Home
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.
Utilities & Libraries
C Library Reference
M
MsgQueueOpen()
Associate a client with a kernel message queue

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
- 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
  - C Library Reference
    - What's in a Function Description?
    - Manifests
    - Character Sets
    - A
    - B
    - C
    - D
    - E
    - F
    - G
    - H
    - I
    - J
    - K
    - L
    - M
      - main()
        Program entry function
      - mallinfo()
        Get memory allocation information
      - malloc()
        Allocate memory
      - malloc_usable_size()
        Obtain the size of an allocated memory block
      - mallopt()
        Control memory allocation and the extra checking for it
      - max()
        Return the greater of two numbers
      - max_align_t
        A type whose alignment requirement is at least as strict as that of every scalar type
      - mblen()
        Count the bytes in a multibyte character
      - mbrlen()
        Count the bytes in a multibyte character (restartable)
      - mbrtoc16(), mbrtoc32()
        Convert a multibyte character (UTF-8) into a wide character (UTF-16 or UTF-32)
      - mbrtowc()
        Convert a multibyte character into a wide character (restartable)
      - mbsinit()
        Determine the status of the conversion object used for restartable mb*() functions
      - mbsrtowcs()
        Convert a multibyte-character string into a wide-character string (restartable)
      - mbstowcs()
        Convert a multibyte-character string into a wide-character string
      - mbtowc()
        Convert a multibyte character into a wide character
      - mem_offset(), mem_offset64()
        Get the physical address and length of a virtually mapped memory block
      - memalign()
        Allocate aligned memory
      - memccpy()
        Copy bytes between buffers until a given byte is found
      - memchr()
        Find the first occurrence of a character in a buffer
      - memcmp()
        Compare the bytes in two buffers
      - memcpy()
        Copy bytes from one buffer to another
      - memcpyv()
        Copy data from one I/O vector to another
      - memicmp()
        Compare two buffers, ignoring case
      - memmem()
        Find the first occurrence of a pattern inside a memory region
      - memmove()
        Copy bytes from one buffer to another, handling overlapping memory correctly
      - memory_order
        Enumerated type for how regular non-atomic memory accesses are to be ordered around an atomic operation
      - memset()
        Set memory to a given value
      - memset_s()
        Securely set memory to a given value
      - message_attach()
        Attach a message range
      - message_connect()
        Create a connection to a channel
      - message_detach()
        Detach a message range
      - min()
        Return the lesser of two numbers
      - mkdir()
        Create a directory
      - mkdirat()
        Create a directory at a given location
      - mkdtemp()
        Make a unique temporary directory name
      - mkfifo()
        Create a FIFO special file
      - mkfifoat()
        Create a FIFO special file at a given location
      - mknod()
        Make a new filesystem entry
      - mknodat()
        Make a new filesystem entry at a given location
      - mkostemp(), mkostemps()
        Make a unique temporary filename and open the file, specifying some flags
      - mkstemp(), mkstemps()
        Make a unique temporary filename and open the file
      - mktemp()
        Make a unique temporary filename
      - mktime()
        Convert local time to calendar time
      - mlock()
        Lock a range of process address space in physical memory
      - mlockall()
        Lock all of a process's address space
      - mmap(), mmap64(), mmap_handle(), mmap_handle_pid(), _mmap2()
        Map a memory region into a process's address space
      - mmap_device_io()
        Gain access to a device's registers
      - mmap_device_memory()
        Map a device's physical memory into a process's address space
      - mmap_peer(), mmap64_peer()
        Create a virtual mapping in the address space of another process
      - modf(), modff(), modfl()
        Break a number into integral and fractional parts
      - mount()
        Mount a filesystem
      - mount_parse_generic_args()
        Strip off common mount arguments
      - mprotect()
        Change memory protection
      - mq_clockreceive()
        Receive a message from a message queue with a timeout measured against a specific clock
      - mq_clocksend()
        Send a message to a message queue with a timeout measured against a specific clock
      - mq_close()
        Close a message queue
      - mq_getattr()
        Get a message queue's attributes
      - mq_notify()
        Ask to be notified when there's a message in the queue
      - mq_open()
        Open a message queue
      - mq_receive()
        Receive a message from a queue
      - mq_send()
        Send a message to a queue
      - mq_setattr()
        Set a queue's attributes
      - mq_timedreceive(), mq_timedreceive_monotonic()
        Receive a message from a message queue
      - mq_timedsend(), mq_timedsend_monotonic()
        Send a message to a message queue
      - mq_unlink()
        Remove a queue
      - mrand48()
        Generate a pseudo-random signed long integer
      - _msg_info
        Information about a message
      - MsgCurrent(), MsgCurrent_r()
        Resume processing of a message
      - MsgDeliverEvent(), MsgDeliverEvent_r()
        Deliver an event through a channel
      - MsgError(), MsgError_r()
        Unblock a client and set its error code
      - MsgInfo(), MsgInfo_r()
        Get additional information about a message
      - MsgPause(), MsgPause_r()
        Pause the processing of a message
      - MsgQueueClose()
        Close a message queue connection
      - MsgQueueCtl()
        Perform message queue operations
      - MsgQueueOpen()
        Associate a client with a kernel message queue
      - MsgQueueSend()
        Send a message on an open queue
      - MsgRead(), MsgRead_r()
        Read data from a message
      - MsgReadv(), MsgReadv_r()
        Read data from a message
      - MsgReceive(), MsgReceive_r()
        Wait for a message or pulse on a channel
      - MsgReceivePulse(), MsgReceivePulse_r()
        Receive a pulse on a channel
      - MsgReceivev(), MsgReceivev_r()
        Wait for a message or pulse on a channel
      - MsgRegisterEvent(), MsgRegisterEvent_r()
        Register a secure event
      - MsgReply(), MsgReply_r()
        Reply with a message
      - MsgReplyv(), MsgReplyv_r()
        Reply with a message
      - MsgSend(), MsgSend_r()
        Send a message to a channel
      - MsgSendnc(), MsgSendnc_r()
        Send a message to a channel (non-cancellation point)
      - MsgSendPulse(), MsgSendPulse_r()
        Send a pulse to a process
      - MsgSendPulsePtr(), MsgSendPulsePtr_r()
        Send a pulse (containing a pointer) to a process
      - MsgSendsv(), MsgSendsv_r()
        Send a message to a channel
      - MsgSendsvnc(), MsgSendsvnc_r()
        Send a message to a channel (non-cancellation point)
      - MsgSendv(), MsgSendv_r()
        Send a message to a channel
      - MsgSendvnc(), MsgSendvnc_r()
        Send a message to a channel (non-cancellation point)
      - MsgSendvs(), MsgSendvs_r()
        Send a message to a channel
      - MsgSendvsnc(), MsgSendvsnc_r()
        Send a message to a channel (non-cancellation point)
      - MsgUnregisterEvent(), MsgUnregisterEvent_r()
        Unregister a secure event
      - MsgVerifyEvent(), MsgVerifyEvent_r()
        Check the validity of a receive ID and an event configuration
      - MsgWrite(), MsgWrite_r()
        Write a reply
      - MsgWritev(), MsgWritev_r()
        Write a reply
      - msync()
        Synchronize memory with physical storage
      - mtx_destroy()
        Destroy a mutex
      - mtx_init()
        Initialize a mutex
      - mtx_lock()
        Lock a mutex
      - mtx_timedlock()
        Lock a mutex, with a time limit
      - mtx_trylock()
        Attempt to lock a mutex
      - mtx_unlock()
        Unlock a mutex
      - munlock()
        Unlock the address space of a process
      - munlockall()
        Unlock a process's address space
      - munmap()
        Unmap previously mapped addresses
      - munmap_device_io()
        Free access to a device's registers
      - munmap_device_memory()
        Unmap previously mapped addresses
      - munmap_flags()
        Unmap previously mapped addresses, exercising more control than possible with munmap()
      - munmap_peer()
        Unmap a part of the virtual address space in another process
    - N
    - O
    - P
    - Q
    - R
    - S
    - T
    - U
    - V
    - W
    - X
    - Y
    - Z
    - Full Safety Information
    - Glossary
  - Devctl and Ioctl Commands
  - JSON Library Reference
  - PAM Reference
  - QNX Helpers Developer's Guide
    These libraries provide QNX helpers, including helpers that assist with logging, string conversion, and number and type sizes.
  - SPI Framework Technote
  - QNX Cryptography Library
    The QNX Cryptography Library (qcrypto) is a flexible cryptography framework that allows you to choose which cryptography provider responds to requests on a per-process basis.
  - QNX devcrypto Service
  - Utilities Reference
  - WPA Control Interface Reference
    This document provides a reference for the Wi-Fi Protected Access (WPA) control interface, which provides a C API for the WPA/IEEE 802.11i protocols in Wi-Fi client stations. Application developers who want to control WPA operations and get associated status and event data should read this reference.
- Migrating to QNX OS 8.0
QNX Software in the Cloud
QNX Software in the Cloud enables developers to use the QNX OS in the Amazon cloud environment.
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.
QNX Hypervisor User's Guide
This User's Guide explains the QNX hypervisor architecture and provides instructions for installing and running a QNX Hypervisor system, changing system components and configuration, and using hypervisor features such as virtual devices (vdevs).
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.

MsgQueueOpen()

QNX SDP8.0C Library ReferenceAPIDeveloper

Associate a client with a kernel message queue

Synopsis:

#include <sys/neutrino.h>

int MsgQueueOpen( rcvid_t rcvid,
                  int queueid,
                  uint32_t msgsize,
                  uint32_t maxmsg,
                  unsigned oflag );

Arguments:

rcvid

The receive ID associated with an open request that a message queue manager has received.

queueid

The identifier of the message queue. Specify a non-negative integer to open an existing queue or -1 to create a new queue.

msgsize

The maximum size of messages that can be sent or received on the message queue.

maxmsg

The maximum number of messages that the message queue can hold.

oflag

Flags specifying the behavior of the open operation:

_IO_FLAG_RD—open the connection for reading.
_IO_FLAG_WR—open the connection for writing.
O_NONBLOCK —specify non-blocking queue access.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The MsgQueueOpen() function is used by message queue managers to connect a client to a message queue in response to an mq_open() call. It performs the necessary steps for locating the queue (and creates it if necessary), creates a new connection between the client (identified by rcvid) of the caller and the queue object, and stores the relevant flags for subsequent operations.

Returns:

≥0: If the queueid argument was -1, the ID of the newly created message queue. Otherwise, 0 to indicate that the queue specified by queueid was successfully opened.
-1: An error occurred (errno is set).

Errors:

EAGAIN: There are too many queues.
EBUSY: The file descriptor in fd is already associated with a queue.
EINVAL: An argument was invalid.
ENOSPC: There's insufficient memory for a new queue.
EPERM: The process tried to create a new queue without having the required permission; go to Abilities.
ESRCH: The queue specified by queueid doesn't exist.

Classification:


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025