mq_open(2) — Linux manual page


MQ_OPEN(3)                Linux Programmer's Manual               MQ_OPEN(3)

NAME         top

       mq_open - open a message queue

SYNOPSIS         top

       #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.

DESCRIPTION         top

       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.

       O_RDWR Open the queue to both send and receive messages.

       Zero or more of the following flags can additionally be ORed in

       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.

              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.

              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-de‐
       fined default attributes.  Since Linux 3.5, two /proc files can be
       used to control these defaults; see mq_overview(7) for details.

RETURN VALUE         top

       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         top

       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
              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.)

       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)).

              name was too long.

       ENFILE The system-wide limit on the total number of open files and
              message 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
              encountered; see mq_overview(7).

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value   │
       │mq_open() │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

   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.

BUGS         top

       In kernels before 2.6.14, the process umask was not applied to the
       permissions specified in mode.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-08-13                       MQ_OPEN(3)

Pages that refer to this page: syscalls(2)mq_overview(7)