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

       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.

       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.

              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-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.

       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).

       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.

   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.

       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 5.13 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                              2021-03-22                         MQ_OPEN(3)