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Showing content from http://www.lispworks.com/documentation/lw71/LW/html/lw-631.htm below:

modify-hash

The function modify-hash locks the hash table hash-table. It then calls the function function with three arguments: key, the value currently associated with key in hash-table (if any), and a flag which is true if the key was in the table. (This last argument is needed in case the associated value is nil).

modify-hash then sets the result of the function function as the value for key in the table. modify-hash returns two values, the new-value and the key.

The overall effect is like:

(with-hash-table-locked

 hash-table

 (multiple-value-bind (value found-p)

     (gethash key
 hash-table
)

   (let ((new-value (funcall function

                             key
 value found-p)))

     (setf (gethash key
 hash-table
) new-value)

     (values new-value key
))))

but modify-hash should be more efficient.

It is guaranteed that no other thread can modify the value associated with key until modify-hash returns.

function is called with hash-table locked, so it should not do anything that may require hanging the modification, or that waits for another process that tries to modify the table.

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