Adds an object to a mailbox.
mailbox-send mailbox object =>
mailbox-send sends object to mailbox. The object is queued in the mailbox for retrieval by the reader.
When objects are read from from the mailbox (using mailbox-read or higher level functions such as mailbox-wait-for-event), they are read in the order in which they were added to the mailbox.
If mailbox is full,
mailbox-send expands it. In situations where the mailbox can grow too much, you can use mailbox-send-limited instead.
LispWorks User Guide and Reference Manual - 20 Sep 2017