Shared Resources

The present working directory is a resource shared by all interpreters in all threads. If one thread changes the present working directory, then that change affects all interpreters and all threads. This can pose a significant problem, as some library routines temporarily change the present working directory during execution, and then restore it before returning. But in a multi-threaded application, another thread could attempt to access the present working directory during this period and get incorrect results. Therefore, the safest approach if your application needs to access the present working directory is to store this value in a global or thread-shared variable before creating any other threads. The following example uses tsv::set to store the current directory in the pwd element of the application shared variable:

package require Thread
# Save the pwd in a thread-shared variable
tsv::set application pwd [pwd]
set t [thread::create {#...}]

Environment variables are another shared resource. If one thread makes a change to an environment variable, then that change affects all threads in your application. This might make it tempting to use the global env array as a method for sharing information between threads. However, you should not do so, because it is far less efficient than thread-shared variables, and there are subtle differences in the way environment variables are handled on different platforms. If you need to share information between threads, you should instead use thread-shared variables, as discussed in "Shared Variables" on page 337.

Although technically not a shared resource, it's important to recognize that the exit command kills the entire application, no matter which thread executes it. Therefore, you should never call exit from a thread when your intention is to terminate only that thread.