Operating Systems -- 2

A pipe is a mechanism to share data between two processes. One process writes to the pipe and another process reads from it. The operating system doesn't do anything with the data except hold it until the reader gets to it. The pipe device works very much like Random for open and close. Where it becomes different is read, write and seek. Of course, the data structure in the device is a little different too. For pipe, instead of the device holding an array of Random objects, it should instead hold an object that has a name string, a count and a array of byte[]. These byte arrays are a buffer - space to hold data. Let's walk through an example: Process 1 starts and calls x = open("pipe fred") -- the pipe device looks for an entry with the name "fred". There is no entry, so it makes one. Count = 1 Process 1 makes some data: byte [] data = someFunction(); Process 1 writes: write (x,data); -- the pipe device uses x (an int) to index into the array and the data is copied into the byte array. Process 2 starts and calls y = open ("pipe fred") -- the pipe device finds fred and return the id for the fred array entry. Count = 2 Process 2 reads: byte[] myData = read(y,10) -- the pipe device returns 10 bytes from the byte[] in the array entry Process 1 closes x -- the pipe device decrements count of the fred entry process 2 closes y -- the pipe device decrements the count, sees that it is 0 and removes the fred entry The FakeFileSystem works very similarly to the Random - it creates a single object (RandomAccessFile) for each entry. When you call read, it returns the read bytes. When you call write, it writes the bytes and returns the number of bytes written.