The Readers / Writers Problem

• Writing is exclusive.

• Several readers can read together.

• Readers have priority over writers.

wait( wrt );

…

writing is performed

…

signal( wrt );

wait( mutex );

rcount := rcount + 1;

if (rcount = 1 ) then wait( wrt );

signal( mutex );

…

reading is performed

…

wait( mutex );

rcount := rcount - 1;

if (rcount = 0 ) then

signal( wrt );

signal( mutex );

Writer

Reader

Initialization

semaphores: mutex = 1;

wrt = 1;

integer : rcount = 0;

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