MQ_OPEN(3C)MQ_OPEN(3C)NAMEmq_open - open a message queue
mqd_t mq_open(const char *name, int oflag,
/* unsigned long mode, mq_attr attr */ ...);
The mq_open() function establishes the connection between a process and
a message queue with a message queue descriptor. It creates a open mes‐
sage 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. The name
argument must conform to the construction rules for a path-name. If
name is not the name of an existing message queue and its creation is
not requested, mq_open() fails and returns an error. The first charac‐
ter of name must be a slash (/) character and the remaining characters
of name cannot include any slash characters. For maximum portability,
name should include no more than 14 characters, but this limit is not
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 is granted if the calling process would be granted
read or write access, respectively, to a file with the equivalent per‐
The value of oflag is the bitwise inclusive OR of values from the fol‐
lowing list. Applications must specify exactly one of the first three
values (access modes) below in the value of oflag:
Open the message queue for receiving messages. The process
can use the returned message queue descriptor with
mq_receive(3C), but not mq_send(3C). A message queue may
be open multiple times in the same or different processes
for receiving messages.
Open the queue for sending messages. The process can use
the returned message queue descriptor with mq_send(3C) but
not mq_receive(3C). A message queue may be open multiple
times in the same or different processes for sending mes‐
Open the queue for both receiving and sending messages.
The process can use any of the functions allowed for
O_RDONLY and O_WRONLY. A message queue may be open multi‐
ple times in the same or different processes for sending
Any combination of the remaining flags may additionally be specified in
the value of oflag:
This option is used to create a message queue, and it
requires two additional arguments: mode, which is of
type mode_t, and attr, which is pointer to a mq_attr
structure. If the pathname, name, has already been used
to create a message queue that still exists, then this
flag has no effect, except as noted under O_EXCL (see
below). Otherwise, a message queue is created without
any messages in it.
The user ID of the message queue is set to the effective
user ID of process, and the group ID of the message
queue is set to the effective group ID of the process.
The file permission bits are set to the value of mode,
and modified by clearing all bits set in the file mode
creation mask of the process (see umask(2)).
If attr is non-NULL and the calling process has the
appropriate privilege on name, the message queue
mq_maxmsg and mq_msgsize attributes are set to the val‐
ues of the corresponding members in the mq_attr struc‐
ture 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 fails and returns an
error without creating the message queue.
If both O_EXCL and O_CREAT are set, mq_open() will fail
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 are atomic with
respect to other processes executing mq_open() naming
the same name with both O_EXCL and O_CREAT set. If
O_EXCL and O_CREAT are not set, the result is undefined.
The setting of this flag is associated with the open
message queue description and determines whether a
mq_send(3C) or mq_receive(3C) waits for resources or
messages that are not currently available, or fails with
errno set to EAGAIN. See mq_send(3C) and mq_receive(3C)
Upon successful completion, mq_open() returns a message queue descrip‐
tor; otherwise the function returns (mqd_t)−1 and sets errno to indi‐
cate the error condition.
The mq_open() function will fail if:
The message queue exists and the permissions specified
by oflag are denied, or the message queue does not
exist and permission to create the message queue is
O_CREAT and O_EXCL are set and the named message queue
The mq_open() operation was interrupted by a signal.
The mq_open() operation 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.
The number of open message queue descriptors in this
process exceeds MQ_OPEN_MAX, of the number of open
file descriptors in this process exceeds OPEN_MAX.
The length of the name string exceeds PATH_MAX, or a
pathname component is longer than NAME_MAX while
_POSIX_NO_TRUNC is in effect.
Too many message queues are currently open in the sys‐
O_CREAT is not set and the named message queue does
There is insufficient space for the creation of the
new message queue.
The mq_open() function is not supported by the system.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Committed │
│MT-Level │ MT-Safe │
│Standard │ See standards(5). │
SEE ALSOexec(2), exit(2), umask(2), sysconf(3C), mqueue.h(3HEAD), mq_close(3C),
mq_receive(3C), mq_send(3C), mq_setattr(3C), mq_unlink(3C),
Due to the manner in which message queues are implemented, they should
not be considered secure and should not be used in security-sensitive
Solaris 2.6 was the first release to support the Asynchronous Input and
Output option. Prior to this release, this function always returned −1
and set errno to ENOSYS.
Feb 5, 2008 MQ_OPEN(3C)