Concurrent Reading and Writing

L e s l i e L a m p o r t M a s s a c h u s e t t s C o m p u t e r A s s o c i a t e s The problem of sharing data among asynchronous processes is considered. It is assumed that only one process at a time can modify the data, but concurrent reading and writing is permitted. Two general theorems are proved, and some algorithms are presented to illustrate their use. These include a solution to the general problem in which a read is repeated if it might have obtained an incorrect result, and two techniques for transmitting messages between processes. These solutions do not assume any synchronizing mechanism other than data which can be written by one process and read by other processes. General permission to republish, but not for profit, all or part of this material is granted provided that ACM's copyright notice is given and that reference is made to the publication, to its date of issue, and to the fact that reprinting privileges were granted by permission of the Association for Computing Machinery.