31.3. Thread-Specific Data

The most efficient way of making a function thread-safe is to make it reentrant. All new library functions should be implemented in this way. However, for an existing nonreentrant library function (one that was perhaps designed before the use of threads became common), this approach usually requires changing the function’s interface, which means modifying all of the programs that use the function.

Thread-specific data is a technique for making an existing function thread-safe without changing its interface. A function that uses thread-specific data may be slightly less efficient than a reentrant function, but allows us to leave the programs that call the function unchanged.