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

       mq_getattr, mq_setattr - get/set message queue attributes

       #include <mqueue.h>

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

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

       Link with -lrt.

       mq_getattr()   and   mq_setattr()   respectively	 retrieve  and	modify
       attributes of the message  queue	 referred  to  by  the	message	 queue
       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_NON-

       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_overview(7).

       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

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

       EBADF  The message queue descriptor specified in mqdes is invalid.

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

       For   an	  explanation	of   the  terms	 used  in  this	 section,  see

       |Interface		   | Attribute	   | Value   |
       |mq_getattr(), mq_setattr() | Thread safety | MT-Safe |
       POSIX.1-2001, POSIX.1-2008.

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

       The program below can be used to show the default mq_maxmsg and mq_msg-
       size values that are assigned to a message queue that is created with a
       call  to	 mq_open(3)  in	 which	the attr argument is NULL.  Here is an
       example run of the program:

	   $ ./a.out /testq
	   Maximum # of messages on queue:   10
	   Maximum message size:	     8192

       Since Linux 3.5, the  following	/proc  files  (described  in  mq_over-
       view(7)) can be used to control the defaults:

	   $ uname -sr
	   Linux 3.8.0
	   $ cat /proc/sys/fs/mqueue/msg_default
	   $ cat /proc/sys/fs/mqueue/msgsize_default

   Program source

       #include <mqueue.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
			       } while (0)

       main(int argc, char *argv[])
	   mqd_t mqd;
	   struct mq_attr attr;

	   if (argc != 2) {
	       fprintf(stderr, "Usage: %s mq-name\n", argv[0]);

	   mqd = mq_open(argv[1], O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, NULL);
	   if (mqd == (mqd_t) -1)

	   if (mq_getattr(mqd, &attr) == -1)

	   printf("Maximum # of messages on queue:   %ld\n", attr.mq_maxmsg);
	   printf("Maximum message size:	     %ld\n", attr.mq_msgsize);

	   if (mq_unlink(argv[1]) == -1)


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

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Linux				  2016-03-15			 MQ_GETATTR(3)