| MQ_OPEN(3) | Library Functions Manual | MQ_OPEN(3) | 
mq_open —
#include <mqueue.h>
mqd_t
  
  mq_open(const
    char *name, int
    oflag);
mqd_t
  
  mq_open(const
    char *name, int
    oflag, mode_t mode,
    struct mq_attr
  *attr);
mq_open() function establishes the connection
  between a process and a message queue with a message queue descriptor. It
  creates an open message queue description that refers to the message queue,
  and a message queue descriptor that refers to that open message queue
  description. The message queue descriptor is used by other functions to refer
  to that message queue. The name argument points to a
  string naming a message queue, which should conform to the construction rules
  for a pathname. The name should begin with a slash
  character. The processes calling mq_open() with the
  same value of name will refer to the same message queue
  object, as long as that name has not been removed. If the
  name argument is not the name of an existing message
  queue and creation is not requested, mq_open() fails
  and returns an error.
The oflag argument requests the desired receive and/or send access to the message queue. The requested access permission to receive messages or send messages are granted if the calling process would be granted read or write access, respectively, to an equivalently protected file.
The value of oflag is the bitwise-inclusive OR of values from the following list. Applications must specify exactly one of the first three values (access modes) below in the value of oflag:
O_RDONLYO_WRONLYO_RDWRO_RDONLY and
      O_WRONLY.In all cases, a message queue may be open multiple times in the same or different processes for sending/receiving messages.
Any combination of the remaining flags may be specified in the value of oflag:
O_CREATO_EXCL. Otherwise, a message queue will be
      created without any messages in it. The user ID of the message queue will
      be set to the effective user ID of the process, and the group ID of the
      message queue will be set to the effective group ID of the process. The
      permission bits of the message queue will be set to the value of the
      mode argument, except those set in the file mode
      creation mask of the process. When bits in mode
      other than the file permission bits are specified, the effect is
      unspecified. If attr is
      NULL, the message queue will be created with
      implementation-defined default message queue attributes. If
      attr is
      non-NULL and the calling
      process has the appropriate privilege on name, the
      message queue mq_maxmsg and
      mq_msgsize attributes will be set to the values of
      the corresponding members in the mq_attr structure
      referred to by attr. If attr
      is non-NULL, but the
      calling process does not have the appropriate privilege on
      name, the mq_open() function
      will fail and return an error without creating the message queue.O_EXCLO_EXCL and O_CREAT are
      set, mq_open() fails if the message queue
      name exists. The check for the existence of the
      message queue and the creation of the message queue if it does not exist
      will be atomic with respect to other threads executing
      mq_open() naming the same
      name with O_EXCL and
      O_CREAT set. If O_EXCL is
      set and O_CREAT is not set, the result is
      undefined.O_NONBLOCKEAGAIN.The mq_open() function does not add or
    remove messages from the queue.
mq_open() returns a message
  queue descriptor. Otherwise, the function returns
  (mqd_t) -1 and sets the global variable
  errno to indicate the error.
mq_open() function fails if:
EACCES]EEXIST]O_CREAT
      and O_EXCL are set and the named message queue
      already exists.EINTR]mq_open() function was interrupted by a
      signal.EINVAL]mq_open() function is not supported for the
      given name, or O_CREAT was specified in
      oflag, the value of attr is
      not NULL, and either
      mq_maxmsg or mq_msgsize was
      less than or equal to zero.EMFILE]ENAMETOOLONG]PATH_MAX} or a pathname component is longer than
      {NAME_MAX}.ENFILE]ENOENT]O_CREAT
      is not set and the named message queue does not exist.ENOSPC]| June 7, 2010 | NetBSD 10.0 |