Portable thread-safety in C?

Question

Purpose

I'm writing a small library for which portability is the biggest concern. It has been designed to assume only a mostly-compliant C90 (ISO/IEC 9899:1990) environment... nothing more. The set of functions provided by the library all operate (read/write) on an internal data structure. I've considered some other design alternatives, but nothing else seems feasible for what the library is trying to achieve.

Question

Are there any portable algorithms, techniques, or incantations which can be used to ensure thread-safety? I am not concerned with making the functions re-entrant. Moreover, I am not concerned with speed or (possibly) wasting resources if the algorithm/technique/incantation is portable. Ideally, I don't want to depend on any libraries (such as GNU Pth) or system-specific operations (like atomic test-and-set).

I have considered modifying Lamport's bakery algorithm, but I do not know how to alter it to work inside of the functions called by the threads instead of working in the threads themselves.

Any help is greatly appreciated.

Answer 1

Lamport's bakery algorithm would probably work; unfortunately, there are still practical problems with it. Specifically, many CPUs implement out-of-order memory operations: even if you've compiled your code into a perfectly correct instruction sequence, the CPU, when executing your code, may decide to reorder the instructions on the fly to achieve better performance. The only way to get around this is to use memory barriers, which are highly system- and CPU-specific.

You really only have two choices here: either (1) keep your library thread-unsafe and make your users aware of that in the documentation, or (2) use a platform-specific mutex. Option 2 can be made easier by using another library that implements mutexes for a large variety of platforms and provides you with a unified, abstract interface.

Answer 2

Without OS/hardware support, at least an atomic CAS, there's nothing you can do that's practical. There are portable libraries that abstract various platforms into a common interface, though.

http://www.gnu.org/software/pth/related.html

Answer 3

Almost all systems (even Windows) can run libpthread these days.

Answer 4

Functions either cannot be thread safe or are innately thread safe, depending on how you want to look at it. And threading/locking is innately platform specific. Really, it is up to users of your library to handle the the threading issues.