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
- 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.
argument specifies attributes for the queue. See
for details. If attr
is NULL, then the queue is
created with implementation-defined default attributes. Since Linux 3.5, two
files can be used to control these defaults; see
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
- name was too long.
- The system limit on the total number of open files and message queues has
- The O_CREAT flag was not specified in oflag, and no queue
with this name exists.
- name was just "/" followed by no other characters.
- Insufficient memory.
- Insufficient space for the creation of a new message queue. This probably
occurred because the queues_max limit was encountered; see
Multithreading (see pthreads(7))¶
() function is thread-safe.
In kernels before 2.6.14, the process umask was not applied to the permissions
specified in mode
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