11.3.1 Thread Cells

A thread cell contains a thread-specific value; that is, it contains a specific value for each thread, but it may contain different values for different threads. A thread cell is created with a default value that is used for all existing threads. When the cell’s content is changed with thread-cell-set!, the cell’s value changes only for the current thread. Similarly, thread-cell-ref obtains the value of the cell that is specific to the current thread.

A thread cell’s value can be preserved, which means that when a new thread is created, the cell’s initial value for the new thread is the same as the creating thread’s current value. If a thread cell is non-preserved, then the cell’s initial value for a newly created thread is the default value (which was supplied when the cell was created).

Within the current thread, the current values of all preserved threads cells can be captured through current-preserved-thread-cell-values. The captured set of values can be imperatively installed into the current thread through another call to current-preserved-thread-cell-values. The capturing and restoring threads can be different.

When called with a thread-cell-vals generated by a previous call to current-preserved-thread-cell-values, the values of all preserved thread cells (in the current thread) are set to the values captured in thread-cell-vals; if a preserved thread cell was created after thread-cell-vals was generated, then the thread cell’s value for the current thread reverts to its initial value.