Operating System - readers to write the problem

I. Description of the problem

 

 

 Claim:

1, allows multiple readers can simultaneously perform a read operation on the file.

2, only allows a writer to write the file information.

3, a writer does not allow any other reader or writer to work before the completion of the write operation.

4, writers perform before a write operation, should make the existing readers and writers all out.

Second, the problem analysis

 

 

 The core issue readers to write the question is how to handle multiple readers can simultaneously read the file.

 

Third, how to achieve

 

 

RW = semaphore . 1 ;    // implement exclusive access to the file 
int count = 0 ; 
semaphore the mutex = . 1 ; // implement exclusive access to the count variable 
int I = 0 ; 
Writer () { 
    the while ( . 1 ) { 
        P ( rw); // before writing the "lock" 
        write file 
        V (rw); // after writing "unlock" 
    } 
} 
Reader () { 
    the while ( 1 ) { 
        P (mutex);      // each reading process exclusive access count 
        IF (COUNT == 0 )   //The first reading process responsible for the "lock" 
        { 
            P (rw); 
        } 
        COUNT ++;       // process accesses the file number +1 
        V (mutex); 
        read file 
        P (mutex);      // each reading process exclusive access cOUNT 
        count--;       // process accesses the file number -1 
        IF (cOUNT == 0 )   // last process is responsible for reading "unlock" 
        { 
            V (rw); 
        } 
        V (mutex); 
    } 
}