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 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 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)

       This  page  is  part of release 4.04 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                             2015-08-08                     MQ_GETATTR(3)
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