Manpages - mq_getattr.3

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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 *restrict newattr,
   struct mq_attr *restrict 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.

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 <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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Author: dt

Created: 2022-02-20 Sun 17:16