Ubuntu Manpage: mq_open - open a message queue

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NAME

       mq_open - open a message queue

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);

       Link with -lrt.

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 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.

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 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)).

ENAMETOOLONG
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

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │Interface │ Attribute     │ Value   │
       ├──────────┼───────────────┼─────────┤
       │mq_open() │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘

CONFORMING TO

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

NOTES

   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

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

COLOPHON

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