MQ_GETATTR(3)                                       Linux Programmer's Manual                                      MQ_GETATTR(3)



NAME
       mq_getattr, mq_setattr - get/set message queue attributes

SYNOPSIS
       #include <mqueue.h>

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

       mqd_t 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
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-09-29                                              MQ_GETATTR(3)