mq_open - open a message queue

#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
#include <mqueue.h>
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);

mq_open() creates a new POSIX message queue or opens an existing queue. The queue is identified by name. For details of the construction of name, see mq_overview(7).

The oflag argument specifies flags that control the operation of the call. (Definitions of the flags values can be obtained by including <fcntl.h>.) Exactly one of the following must be specified in 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 ORed in oflag:

O_CLOEXEC (since Linux 2.6.26)

Set the close-on-exec flag for the message queue descriptor. See open(2) for a discussion of why this flag is useful.

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 oflag, and a queue with the given name already exists, then fail with the error EEXIST.

O_NONBLOCK

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.

If O_CREAT 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>.) The permissions settings are masked against the process umask.

The fields of the struct mq_attr pointed to attr specify the maximum number of messages and the maximum size of messages that the queue will allow. This structure is defined as follows:

struct mq_attr {
    long mq_flags;       /* Flags (ignored for mq_open()) */
    long mq_maxmsg;      /* Max. # of messages on queue */
    long mq_msgsize;     /* Max. message size (bytes) */
    long mq_curmsgs;     /* # of messages currently in queue
                            (ignored for mq_open()) */
};

Only the mq_maxmsg and mq_msgsize fields are employed when calling mq_open(); the values in the remaining fields are ignored.

If attr is NULL, then the queue is created with implementation-defined default attributes. Since Linux 3.5, two /proc files can be used to control these defaults; see mq_overview(7) for details.

On success, mq_open() returns a message queue descriptor for use by other message queue functions. On error, mq_open() returns (mqd_t) -1, with errno set to indicate the error.

mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)