marioemmanuel / messagequeues

messagequeues

System V IPC Queues X11/Motif Example

Introduction

This repository contains a simplified example on message Queues. It uses System V IPC as message queues and Motif as X11 Toolkit. It is not intended as a ready to use library but as a basic example on how to use SysV queues.

Compilation

Requieres Motif and X11 development libraries. File /etc/xbus is used for ftok

# touch /etc/xbus
$ make xbus
$ make xclient

Usage

Clients allow registering against the bus and sending messages. Bus reads client register requests and distributes to all registered clients the messages. Below image shows one bus and four clients launched. Notice how the fourth client did not get registered and hence it is not receiving the messages.

Caveats

This repository is just a basic example and it lacks certain functionality:

• Deregistration of clients.
• Timeouts/Keepalive messages.
• Proper termination of queues.

Additionally Andrew from #FreeBSD at Freenode correctly pointed out some issues:

• There is no need to open one queue per client in the bus process, as mtype value could be used to identify destination client. Hence all clients and bus would share the same queue. The bus would send as many messages as clients are using mtype as destination.
• ftok takes only the 8 least significant bits from proj_id to generate the queue key, in the current implementation it would not be that unlikely to get collisions on the lowest byte of the PID, as PIDs are used as proj_id.
• There is at least one bug in the FreeBSD kernel which deals key_t as int instead of as long. Apparently that is affecting ipcs but not code written in C.