#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);
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().
| Interface | Attribute | Value |
| mq_getattr(), mq_setattr() | Thread safety | MT-Safe |
$ ./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
#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");