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_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

       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().

       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 attributes(7).

       │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_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
           $ 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)

       This page is part of release 5.10 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

Linux                              2020-06-09                      MQ_GETATTR(3)