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mq_open(3c)
mq_open(3C) Standard C Library Functions mq_open(3C)
NAME
mq_open - open a message queue
SYNOPSIS
#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 characters
of name cannot include any slash characters. For maximum portability,
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_re‐
ceive(3C), but not mq_send(3C). A message queue may be open
multiple times in the same or different processes for re‐
ceiving messages.
O_WRONLY 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‐
sages.
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 re‐
quires two additional arguments: mode, which is of type
mode_t, and attr, which is pointer to a mq_attr struc‐
ture. 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 mes‐
sages 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 modi‐
fied 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 ap‐
propriate privilege on name, the message queue mq_maxmsg
and mq_msgsize attributes are 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 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 exis‐
tence of the message queue and the creation of the mes‐
sage 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.
O_NONBLOCK The setting of this flag is associated with the open mes‐
sage queue description and determines whether a
mq_send(3C) or mq_receive(3C) waits for resources or mes‐
sages that are not currently available, or fails with er‐
rno set to EAGAIN. See mq_send(3C) and mq_receive(3C) 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 ex‐
ist and permission to create the message queue is de‐
nied.
EEXIST O_CREAT and O_EXCL are set and the named message 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 sys‐
tem.
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(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe _ Stan‐
dardSee standards(7).
SEE ALSO
exec(2), exit(2), umask(2), mq_close(3C), mq_receive(3C), mq_send(3C),
mq_setattr(3C), mq_unlink(3C), sysconf(3C), mqueue.h(3HEAD), attrib‐
utes(7), standards(7)
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.
Oracle Solaris 11.4 31 Jan 2012 mq_open(3C)