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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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