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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_NON-
       BLOCK.
       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_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
           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
       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)