mq_getattr(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

MQ_GETATTR(3)           Linux Programmer's Manual          MQ_GETATTR(3)

NAME         top

       mq_getattr, mq_setattr - get/set message queue attributes

SYNOPSIS         top

       #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);

       Link with -lrt.

DESCRIPTION         top

       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         top

       On success mq_getattr() and mq_setattr() return 0; on error, -1
       is returned, with errno set to indicate the error.

ERRORS         top

       EBADF  The message queue descriptor specified in mqdes is
              invalid.

       EINVAL newattr->mq_flags contained set bits other than
              O_NONBLOCK.

ATTRIBUTES         top

       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         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       On Linux, mq_getattr() and mq_setattr() are library functions
       layered on top of the mq_getsetattr(2) system call.

EXAMPLES         top

       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         top

       mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3),
       mq_unlink(3), mq_overview(7)

COLOPHON         top

       This page is part of release 5.13 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                          2021-03-22                  MQ_GETATTR(3)

Pages that refer to this page: mq_getsetattr(2)mq_close(3)mq_notify(3)mq_open(3)mq_receive(3)mq_send(3)mq_unlink(3)systemd.socket(5)mq_overview(7)