线程特定数据 Thread-Specific Data

 

 Thread-Specific Data

When converting existing functions to run in a threads environment, a common problem encountered is due to static variables. A function that keeps state in a private buffer, or one that returns a result in the form of a pointer to a static buffer, is not thread-safe because multiple threads cannot use the buffer to hold different things at the same time. When faced with this problem, there are various solutions:

• Use thread-specific data. This is nontrivial and then converts the function into one that works only on systems with threads support. The advantage to this approach is that the calling sequence does not change and all the changes go into the library function and not the applications that call the function. We show a version of readline that is thread-safe by using thread-specific data later in this section.

• Change the calling sequence so that the caller packages all the arguments into a structure, and also store in that structure the static variables from Figure 3.18. This was also done, and Figure 26.6 shows the new structure and new function prototypes.