1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
|
'\" t
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH mq_getattr 3 2024-05-02 "Linux man-pages 6.8"
.SH NAME
mq_getattr, mq_setattr \- get/set message queue attributes
.SH LIBRARY
Real-time library
.RI ( librt ", " \-lrt )
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.P
.BI "int mq_getattr(mqd_t " mqdes ", struct mq_attr *" attr );
.BI "int mq_setattr(mqd_t " mqdes ", const struct mq_attr *restrict " newattr ,
.BI " struct mq_attr *restrict " oldattr );
.fi
.SH DESCRIPTION
.BR mq_getattr ()
and
.BR mq_setattr ()
respectively retrieve and modify attributes of the message queue
referred to by the message queue descriptor
.IR mqdes .
.P
.BR mq_getattr ()
returns an
.I mq_attr
structure in the buffer pointed by
.IR attr .
This structure is defined as:
.P
.in +4n
.EX
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 */
};
.EE
.in
.P
The
.I mq_flags
field contains flags associated with the open message queue description.
This field is initialized when the queue is created by
.BR mq_open (3).
The only flag that can appear in this field is
.BR O_NONBLOCK .
.P
The
.I mq_maxmsg
and
.I mq_msgsize
fields are set when the message queue is created by
.BR mq_open (3).
The
.I mq_maxmsg
field is an upper limit on the number of messages
that may be placed on the queue using
.BR mq_send (3).
The
.I 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
.I /proc
files that place ceilings on the values for these fields are described in
.BR mq_overview (7).
.P
The
.I mq_curmsgs
field returns the number of messages currently held in the queue.
.P
.BR mq_setattr ()
sets message queue attributes using information supplied in the
.I mq_attr
structure pointed to by
.IR newattr .
The only attribute that can be modified is the setting of the
.B O_NONBLOCK
flag in
.IR mq_flags .
The other fields in
.I newattr
are ignored.
If the
.I oldattr
field is not NULL,
then the buffer that it points to is used to return an
.I mq_attr
structure that contains the same information that is returned by
.BR mq_getattr ().
.SH RETURN VALUE
On success
.BR mq_getattr ()
and
.BR mq_setattr ()
return 0; on error, \-1 is returned, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EBADF
The message queue descriptor specified in
.I mqdes
is invalid.
.TP
.B EINVAL
.I newattr\->mq_flags
contained set bits other than
.BR O_NONBLOCK .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR mq_getattr (),
.BR mq_setattr ()
T} Thread safety MT-Safe
.TE
.SH VERSIONS
On Linux,
.BR mq_getattr ()
and
.BR mq_setattr ()
are library functions layered on top of the
.BR mq_getsetattr (2)
system call.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH EXAMPLES
The program below can be used to show the default
.I mq_maxmsg
and
.I mq_msgsize
values that are assigned to a message queue that is created with a call to
.BR mq_open (3)
in which the
.I attr
argument is NULL.
Here is an example run of the program:
.P
.in +4n
.EX
$ \fB./a.out /testq\fP
Maximum # of messages on queue: 10
Maximum message size: 8192
.EE
.in
.P
Since Linux 3.5, the following
.I /proc
files (described in
.BR mq_overview (7))
can be used to control the defaults:
.P
.in +4n
.EX
$ \fBuname \-sr\fP
Linux 3.8.0
$ \fBcat /proc/sys/fs/mqueue/msg_default\fP
10
$ \fBcat /proc/sys/fs/mqueue/msgsize_default\fP
8192
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (mq_getattr.c)
.EX
#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); \e
} while (0)
\&
int
main(int argc, char *argv[])
{
mqd_t mqd;
struct mq_attr attr;
\&
if (argc != 2) {
fprintf(stderr, "Usage: %s mq\-name\en", 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\en", attr.mq_maxmsg);
printf("Maximum message size: %ld\en", attr.mq_msgsize);
\&
if (mq_unlink(argv[1]) == \-1)
errExit("mq_unlink");
\&
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR mq_close (3),
.BR mq_notify (3),
.BR mq_open (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR mq_overview (7)
|
