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



PPRROOLLOOGG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NNAAMMEE
       mq_open -- open a message queue (RREEAALLTTIIMMEE)

SSYYNNOOPPSSIISS
       #include <mqueue.h>

       mqd_t mq_open(const char *_n_a_m_e, int _o_f_l_a_g, ...);

DDEESSCCRRIIPPTTIIOONN
       The _m_q___o_p_e_n() function shall establish the connection between a process
       and a message queue with a message queue descriptor. It shall create an
       open message queue description that refers to the message queue, and a
       message queue descriptor that refers to that open message queue
       description. The message queue descriptor is used by other functions to
       refer to that message queue. The _n_a_m_e argument points to a string
       naming a message queue. It is unspecified whether the name appears in
       the file system and is visible to other functions that take pathnames
       as arguments. The _n_a_m_e argument conforms to the construction rules for
       a pathname, except that the interpretation of <slash> characters other
       than the leading <slash> character in _n_a_m_e is implementation-defined,
       and that the length limits for the _n_a_m_e argument are implementation-
       defined and need not be the same as the pathname limits {PATH_MAX} and
       {NAME_MAX}.  If _n_a_m_e begins with the <slash> character, then processes
       calling _m_q___o_p_e_n() with the same value of _n_a_m_e shall refer to the same
       message queue object, as long as that name has not been removed. If
       _n_a_m_e does not begin with the <slash> character, the effect is
       implementation-defined. If the _n_a_m_e argument is not the name of an
       existing message queue and creation is not requested, _m_q___o_p_e_n() shall
       fail and return an error.

       A message queue descriptor may be implemented using a file descriptor,
       in which case applications can open up to at least {OPEN_MAX} file and
       message queues.

       The _o_f_l_a_g argument requests the desired receive and/or send access to
       the message queue. The requested access permission to receive messages
       or send messages shall be granted if the calling process would be
       granted read or write access, respectively, to an equivalently
       protected file.

       The value of _o_f_l_a_g is the bitwise-inclusive OR of values from the
       following list.  Applications shall specify exactly one of the first
       three values (access modes) below in the value of _o_f_l_a_g:

       O_RDONLY    Open the message queue for receiving messages. The process
                   can use the returned message queue descriptor with
                   _m_q___r_e_c_e_i_v_e(), but not _m_q___s_e_n_d().  A message queue may be
                   open multiple times in the same or different processes for
                   receiving messages.

       O_WRONLY    Open the queue for sending messages. The process can use
                   the returned message queue descriptor with _m_q___s_e_n_d() but
                   not _m_q___r_e_c_e_i_v_e().  A message queue may be open multiple
                   times in the same or different processes for sending
                   messages.

       O_RDWR      Open the queue for both receiving and sending messages. The
                   process can use any of the functions allowed for O_RDONLY
                   and O_WRONLY. A message queue may be open multiple times in
                   the same or different processes for sending messages.

       Any combination of the remaining flags may be specified in the value of
       _o_f_l_a_g:

       O_CREAT     Create a message queue. It requires two additional
                   arguments: _m_o_d_e, which shall be of type mmooddee__tt, and _a_t_t_r,
                   which shall be a pointer to an mmqq__aattttrr structure. If the
                   pathname _n_a_m_e has already been used to create a message
                   queue that still exists, then this flag shall have no
                   effect, except as noted under O_EXCL.  Otherwise, a message
                   queue shall be created without any messages in it. The user
                   ID of the message queue shall be set to the effective user
                   ID of the process. The group ID of the message queue shall
                   be set to the effective group ID of the process; however,
                   if the _n_a_m_e argument is visible in the file system, the
                   group ID may be set to the group ID of the containing
                   directory. When bits in _m_o_d_e other than the file permission
                   bits are specified, the effect is unspecified. If _a_t_t_r is
                   NULL, the message queue shall be created with
                   implementation-defined default message queue attributes. If
                   _a_t_t_r is non-NULL and the calling process has appropriate
                   privileges on _n_a_m_e, the message queue _m_q___m_a_x_m_s_g and
                   _m_q___m_s_g_s_i_z_e attributes shall be set to the values of the
                   corresponding members in the mmqq__aattttrr structure referred to
                   by _a_t_t_r.  The values of the _m_q___f_l_a_g_s and _m_q___c_u_r_m_s_g_s members
                   of the mmqq__aattttrr structure shall be ignored. If _a_t_t_r is non-
                   NULL, but the calling process does not have appropriate
                   privileges on _n_a_m_e, the _m_q___o_p_e_n() function shall fail and
                   return an error without creating the message queue.

       O_EXCL      If O_EXCL and O_CREAT are set, _m_q___o_p_e_n() shall fail if the
                   message queue _n_a_m_e exists. The check for the existence of
                   the message queue and the creation of the message queue if
                   it does not exist shall be atomic with respect to other
                   threads executing _m_q___o_p_e_n() naming the same _n_a_m_e with
                   O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not
                   set, the result is undefined.

       O_NONBLOCK  Determines whether an _m_q___s_e_n_d() or _m_q___r_e_c_e_i_v_e() waits for
                   resources or messages that are not currently available, or
                   fails with _e_r_r_n_o set to [[EEAAGGAAIINN]]; see _m_q___s_e_n_d() and
                   _m_q___r_e_c_e_i_v_e() for details.

       The _m_q___o_p_e_n() function does not add or remove messages from the queue.

RREETTUURRNN VVAALLUUEE
       Upon successful completion, the function shall return a message queue
       descriptor; otherwise, the function shall return (mmqqdd__tt)-1 and set
       _e_r_r_n_o to indicate the error.

EERRRROORRSS
       The _m_q___o_p_e_n() function shall fail if:

       EEAACCCCEESS The message queue exists and the permissions specified by _o_f_l_a_g
              are denied, or the message queue does not exist and permission
              to create the message queue is denied.

       EEEEXXIISSTT O_CREAT and O_EXCL are set and the named message queue already
              exists.

       EEIINNTTRR  The _m_q___o_p_e_n() function was interrupted by a signal.

       EEIINNVVAALL The _m_q___o_p_e_n() function is not supported for the given name.

       EEIINNVVAALL O_CREAT was specified in _o_f_l_a_g, the value of _a_t_t_r is not NULL,
              and either _m_q___m_a_x_m_s_g or _m_q___m_s_g_s_i_z_e was less than or equal to
              zero.

       EEMMFFIILLEE Too many message queue descriptors or file descriptors are
              currently in use by this process.

       EENNFFIILLEE Too many message queues are currently open in the system.

       EENNOOEENNTT O_CREAT is not set and the named message queue does not exist.

       EENNOOSSPPCC There is insufficient space for the creation of the new message
              queue.

       If any of the following conditions occur, the _m_q___o_p_e_n() function may
       return (mmqqdd__tt)-1 and set _e_r_r_n_o to the corresponding value.

       EENNAAMMEETTOOOOLLOONNGG
              The length of the _n_a_m_e argument exceeds {_POSIX_PATH_MAX} on
              systems that do not support the XSI option or exceeds
              {_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
              that is longer than {_POSIX_NAME_MAX} on systems that do not
              support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI
              systems.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       None.

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       A future version might require the _m_q___o_p_e_n() and _m_q___u_n_l_i_n_k() functions
       to have semantics similar to normal file system operations.

SSEEEE AALLSSOO
       _m_q___c_l_o_s_e(), _m_q___g_e_t_a_t_t_r(), _m_q___r_e_c_e_i_v_e(), _m_q___s_e_n_d(), _m_q___s_e_t_a_t_t_r(),
       _m_q___u_n_l_i_n_k(), _m_s_g_c_t_l(), _m_s_g_g_e_t(), _m_s_g_r_c_v(), _m_s_g_s_n_d()

       The Base Definitions volume of POSIX.1-2017, <<mmqquueeuuee..hh>>

CCOOPPYYRRIIGGHHTT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           MQ_OPEN(3)
