Function

Summary

Reads and writes an entry in a hash table atomically.

Package

hcl

Signature

modify-hash hash-table key function => new-value, key

Arguments

hash-table

A hash table.

key

An object.

function

A function designator.

Values

new-value

An object.

key

An object.

Description

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.

Notes

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.

See also

make-hash-table
with-hash-table-locked
Atomicity and thread-safety of the LispWorks implementation
Modifying a hash table with multiprocessing