mq_open - Establishes the connection between a message
queue and a message queue descriptor (P1003.1b)
#include <mqueue.h>
mqd_t mq_open ( const char *name, int ooption, ...);
Realtime Library (librt.so, librt.a)
Specifies a pointer to the name of a string naming the
message queue. The string can be a pathname. Specifies
the request for receive or send access to the message
queue, as well as the blocking attributes of the resulting
open descriptor for the calling process.
This value of this argument is the bitwise inclusive OR of
the values from the following list of options. An application
must specify exactly one of the first three
options: Opens the message queue for receiving messages.
This enables calls to the mq_receive function but not
calls to the mq_send function for the calling process.
Note that each process has its own read/write access to a
queue. Opens the queue for sending messages.
This enables calls to the mq_send function but not calls
to the mq_receive function for the calling process. Opens
the queue for receiving and sending messages for the calling
process.
Any combination of the following options is valid: Creates
a message queue. Use of this option requires specification
of two additional arguments: mode, which is of type
mode_t, and attr, which points to a mq_attr structure. If
name has already been used to create a message queue that
is still open, this option has no effect, except as noted
under O_EXCL. Otherwise, a message queue is created. The
ID of the message queue becomes the effective user ID, and
the group ID of the message queue becomes the effective
group ID of the process. The mode argument sets the file
permission bits. If the attr argument is NULL, the function
creates the message queue with default attributes.
If the attr argument is not NULL, and the calling process
has appropriate privilege or permission, the message queue
attributes are set as if the mq_setattr function were
called with the attribute argument attr. If the attr
argument is not NULL, but the calling process does not
have the appropriate privilege or permission, the mq_open
argument fails and returns an error. Prevents the opening
of a message queue if O_CREAT is set and the message queue
already exists. Use this option only in combination with
O_CREAT. Allows calls to the mq_send and mq_receive functions
to continue execution while waiting for resources or
messages. When this option is set and resources are not
available, the mq_send and mq_receive functions fail and
errno is set to EAGAIN. This option is a per-open descriptor
option, and applies to the calling process only. Each
process has its own blocking attributes for a queue.
The mq_open function establishes the connection between a
message queue and a message queue descriptor. This function
creates a new open message queue description that
refers to a specified message queue and a message queue
descriptor that refers to that open message queue description.
The message queue descriptor is then available for use by
other functions for reference to the message queue from
the calling process.
On successful completion, this function returns a message
queue descriptor.
Otherwise, the function returns -1 and sets errno to indicate
the error.
The mq_open function fails under the following conditions:
The message queue exists and the permissions specified by
ooption are denied, or the message queue does not exist
and permission to create the message queue is denied.
O_CREAT and O_EXCL are set, and the named message queue
already exists. An internal virtual memory error
occurred. The mq_open operation was interrupted by a signal.
The mq_open operation is not supported for the given
name. O_CREAT was specified in ooption, the value of attr
is not NULL, and either mq_maxmsg or mq_msgsize was less
than or equal to zero.
POSIX message queues should be created and modified
only through POSIX message queue routines. It is
possible for this error to be returned if the given
name refers to a file that cannot be recognized,
perhaps because it was created or modified by an
unsupported call. Too many message queue descriptors
or file descriptors are currently in use by
this process. 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 system. O_CREAT is not set and the
named message queue does not exist. There is
insufficient space for the creation of the new message
queue.
Functions: mq_close(3), mq_getattr(3), mq_receive(3),
mq_send(3), mq_setattr(3), mq_unlink(3), mkfifo(3)
Guide to Realtime Programming
mq_open(3)
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