NAME

       mq_getattr, mq_setattr - get/set message queue attributes

SYNOPSIS

       #include <mqueue.h>

       int mq_getattr(mqd_t mqdes, struct mq_attr *attr);

       int mq_setattr(mqd_t mqdes, struct mq_attr *newattr,
                        struct mq_attr *oldattr);

       Link with -lrt.

DESCRIPTION

       mq_getattr()  and  mq_setattr() respectively retrieve and modify attributes of the message queue referred
       to by the descriptor mqdes.

       mq_getattr() returns an mq_attr structure in the buffer pointed by attr.  This structure is defined as:

           struct mq_attr {
               long mq_flags;       /* Flags: 0 or O_NONBLOCK */
               long mq_maxmsg;      /* Max. # of messages on queue */
               long mq_msgsize;     /* Max. message size (bytes) */
               long mq_curmsgs;     /* # of messages currently in queue */
           };

       The mq_flags field contains flags associated with the open message  queue  description.   This  field  is
       initialized  when  the  queue  is  created by mq_open(3).  The only flag that can appear in this field is
       O_NONBLOCK.

       The mq_maxmsg and mq_msgsize fields are set when  the  message  queue  is  created  by  mq_open(3).   The
       mq_maxmsg  field  is  an  upper  limit  on  the  number of messages that may be placed on the queue using
       mq_send(3).  The mq_msgsize field is an upper limit on the size of messages that may  be  placed  on  the
       queue.  Both of these fields must have a value greater than zero.  Two /proc files that place ceilings on
       the values for these fields are described in mq_open(3).

       The mq_curmsgs field returns the number of messages currently held in the queue.

       mq_setattr() sets message queue attributes using information supplied in the mq_attr structure pointed to
       by  newattr.   The only attribute that can be modified is the setting of the O_NONBLOCK flag in mq_flags.
       The other fields in newattr are ignored.  If the oldattr field is not  NULL,  then  the  buffer  that  it
       points  to  is used to return an mq_attr structure that contains the same information that is returned by
       mq_getattr().

RETURN VALUE

       On success mq_getattr() and mq_setattr() return 0; on error, -1 is returned, with errno set  to  indicate
       the error.

ERRORS

       EBADF  The descriptor specified in mqdes is invalid.

       EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.

CONFORMING TO

       POSIX.1-2001.

NOTES

       On  Linux,  mq_getattr()  and  mq_setattr()  are library functions layered on top of the mq_getsetattr(2)
       system call.

SEE ALSO

       mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)

COLOPHON

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