mq_getattr

mq_getattr, mq_setattr - get/set message queue attributes

#include <mqueue.h>
 I mqd_t mq_getattr(mqd_t  mqdes , struct mq_attr * attr );
I mqd_t mq_setattr(mqd_t  mqdes , struct mq_attr * newattr ,
I                  struct mq_attr * oldattr );

R mq_getattr () and R mq_setattr () respectively retrieve and modify attributes of the message queue referred to by the descriptor R mqdes . R mq_getattr () returns an mq_attr structure in the buffer pointed by R 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 R 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_open(3). The mq_curmsgs field returns the number of messages currently held in the queue. R mq_setattr () sets message queue attributes using information supplied in the mq_attr structure pointed to by R newattr . The only attribute that can be modified is the setting of the O_NONBLOCK flag in R 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 R mq_getattr ().

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

EBADF

EINVAL

POSIX.1-2001.

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