mq_open(3RT) Realtime Library Functions mq_open(3RT)NAMEmq_open - open a message queue
SYNOPSIS
cc [ flag... ] file... -lrt [ library... ]
#include <mqueue.h>
mqd_t mq_open(const char *name, int oflag, /* unsigned long mode,
mq_attr attr */ ...);
DESCRIPTION
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 charac‐
ters of name cannot include any slash characters. For maximum porta‐
bility, name should include no more than 14 characters, but this limit
is not enforced.
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‐
missions.
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:
O_RDONLY Open the message queue for receiving messages. The
process can use the returned message queue descriptor
with mq_receive(3RT), but not mq_send(3RT). A message
queue may be open multiple times in the same or differ‐
ent processes for receiving messages.
O_WRONLY Open the queue for sending messages. The process can
use the returned message queue descriptor with
mq_send(3RT) but not mq_receive(3RT). A message queue
may be open multiple times in the same or different
processes for sending messages.
O_RDWR 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
multiple times in the same or different processes for
sending messages.
Any combination of the remaining flags may additionally be specified in
the value of oflag:
O_CREAT 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 effec‐
tive user ID of process, and the group ID of the mes‐
sage 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.
O_EXCL 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 unde‐
fined.
O_NONBLOCK The setting of this flag is associated with the open
message queue description and determines whether a
mq_send(3RT) or mq_receive(3RT) waits for resources or
messages that are not currently available, or fails
with errno set to EAGAIN. See mq_send(3RT) and
mq_receive(3RT) for details.
RETURN VALUES
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.
ERRORS
The mq_open() function will fail if:
EACCES 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 denied.
EEXIST O_CREAT and O_EXCL are set and the named mes‐
sage queue already exists.
EINTR The mq_open() operation was interrupted by a
signal.
EINVAL 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.
EMFILE 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.
ENAMETOOLONG 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.
ENFILE Too many message queues are currently open in
the system.
ENOENT O_CREAT is not set and the named message queue
does not exist.
ENOSPC There is insufficient space for the creation of
the new message queue.
ENOSYS The mq_open() function is not supported by the
system.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOexec(2), exit(2), umask(2), sysconf(3C), mqueue.h(3HEAD),
mq_close(3RT), mq_receive(3RT), mq_send(3RT), mq_setattr(3RT),
mq_unlink(3RT), attributes(5), standards(5)NOTES
Due to the manner in which message queues are implemented, they should
not be considered secure and should not be used in security-sensitive
applications.
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.
SunOS 5.10 28 Jun 2002 mq_open(3RT)
