mq_getattr

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 at-
       tributes of the message queue referred to by the message queue descrip-
       tor 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  re-
       turned, 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 at-
       tributes(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 ex-
       ample 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
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       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                             2019-03-06                     MQ_GETATTR(3)