Man page - mq_setattr(3)

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Manual

mq_getattr

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
VERSIONS
STANDARDS
HISTORY
EXAMPLES
Program source
SEE ALSO

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).

Image grohtml-552761-1.png

VERSIONS

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

STANDARDS

POSIX.1-2008.

HISTORY

POSIX.1-2001.

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);
}