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

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, const 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 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_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_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
       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 message queue descriptor specified in mqdes is invalid.

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

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       +---------------------------+---------------+---------+
       |Interface		   | Attribute	   | Value   |
       +---------------------------+---------------+---------+
       |mq_getattr(), mq_setattr() | Thread safety | MT-Safe |
       +---------------------------+---------------+---------+
CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

NOTES

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

EXAMPLE

       The program below can be used to show the default mq_maxmsg and mq_msgsize 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_overview(7)) can be used to control the defaults:

	   $ uname -sr
	   Linux 3.8.0
	   $ cat /proc/sys/fs/mqueue/msg_default
	   10
	   $ cat /proc/sys/fs/mqueue/msgsize_default
	   8192

   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)

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

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

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

	   if (mq_getattr(mqd, &attr) == -1)
	       errExit("mq_getattr");

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

	   if (mq_unlink(argv[1]) == -1)
	       errExit("mq_unlink");

	   exit(EXIT_SUCCESS);
       }

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 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15							     MQ_GETATTR(3)