The functions for atomic operations described above are either implemented via compiler intrinsics (if the underlying host is capable) or by library fallbacks.

Compiler intrinsics (builtins) are always preferred. However, as the compiler builtins for atomics are not universally implemented, using them directly is problematic, and can result in undefined function calls. (An example of an undefined symbol from the use of __sync_fetch_and_add on an unsupported host is a missing reference to __sync_fetch_and_add_4.)

In addition, on some hosts the compiler intrinsics are enabled conditionally, via the -march command line flag. This makes usage vary depending on the target hardware and the flags used during compile.

If builtins are possible for bool-sized integral types, _GLIBCXX_ATOMIC_BUILTINS_1 will be defined. If builtins are possible for int-sized integral types, _GLIBCXX_ATOMIC_BUILTINS_4 will be defined.

For the following hosts, intrinsics are enabled by default.

For others, some form of -march may work. On non-ancient x86 hardware, -march=native usually does the trick.

For hosts without compiler intrinsics, but with capable hardware, hand-crafted assembly is selected. This is the case for the following hosts:

And for the rest, a simulated atomic lock via pthreads.

Detailed information about compiler intrinsics for atomic operations can be found in the GCC documentation.

More details on the library fallbacks from the porting section.

A thin layer above IEEE 1003.1 (i.e. pthreads) is used to abstract the thread interface for GCC. This layer is called "gthread," and is comprised of one header file that wraps the host's default thread layer with a POSIX-like interface.

The file <gthr-default.h> points to the deduced wrapper for the current host. In libstdc++ implementation files, <bits/gthr.h> is used to select the proper gthreads file.

Within libstdc++ sources, all calls to underlying thread functionality use this layer. More detail as to the specific interface can be found in the source documentation.

By design, the gthread layer is interoperable with the types, functions, and usage found in the usual <pthread.h> file, including pthread_t, pthread_once_t, pthread_create, etc.