Frequently Asked Questions about zlib
Is zlib thread-safe? Yes. However any library routines that zlib uses and any application- provided memory allocation routines must also be thread-safe. zlib's gz* functions use stdio library routines, and most of zlib's functions use the library memory allocation routines by default. zlib's *Init* functions allow for the application to provide custom memory allocation routines.
Of course, you should only operate on any given zlib or gzip stream from a single thread at a time.