Manual Reference Pages  - MQ_GETATTR (3)


mq_getattr, mq_setattr - get/set message queue attributes


Return Value
Conforming To


#include <mqueue.h> 

int mq_getattr(mqd_t mqdes, struct mq_attr *attr);

int mq_setattr(mqd_t mqdes, struct mq_attr *newattr, struct mq_attr *oldattr);

Link with -lrt.


mq_getattr() and mq_setattr() respectively retrieve and modify attributes of the message queue referred to by the 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_open(3).

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().


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


EBADF The descriptor specified in mqdes is invalid.
EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.




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


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


This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

Linux MQ_GETATTR (3) 2010-08-29
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