MQ_RECEIVE(3)              POSIX Programmer's Manual             MQ_RECEIVE(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_receive, mq_timedreceive -- receive a message from a message queue
       (RREEAALLTTIIMMEE)

SSYYNNOOPPSSIISS
       #include <mqueue.h>

       ssize_t mq_receive(mqd_t _m_q_d_e_s, char *_m_s_g___p_t_r, size_t _m_s_g___l_e_n,
           unsigned *_m_s_g___p_r_i_o);

       #include <mqueue.h>
       #include <time.h>

       ssize_t mq_timedreceive(mqd_t _m_q_d_e_s, char *restrict _m_s_g___p_t_r,
           size_t _m_s_g___l_e_n, unsigned *restrict _m_s_g___p_r_i_o,
           const struct timespec *restrict _a_b_s_t_i_m_e);

DDEESSCCRRIIPPTTIIOONN
       The _m_q___r_e_c_e_i_v_e() function shall receive the oldest of the highest
       priority message(s) from the message queue specified by _m_q_d_e_s.  If the
       size of the buffer in bytes, specified by the _m_s_g___l_e_n argument, is less
       than the _m_q___m_s_g_s_i_z_e attribute of the message queue, the function shall
       fail and return an error. Otherwise, the selected message shall be
       removed from the queue and copied to the buffer pointed to by the
       _m_s_g___p_t_r argument.

       If the value of _m_s_g___l_e_n is greater than {SSIZE_MAX}, the result is
       implementation-defined.

       If the argument _m_s_g___p_r_i_o is not NULL, the priority of the selected
       message shall be stored in the location referenced by _m_s_g___p_r_i_o.

       If the specified message queue is empty and O_NONBLOCK is not set in
       the message queue description associated with _m_q_d_e_s, _m_q___r_e_c_e_i_v_e() shall
       block until a message is enqueued on the message queue or until
       _m_q___r_e_c_e_i_v_e() is interrupted by a signal. If more than one thread is
       waiting to receive a message when a message arrives at an empty queue
       and the Priority Scheduling option is supported, then the thread of
       highest priority that has been waiting the longest shall be selected to
       receive the message. Otherwise, it is unspecified which waiting thread
       receives the message. If the specified message queue is empty and
       O_NONBLOCK is set in the message queue description associated with
       _m_q_d_e_s, no message shall be removed from the queue, and _m_q___r_e_c_e_i_v_e()
       shall return an error.

       The _m_q___t_i_m_e_d_r_e_c_e_i_v_e() function shall receive the oldest of the highest
       priority messages from the message queue specified by _m_q_d_e_s as
       described for the _m_q___r_e_c_e_i_v_e() function. However, if O_NONBLOCK was not
       specified when the message queue was opened via the _m_q___o_p_e_n() function,
       and no message exists on the queue to satisfy the receive, the wait for
       such a message shall be terminated when the specified timeout expires.
       If O_NONBLOCK is set, this function is equivalent to _m_q___r_e_c_e_i_v_e().

       The timeout expires when the absolute time specified by _a_b_s_t_i_m_e passes,
       as measured by the clock on which timeouts are based (that is, when the
       value of that clock equals or exceeds _a_b_s_t_i_m_e), or if the absolute time
       specified by _a_b_s_t_i_m_e has already been passed at the time of the call.

       The timeout shall be based on the CLOCK_REALTIME clock.  The resolution
       of the timeout shall be the resolution of the clock on which it is
       based. The _t_i_m_e_s_p_e_c argument is defined in the _<_t_i_m_e_._h_> header.

       Under no circumstance shall the operation fail with a timeout if a
       message can be removed from the message queue immediately. The validity
       of the _a_b_s_t_i_m_e parameter need not be checked if a message can be
       removed from the message queue immediately.

RREETTUURRNN VVAALLUUEE
       Upon successful completion, the _m_q___r_e_c_e_i_v_e() and _m_q___t_i_m_e_d_r_e_c_e_i_v_e()
       functions shall return the length of the selected message in bytes and
       the message shall be removed from the queue. Otherwise, no message
       shall be removed from the queue, the functions shall return a value of
       -1, and set _e_r_r_n_o to indicate the error.

EERRRROORRSS
       These functions shall fail if:

       EEAAGGAAIINN O_NONBLOCK was set in the message description associated with
              _m_q_d_e_s, and the specified message queue is empty.

       EEBBAADDFF  The _m_q_d_e_s argument is not a valid message queue descriptor open
              for reading.

       EEMMSSGGSSIIZZEE
              The specified message buffer size, _m_s_g___l_e_n, is less than the
              message size attribute of the message queue.

       EEIINNTTRR  The _m_q___r_e_c_e_i_v_e() or _m_q___t_i_m_e_d_r_e_c_e_i_v_e() operation was interrupted
              by a signal.

       EEIINNVVAALL The process or thread would have blocked, and the _a_b_s_t_i_m_e
              parameter specified a nanoseconds field value less than zero or
              greater than or equal to 1000 million.

       EETTIIMMEEDDOOUUTT
              The O_NONBLOCK flag was not set when the message queue was
              opened, but no message arrived on the queue before the specified
              timeout expired.

       These functions may fail if:

       EEBBAADDMMSSGG
              The implementation has detected a data corruption problem with
              the message.

       _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
       None.

SSEEEE AALLSSOO
       _m_q___o_p_e_n(), _m_q___s_e_n_d(), _m_s_g_c_t_l(), _m_s_g_g_e_t(), _m_s_g_r_c_v(), _m_s_g_s_n_d(), _t_i_m_e()

       The Base Definitions volume of POSIX.1-2017, <<mmqquueeuuee..hh>>, <<ttiimmee..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_RECEIVE(3)
