1mq_open(3C) Standard C Library Functions mq_open(3C)
2
6 mq_open - open a message queue
7
9 #include <mqueue.h>
10 mqd_t mq_open(const char *name, int oflag,
12 /* unsigned long mode, mq_attr attr */ ...);
13
16 The mq_open() function establishes the connection between a process and
17 a message queue with a message queue descriptor. It creates a open mes‐
18 sage queue description that refers to the message queue, and a message
19 queue descriptor that refers to that open message queue description.
20 The message queue descriptor is used by other functions to refer to
21 that message queue.
22 The name argument points to a string naming a message queue. The name
25 argument must conform to the construction rules for a path-name. If
26 name is not the name of an existing message queue and its creation is
27 not requested, mq_open() fails and returns an error. The first charac‐
28 ter of name must be a slash (/) character and the remaining charac‐
29 ters of name cannot include any slash characters. For maximum porta‐
30 bility, name should include no more than 14 characters, but this limit
31 is not enforced.
32 The oflag argument requests the desired receive and/or send access to
35 the message queue. The requested access permission to receive messages
36 or send messages is granted if the calling process would be granted
37 read or write access, respectively, to a file with the equivalent per‐
38 missions.
39 The value of oflag is the bitwise inclusive OR of values from the fol‐
42 lowing list. Applications must specify exactly one of the first three
43 values (access modes) below in the value of oflag:
44 O_RDONLY Open the message queue for receiving messages. The process
46 can use the returned message queue descriptor with
47 mq_receive(3C), but not mq_send(3C). A message queue may
48 be open multiple times in the same or different processes
49 for receiving messages.
50 O_WRONLY Open the queue for sending messages. The process can use
53 the returned message queue descriptor with mq_send(3C) but
54 not mq_receive(3C). A message queue may be open multiple
55 times in the same or different processes for sending mes‐
56 sages.
57 O_RDWR Open the queue for both receiving and sending messages.
60 The process can use any of the functions allowed for
61 O_RDONLY and O_WRONLY. A message queue may be open multi‐
62 ple times in the same or different processes for sending
63 messages.
64 Any combination of the remaining flags may additionally be specified in
68 the value of oflag:
69 O_CREAT This option is used to create a message queue, and it
71 requires two additional arguments: mode, which is of
72 type mode_t, and attr, which is pointer to a mq_attr
73 structure. If the pathname, name, has already been used
74 to create a message queue that still exists, then this
75 flag has no effect, except as noted under O_EXCL (see
76 below). Otherwise, a message queue is created without
77 any messages in it.
78 The user ID of the message queue is set to the effective
80 user ID of process, and the group ID of the message
81 queue is set to the effective group ID of the process.
82 The file permission bits are set to the value of mode,
83 and modified by clearing all bits set in the file mode
84 creation mask of the process (see umask(2)).
85 If attr is non-NULL and the calling process has the
87 appropriate privilege on name, the message queue
88 mq_maxmsg and mq_msgsize attributes are set to the val‐
89 ues of the corresponding members in the mq_attr struc‐
90 ture referred to by attr. If attr is non-NULL, but the
91 calling process does not have the appropriate privilege
92 on name, the mq_open() function fails and returns an
93 error without creating the message queue.
94 O_EXCL If both O_EXCL and O_CREAT are set, mq_open() will fail
97 if the message queue name exists. The check for the
98 existence of the message queue and the creation of the
99 message queue if it does not exist are atomic with
100 respect to other processes executing mq_open() naming
101 the same name with both O_EXCL and O_CREAT set. If
102 O_EXCL and O_CREAT are not set, the result is undefined.
103 O_NONBLOCK The setting of this flag is associated with the open
106 message queue description and determines whether a
107 mq_send(3C) or mq_receive(3C) waits for resources or
108 messages that are not currently available, or fails with
109 errno set to EAGAIN. See mq_send(3C) and mq_receive(3C)
110 for details.
111
114 Upon successful completion, mq_open() returns a message queue descrip‐
115 tor; otherwise the function returns (mqd_t)−1 and sets errno to indi‐
116 cate the error condition.
117
119 The mq_open() function will fail if:
120 EACCES The message queue exists and the permissions specified
122 by oflag are denied, or the message queue does not
123 exist and permission to create the message queue is
124 denied.
125 EEXIST O_CREAT and O_EXCL are set and the named message queue
128 already exists.
129 EINTR The mq_open() operation was interrupted by a signal.
132 EINVAL The mq_open() operation is not supported for the given
135 name, or O_CREAT was specified in oflag, the value of
136 attr is not NULL, and either mq_maxmsg or mq_msgsize
137 was less than or equal to zero.
138 EMFILE The number of open message queue descriptors in this
141 process exceeds MQ_OPEN_MAX, of the number of open
142 file descriptors in this process exceeds OPEN_MAX.
143 ENAMETOOLONG The length of the name string exceeds PATH_MAX, or a
146 pathname component is longer than NAME_MAX while
147 _POSIX_NO_TRUNC is in effect.
148 ENFILE Too many message queues are currently open in the sys‐
151 tem.
152 ENOENT O_CREAT is not set and the named message queue does
155 not exist.
156 ENOSPC There is insufficient space for the creation of the
159 new message queue.
160 ENOSYS The mq_open() function is not supported by the system.
163
166 See attributes(5) for descriptions of the following attributes:
167 ┌─────────────────────────────┬─────────────────────────────┐
172 │ATTRIBUTE TYPE │ATTRIBUTE VALUE │
173 ├─────────────────────────────┼─────────────────────────────┤
174 │Interface Stability │Committed │
175 ├─────────────────────────────┼─────────────────────────────┤
176 │MT-Level │MT-Safe │
177 ├─────────────────────────────┼─────────────────────────────┤
178 │Standard │See standards(5). │
179 └─────────────────────────────┴─────────────────────────────┘
180
182 exec(2), exit(2), umask(2), sysconf(3C), mqueue.h(3HEAD), mq_close(3C),
183 mq_receive(3C), mq_send(3C), mq_setattr(3C), mq_unlink(3C),
184 attributes(5), standards(5)
185
187 Due to the manner in which message queues are implemented, they should
188 not be considered secure and should not be used in security-sensitive
189 applications.
190 Solaris 2.6 was the first release to support the Asynchronous Input and
193 Output option. Prior to this release, this function always returned −1
194 and set errno to ENOSYS.
195SunOS 5.11 5 Feb 2008 mq_open(3C)