Also, as someone who wrote code that had to support C89-only compilers as recently as 5 years ago, I noticed immediately that this code mixes code and variable

Also, as someone who wrote code that had to support C89-only compilers as recently as 5 years ago, I noticed immediately that this code mixes code and variable declarations which strictly C89 compatible compilers will barf on. That said, this code also uses pthreads which is not exactly as portable as "pure C89" implies either.