[MaxBarraclough]

  > The code wasn't standard C, and it didn't just "happen" to work

Thanks, that makes sense.

At the risk of sounding pedantic, you did say using nothing but C99 or C90, implying use of standard features only.

  > You probably wouldn't want to build an atomic compare-exchange in
  > standard C, even if it were possible; you find out what the
  > hardware provides and work with that.

Agreed.

[kazinator]

Right; I could do it using nothing but standard C features in C source files, by defining some C compatible assembly language routines. The compare_swap primitive can be an external function, e.g.:

  bool cmpswap64(uint64_t *location, uint64_t if_old, uint64_t then_new);

Code that relies on the header doesn't have to process anything compiler-specific. FFI could be used to bind to that function from non-C languages.

[vbtemp]

Interesting discussion, lots of good points all around. Thanks to you both.