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QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
Programming
Writing a Resource Manager
Using the Receive Buffer as a Reply Buffer
When building a response to a client request, you often need a temporary buffer in which to build this response.

QNX Momentics IDE User's Guide
This User's Guide describes version 8.0 of the Integrated Development Environment (IDE) that's part of the QNX Tool Suite.
QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
- Quickstart Guide
- System Architecture
  The System Architecture guide accompanies the QNX OS and is intended for both application developers and end-users.
- OS Components & Operations
- Graphics
- Programming
  - Getting Started with the QNX OS
    Getting Started with the QNX OS: A Guide for Realtime Programmers is intended to introduce you to the QNX OS and help you develop applications and resource managers for it.
  - Programmer's Guide
    The QNX OS Programmer's Guide covers a variety of topics that might interest developers who are building applications that will run under the QNX OS.
  - Writing a Resource Manager
    - What Is a Resource Manager?
      In general terms, a resource manager is a process that registers a pathname in the filesystem name space. Other processes use that pathname to communicate with the resource manager.
    - The Bones of a Resource Manager
      Let's start with the overall structure of a resource manager. First we'll look at what happens under the covers on both the client side and the server side. After that, we'll go into the layers in a resource manager and then look at some examples.
    - Fleshing Out the Skeleton
      It's time now to start adding some flesh to the basic bones of the resource manager. We'll look at the types of messages you might have to handle, how to set the resource manager's attributes, how to add functionality, and some security issues you should consider.
    - POSIX-Layer Data Structures
      The resource manager library defines (in <sys/iofunc.h>) several key structures that are related to the POSIX-layer support routines.
    - Handling Read and Write Messages
      A resource manager must define routines or handlers to process requests by the client to receive data (read) or send data to the underlying device (write). Here, we give detailed examples of such handlers.
    - Resource Managers that Handle Multiple Devices
      Resource managers can easily be adapted to handle multiple devices.
    - Combine Messages
      To conserve bandwidth and to support atomic operations, combine messages are supported. A combine message is constructed by the client's C library and consists of multiple I/O and/or connect messages packaged together into one.
    - Using the Receive Buffer as a Reply Buffer
      When building a response to a client request, you often need a temporary buffer in which to build this response.
    - Handling Other Messages
      Your resource manager will likely need to handle messages for operations other than read and write. Here, we explain and demonstrate how to handle other common message types.
    - Unblocking Clients and Handling Interrupts
      Your resource manager needs to avoid leaving clients blocked indefinitely, and it might need to handle interrupts.
    - Multithreaded Resource Managers
      When you create a multithreaded resource manager, you should design it so that there's always at least one RECEIVE-blocked thread.
    - Filesystem Resource Managers
      Some resource managers need to implement filesystems by responding to access requests for any pathname within a directory structure.
    - Glossary
- System Security Guide
  The QNX System Security Guide is intended for both system integrators who are responsible for the security of a QNX OS system and developers who want to create a QNX OS resource manager free from vulnerabilities.
- Utilities & Libraries
- Migrating to QNX OS 8.0
QNX Software in the Cloud
QNX Toolkit for Visual Studio Code
This User's Guide describes the QNX^® Toolkit for Visual Studio Code. The guide introduces you to the QNX Toolkit by explaining the QNX development environment and how to build, run, and debug your QNX^® Operating System (OS) applications and systems.
Typographical Conventions, Support, and Licensing
This section describes the typographical conventions used throughout the documentation, explains how to obtain technical support, and provides some information about licensing.

Using the Receive Buffer as a Reply Buffer

QNX SDP8.0Writing a Resource ManagerDeveloper

When building a response to a client request, you often need a temporary buffer in which to build this response.

Using the receive buffer, ctp->msg, can be a convenient choice because:

It does not require allocating or freeing
It is local to the thread doing the work
Unlike a stack buffer, it is valid after a return from the function, which is important if the library replies on your behalf (e.g., by returning _RESMGR_PTR())

The ctp->msg buffer has ctp->msg_max_size bytes of space available in which to build this response. The default value is a bit more than 2KiB. If you find you need more, you can configure a larger receive buffer at initialization by setting the msg_max_size field in the resmgr_attr_t structure given to resmgr_attach().

Note:

When you're handling parts of combine messages that are not the last part of the message (i.e., when ctp->rcvid == 0), you are not allowed to reply, and should not touch the receive buffer at all.

While using the receive buffer as your reply buffer, you should:

Make sure to extract any information you need from the receive buffer before overwriting it. For example, if you need the client's number of bytes (msg->i.nbytes in several cases), store that value before you overwrite it. Some resource manager library helper functions (e.g., iofunc_stat() or resmgr_msgget()) also examine or extract data from the receive buffer, and should not be used after you have modified the contents of the buffer.
Take care to not exceed your available space. Take special caution when assembling a response from multiple parts with variable size, such as when assembling a path for a symbolic link reply or when building a list of directory entries in response to a readdir() request.
Remember that while many of the message types provide a msg->o entry in their union for building a response, you should always use ctp->msg to have the full receive buffer available. If you're handling later messages in a combine message, the msg->o may be pointing arbitrarily close to the end of your receive buffer, not giving you the space you expect.

Page updated: August 09, 2024