mq_open - open a message queue
#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
mqd_t mq_open(const char *name, int oflag);
mqd_t mq_open(const char *name, int oflag, mode_t mode,
struct mq_attr *attr);
Link with -lrt
() creates a new POSIX message queue or opens an existing queue.
The queue is identified by name
. For details of the construction of
, see mq_overview(7)
argument specifies flags that control the operation of the
call. (Definitions of the flags values can be obtained by including
.) Exactly one of the following must be specified in
- Open the queue to receive messages only.
- Open the queue to send messages only.
- Open the queue to both send and receive messages.
Zero or more of the following flags can additionally be OR
- Open the queue in nonblocking mode. In circumstances where
mq_receive(3) and mq_send(3) would normally block, these
functions instead fail with the error EAGAIN.
- 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.
- If O_CREAT was specified in oflag, and a
queue with the given name already exists, then fail with the error
is specified in 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)
. (Symbolic definitions for the
permissions bits can be obtained by including <sys/stat.h>
permissions settings are masked against the process umask. The attr
argument specifies attributes for the queue. See mq_getattr(3)
details. If attr
is NULL, then the queue is created with
implementation-defined default attributes.
On success, mq_open
() returns a message queue descriptor for use by other
message queue functions. On error, mq_open
, with errno
set to indicate the error.
- The queue exists, but the caller does not have permission
to open it in the specified mode.
- name contained more than one slash.
- Both O_CREAT and O_EXCL were specified in
oflag, but a queue with this name already exists.
- O_CREAT was specified in 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.)
- The process already has the maximum number of files and
message queues open.
- name was too long.
- The system limit on the total number of open files and
message queues has been reached.
- The O_CREAT flag was not specified in oflag,
and no queue with this name exists.
- name was just "/" followed by no other
- Insufficient memory.
- Insufficient space for the creation of a new message queue.
This probably occurred because the queues_max limit was
encountered; see mq_overview(7).
In kernels before 2.6.14, the process umask was not applied to the permissions
specified in mode
This page is part of release 3.44 of the Linux man-pages
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