# mq_open

## 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 _OR_ ed 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 │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## 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.
    


## BUGS



           Before Linux 2.6.14, the process umask was not applied to the
           permissions specified in _mode_.
    


* * *