mq_open(3) Library Functions Manual mq_open(3)
NAME
mq_open - open a message queue
LIBRARY
Real-time library (librt, -lrt)
SYNOPSIS
#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);
DESCRIPTION
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 func-
tions 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 permis-
sions 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.
RETURN VALUE
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.
ERRORS
EACCES The queue exists, but the caller does not have permission to
open it in the specified mode.
EACCES name contained more than one slash.
EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a queue
with this name already exists.
EINVAL name doesn't follow the format in mq_overview(7).
EINVAL 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 unprivi-
leged (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 msg-
size_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.)
EMFILE 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)).
ENAMETOOLONG
name was too long.
ENFILE The system-wide limit on the total number of open files and mes-
sage queues has been reached.
ENOENT The O_CREAT flag was not specified in oflag, and no queue with
this name exists.
ENOENT name was just "/" followed by no other characters.
ENOMEM Insufficient memory.
ENOSPC Insufficient space for the creation of a new message queue.
This probably occurred because the queues_max limit was encoun-
tered; see mq_overview(7).
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ mq_open() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
VERSIONS
C library/kernel differences
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.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001.
BUGS
Before Linux 2.6.14, the process umask was not applied to the permis-
sions specified in mode.
SEE ALSO
mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3),
mq_unlink(3), mq_overview(7)
Linux man-pages 6.7 2023-10-31 mq_open(3)
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