I'm compiling with GCC on a Linux host for an ELF target, where are global static constructors and destructors …

Read More
"static const int" causes linking error (undefined-reference)

I am baffled by the linker error when using the following code: // static_const.cpp -- complete code #include <vector> struct Elem { static const int value = 0; }; int main(int argc, char *argv[]) { std::vector<Elem> v(1); …

Read More