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The readers-writers problem is a classical problem of process synchronization, it relates to a data set such as a file that is shared between more than one process at a time. Among these various processes, some are Readers - which can only read the data set; they do not perform any updates, some are Writers - can both read and write in the data sets.

The readers-writers problem is used for managing synchronization among various reader and writer process so that there are no problems with the data sets, i.e. no inconsistency is generated.

Let's understand with an example - If two or more than two readers want to access the file at the same point in time there will be no problem. However, in other situations like when two writers or one reader and one writer wants to access the file at the same point of time, there may occur some problems, hence the task is to design the code in such a manner that if one reader is reading then no writer is allowed to update at the same point of time, similarly, if one writer is writing no reader is allowed to read the file at that point of time and if one writer is updating a file other writers should not be allowed to update the file at the same point of time. However, multiple readers can access the object at the same time.

Let us understand the possibility of reading and writing with the table given below:


Case Process 1 Process 2 Allowed / Not Allowed
Case 1 Writing Writing Not Allowed
Case 2 Reading Writing Not Allowed
Case 3 Writing Reading Not Allowed
Case 4 Reading Reading Allowed

The solution of readers and writers can be implemented using binary semaphores.

We use two binary semaphores "write" and "mutex", where binary semaphore can be defined as:

Semaphore: A semaphore is an integer variable in S, that apart from initialization is accessed by only two standard atomic operations - wait and signal, whose definitions are as follows:

From the above definitions of wait, it is clear that if the value of S <= 0 then it will enter into an infinite loop (because of the semicolon; after while loop). Whereas the job of the signal is to increment the value of S.

The below code will provide the solution of the reader-writer problem, reader and writer process codes are given as follows -

Code for Reader Process

The code of the reader process is given below -

In the above code of reader, mutex and write are semaphores that have an initial value of 1, whereas the readcount variable has an initial value as 0. Both mutex and write are common in reader and writer process code, semaphore mutex ensures mutual exclusion and semaphore write handles the writing mechanism.

The readcount variable denotes the number of readers accessing the file concurrently. The moment variable readcount becomes 1, wait operation is used to write semaphore which decreases the value by one. This means that a writer is not allowed how to access the file anymore. On completion of the read operation, readcount is decremented by one. When readcount becomes 0, the signal operation which is used to write permits a writer to access the file.

Code for Writer Process

The code that defines the writer process is given below:

If a writer wishes to access the file, wait operation is performed on write semaphore, which decrements write to 0 and no other writer can access the file. On completion of the writing job by the writer who was accessing the file, the signal operation is performed on write.

Let's see the proof of each case mentioned in Table 1

CASE 1: WRITING - WRITING → NOT ALLOWED. That is when two or more than two processes are willing to write, then it is not allowed. Let us see that our code is working accordingly or not?

CASE 2: READING - WRITING → NOT ALLOWED. That is when one or more than one process is reading the file, then writing by another process is not allowed. Let us see that our code is working accordingly or not?

CASE 3: WRITING -- READING → NOT ALLOWED. That is when if one process is writing into the file, then reading by another process is not allowed. Let us see that our code is working accordingly or not?

CASE 4: READING - READING → ALLOWED. That is when one process is reading the file, and other process or processes is willing to read, then they all are allowed i.e. reading - reading is not mutually exclusive. Let us see that our code is working accordingly or not?

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