NAME
mq_open - open a message queue
SYNOPSIS
#include <mqueue.h>
I mqd_t mq_open(const char * name , int oflag );
I mqd_t mq_open(const char * name , int oflag , mode_t mode ,
I struct mq_attr * attr );
DESCRIPTION
R mq_open ()
creates a new POSIX message queue or opens an existing queue.
The queue is identified by
R name .
For details of the construction of
R name ,
see
mq_overview(7).
The
oflag
argument specifies flags that control the operation of the call.
Exactly one of the following must be specified in
R oflag:
O_RDONLY
Open the queue to receive messages only.
O_WRONLY
Open the queue to send messages only.
O_RDWR
Open the queue to both send and receive messages.
Zero or more of the following flags can additionally be
R OR ed
in
R oflag :
O_NONBLOCK
Open the queue in non-blocking mode.
In circumstances where
mq_receive(3)
and
mq_send(3)
would normally block, these functions instead fail with the error
R EAGAIN .
O_CREAT
Create the message queue if it does not exist.
The owner (user ID) of the message queue is set to the effective
user ID of the calling process.
The group ownership (group ID) is set to the effective group ID
of the calling process.
O_EXCL
If
O_CREAT
was specified in
R oflag ,
and a queue with the given
name
already exists, then fail with the error
R EEXIST .
If
O_CREAT
is specified in
R oflag ,
then two additional arguments must be supplied.
The
mode
argument specifies the permissions to be placed on the new queue,
as for
open(2).
The permissions settings are masked against the process umask.
The
attr
argument specifies attributes for the queue.
See
mq_getattr(3)
for details.
If
attr
is NULL, then the queue is created with implementation-defined
default attributes.
RETURN VALUE
On success,
R mq_open ()
returns a message queue descriptor for use by other
message queue functions.
On error,
R mq_open ()
returns
R (mqd_t) -1,
with
errno
set to indicate the error.
ERRORS
EACCES
The queue exists, but the caller does not have permission to
open it in the specified mode.
EINVAL
O_CREAT
was specified in
R oflag ,
and
attr
was not NULL, but
attr->mq_maxmsg
or
attr->mq_msqsize
was invalid.
Both of these fields must be greater than zero.
In a process that is unprivileged (does not have the
CAP_SYS_RESOURCE
capability),
attr->mq_maxmsg
must be less than or equal to the
msg_max
limit, and
attr->mq_msgsize
must be less than or equal to the
msgsize_max
limit.
In addition, even in a privileged process,
attr->mq_maxmsg
cannot exceed the
HARD_MAX
limit.
(See
mq_overview(7)
for details of these limits.)
EEXIST
Both
O_CREAT
and
O_EXCL
were specified in
R oflag ,
but a queue with this
name
already exists.
EMFILE
The process already has the maximum number of files and
message queues open.
ENAMETOOLONG
R name
was too long.
ENFILE
The system limit on the total number of open files and message queues
has been reached.
ENOENT
The
O_CREAT
flag was not specified in
R oflag ,
and no queue with this
name
exists.
ENOMEM
Insufficient memory.
ENOSPC
Insufficient space for the creation of a new message queue.
This probably occurred because the
queues_max
limit was encountered; see
mq_overview(7).
CONFORMING TO
POSIX.1-2001.
BUGS
In kernels before 2.6.14,
the process umask was not applied to the permissions specified in
R mode .
SEE ALSO
ČLÁNKY DO MAILU