University:
California State University, Northridge
Course:
COMP 222 | Computer Organization
Academic year:

2024
Views:

182

Pages:

13
Author:

Bailey Hester

Chapter 11 Interprocess Communication 1 TOPIC III Mailslots 38 MAILSLOTS (1 of 3)      Have names, so unrelated processes can use them for communication Broadcast mechanisms which behave differently from named pipes One-directional Can have multiple readers and writers but often used in one-to-many configurations A writer (“client”) does not know whether readers (“servers”) actually received a message 39 MAILSLOTS (2 of 3)      Can be located over a network domain Message lengths are limited Each mailslot server (don’t confuse with the application or named pipe server) creates a mailslot handle with CreateMailslot The server waits to receive a mailslot message with a ReadFile call A write-only client should open the mailslot with CreateFile and write messages with WriteFile 40 MAILSLOTS (3 of 3)   The open will fail (name not found) if there are no waiting readers A client’s message can be read by all servers   They all receive the same message The client, in performing the CreateFile, can use the form \\*\mailslot\mailslotname to connect to all mailslots with the name 41 USING A MAILSLOT Mailslot Servers This Message is Sent Periodically Application Client 0 hMS = CreateMailslot ("\\.\mailslot\status"); ReadFile (hMS, &ServerStatus) /* Connect to this Server */ · · · Application Client N Mailslot Client Application Server while (...) { Sleep (...); hMS = CreateFile ("\\*\mailslot\status"); · · · WriteFile (hMS, &StatusInformation) } hMS = CreateMailslot ("\\.\mailslot\status"); ReadFile (hMS, &ServerStatus) /* Connect to this Server */ 42 USING MAILSLOTS (1 of 2) One scenario: An application server, such as a database server, acting as a mailslot client, periodically broadcasts its name, named pipe name, and status information   Any application client that wants to find a server can receive this name by being a mailslot server Note how the “client” and “server” terms are used An application server can periodically broadcast its status (utilization, database size, etc.) to the application clients 43 USING MAILSLOTS (2 of 2) As shown, there is a single mailslot client (writer) and multiple readers (servers)    With multiple application servers, you would have multiple mailslot readers and writers A bulletin board service might have a single mailslot reader and multiple writers Alternative: The application client also acts as the MS client and broadcasts a pipe name to mailslot (and application) servers 44 CREATING AND OPENING A MAILSLOT (1 of 3) HANDLE CreateMailslot (LPCTSTR lpszName, DWORD cbMaxMsg, DWORD dwReadTimeout, LPSECURITY_ATTRIBUTES lpsa) lpszName — Points to a mailslot name of the form: \\.\mailslot\[path]name cbMaxMsg   Maximum size (in bytes) for messages a client can write 0 means no limit 45 CREATING AND OPENING A MAILSLOT (2 of 3) dwReadTimeout — Number of milliseconds a read operation will wait   0 causes an immediate return MAILSLOT_WAIT_FOREVER is an infinite wait (no timeout) The client must specify the FILE_SHARE_READ flag Functions GetMailslotInfo and SetMailslotInfo are similar to their named pipe counterparts 46 CREATING AND OPENING A MAILSLOT (3 of 3) The client (writer), when opening a mailslot with CreateFile, uses the following name forms:  \\.\mailslot\[path]name   \\computername\mailslot\[path]name   To retrieve a handle for a mailslot on a specified machine \\domainname\mailslot\[path]name   To retrieve a handle for a local mailslot To return a handle representing all mailslots on machines in the domain \\*\mailslot\[path]name  To return a handle representing mailslots on machines in the system’s primary domain 47 LAB 6–B (1 of 2) Use named pipes to connect clients (the client program) to a “command line server”    The clients take a command line and send it to a known server for execution The server returns the results to the client over the named pipe Design your named pipe model carefully; there are several alternatives 48 LAB 6–B (2 of 2) The solution consists of several components:  Client.c is built with locsrver.c     Run Client.exe in its own window Server.c is built as Server.exe and runs in its own window or is controlled with the job management commands. Place the commands to be executed in the same directory as Server.exe. Server.exe runs SrvrBcst.exe as a process that broadcasts the pipe name over a mailslot  Use SrvrBcst.c to build this executable The header file, ClntSrvr.h, contains definitions used in the various programs 49