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\n", 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\n", attr.mq_maxmsg);
           printf("Maximum message size:             %ld\n", 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

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