1MQ_GETATTR(3) Linux Programmer's Manual MQ_GETATTR(3)
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6 mq_getattr, mq_setattr - get/set message queue attributes
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9 #include <mqueue.h>
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11 mqd_t mq_getattr(mqd_t mqdes, struct mq_attr *attr);
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13 mqd_t mq_setattr(mqd_t mqdes, struct mq_attr *newattr,
14 struct mq_attr *oldattr);
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16 Link with -lrt.
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19 mq_getattr() and mq_setattr() respectively retrieve and modify
20 attributes of the message queue referred to by the descriptor mqdes.
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22 mq_getattr() returns an mq_attr structure in the buffer pointed by
23 attr. This structure is defined as:
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25 struct mq_attr {
26 long mq_flags; /* Flags: 0 or O_NONBLOCK */
27 long mq_maxmsg; /* Max. # of messages on queue */
28 long mq_msgsize; /* Max. message size (bytes) */
29 long mq_curmsgs; /* # of messages currently in queue */
30 };
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32 The mq_flags field contains flags associated with the open message
33 queue description. This field is initialized when the queue is created
34 by mq_open(3). The only flag that can appear in this field is O_NON‐
35 BLOCK.
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37 The mq_maxmsg and mq_msgsize fields are set when the message queue is
38 created by mq_open(3). The mq_maxmsg field is an upper limit on the
39 number of messages that may be placed on the queue using mq_send(3).
40 The mq_msgsize field is an upper limit on the size of messages that may
41 be placed on the queue. Both of these fields must have a value greater
42 than zero. Two /proc files that place ceilings on the values for these
43 fields are described in mq_open(3).
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45 The mq_curmsgs field returns the number of messages currently held in
46 the queue.
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48 mq_setattr() sets message queue attributes using information supplied
49 in the mq_attr structure pointed to by newattr. The only attribute
50 that can be modified is the setting of the O_NONBLOCK flag in mq_flags.
51 The other fields in newattr are ignored. If the oldattr field is not
52 NULL, then the buffer that it points to is used to return an mq_attr
53 structure that contains the same information that is returned by
54 mq_getattr().
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57 On success mq_getattr() and mq_setattr() return 0; on error, -1 is
58 returned, with errno set to indicate the error.
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61 EBADF The descriptor specified in mqdes is invalid.
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63 EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.
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66 POSIX.1-2001.
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69 On Linux, mq_getattr() and mq_setattr() are library functions layered
70 on top of the mq_getsetattr(2) system call.
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73 mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3),
74 mq_unlink(3), mq_overview(7)
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77 This page is part of release 3.25 of the Linux man-pages project. A
78 description of the project, and information about reporting bugs, can
79 be found at http://www.kernel.org/doc/man-pages/.
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83Linux 2008-09-29 MQ_GETATTR(3)