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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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