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);
Link with -lrt
.
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
:
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
ed in oflag
:
Set the close-on-exec flag for the message queue descriptor. See open(2) for a discussion of why this flag is useful.
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 EEXIST.
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.
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.
name
doesn't follow the format in mq_overview(7).
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 per-process limit on the number of open file and message queue descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
name
was too long.
The system-wide 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 characters.
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).
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
mq_open() | Thread safety | MT-Safe |
POSIX.1-2001, POSIX.1-2008.
The mq_open() library function is implemented on top of a system call of the same name. The library function performs the check that the name
starts with a slash (/), giving the EINVAL error if it does not. The kernel system call expects name
to contain no preceding slash, so the C library function passes name
without the preceding slash (i.e., name+1
) to the system call.
In kernels before 2.6.14, the process umask was not applied to the permissions specified in mode
.
mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.