Fair solution of Starve-free-Readers-Writers Problem

Instructions to run the code in code.cpp

Compilation command:

g++ -pthreads code.cpp

Execution Command:

./a.out

Pseudocode descripton

Variable Descriptions

• begin: This semaphore allows only one process at a time to execute ENTRY section for readers, and protects readers from entering while writer is in CRITICAL section
• finish: This semaphore allows only one reader process to execute EXIT section.
• writer_wait: This semaphore is allows waiting writer to get priority if readers are in CRITICAL section.
• reader_enter: This integer variable stores the number of reader processes that executed ENTRY section.
• reader_exit: This integer variable stores the number of reader processes that executed the EXIT section.
• writer_waiting: This boolean variable stores whether any writer is waiting while readers are in CRITICAL section.
• DATA: Global shared data. Here it is integer type for simplicity.

Reader function

• ENTRY SECTION: Takes control of begin semaphore to enter ENTRY section if possible, otherwise wait till it gets control. Increment the reader_enter variable. Release the begin semaphotre.
• CRITICAL SECTION: Read the DATA.
• EXIT SECTION: Take control of finish semaphore to enter EXIT section if possible, otherwise wait till it gets control. Increment the reader_exit variable. Check if a writer process is waiting and no other reader process is in CRITICAL SECTION, then Signal the writer_wait semaphore. Finally, release the finish semaphore.

Writer function

• ENTRY SECTION: Take control of begin semaphore and then finish semaphore, if possible, otherwise wait till it gets control. This ensure no reader process is in ENTRY and EXIT sections. Also, check if no Reader process is in CRITICAL SECTION. If true, Release finish semaphore. Otherwise, Update the writer_waiting variable to true, to indicate to reader processes that a writer process is waiting; Also, release finish semaphore to allow reader processes in CRITICAL section to exit; Wait till writer_wait semaphore is taken to control; Also update writer_waiting to false.

• CRITICAL SECTION: Process or Modify DATA

• EXIT SECTION: Release the semaphore begin, to allow other readers and writers process to enter.