NAME

       mq_getattr, mq_setattr - get/set message queue attributes

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

       #include <mqueue.h>

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

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 │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       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.

EXAMPLES

       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 <fcntl.h>
       #include <mqueue.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/stat.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, 0600, 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)