mq_getattr(3)

1MQ_GETATTR(3)              Linux Programmer's Manual             MQ_GETATTR(3)
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NAME

6       mq_getattr, mq_setattr - get/set message queue attributes
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SYNOPSIS

9       #include <mqueue.h>
10
11       int mq_getattr(mqd_t mqdes, struct mq_attr *attr);
12       int mq_setattr(mqd_t mqdes, const struct mq_attr *restrict newattr,
13                      struct mq_attr *restrict oldattr);
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15       Link with -lrt.
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DESCRIPTION

18       mq_getattr()  and  mq_setattr()  respectively  retrieve  and modify at‐
19       tributes of the message queue referred to by the message queue descrip‐
20       tor 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_overview(7).
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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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RETURN VALUE

57       On success mq_getattr() and mq_setattr() return 0; on error, -1 is  re‐
58       turned, with errno set to indicate the error.
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ERRORS

61       EBADF  The message queue 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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ATTRIBUTES

66       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
67       tributes(7).
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69       ┌────────────────────────────────────────────┬───────────────┬─────────┐
70       │Interface                                   │ Attribute     │ Value   │
71       ├────────────────────────────────────────────┼───────────────┼─────────┤
72       │mq_getattr(), mq_setattr()                  │ Thread safety │ MT-Safe │
73       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

76       POSIX.1-2001, POSIX.1-2008.
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NOTES

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

83       The program below can be used to show the default mq_maxmsg and mq_msg‐
84       size values that are assigned to a message queue that is created with a
85       call  to mq_open(3) in which the attr argument is NULL.  Here is an ex‐
86       ample run of the program:
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88           $ ./a.out /testq
89           Maximum # of messages on queue:   10
90           Maximum message size:             8192
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92       Since Linux 3.5, the  following  /proc  files  (described  in  mq_over‐
93       view(7)) can be used to control the defaults:
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95           $ uname -sr
96           Linux 3.8.0
97           $ cat /proc/sys/fs/mqueue/msg_default
98           10
99           $ cat /proc/sys/fs/mqueue/msgsize_default
100           8192
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102   Program source
103
104       #include <mqueue.h>
105       #include <sys/stat.h>
106       #include <fcntl.h>
107       #include <stdio.h>
108       #include <stdlib.h>
109       #include <unistd.h>
110
111       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
112                               } while (0)
113
114       int
115       main(int argc, char *argv[])
116       {
117           mqd_t mqd;
118           struct mq_attr attr;
119
120           if (argc != 2) {
121               fprintf(stderr, "Usage: %s mq-name\n", argv[0]);
122               exit(EXIT_FAILURE);
123           }
124
125           mqd = mq_open(argv[1], O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, NULL);
126           if (mqd == (mqd_t) -1)
127               errExit("mq_open");
128
129           if (mq_getattr(mqd, &attr) == -1)
130               errExit("mq_getattr");
131
132           printf("Maximum # of messages on queue:   %ld\n", attr.mq_maxmsg);
133           printf("Maximum message size:             %ld\n", attr.mq_msgsize);
134
135           if (mq_unlink(argv[1]) == -1)
136               errExit("mq_unlink");
137
138           exit(EXIT_SUCCESS);
139       }
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COLOPHON

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153Linux                             2021-03-22                     MQ_GETATTR(3)