1MQ_OPEN(P) POSIX Programmer's Manual MQ_OPEN(P)
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6 mq_open - open a message queue (REALTIME)
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9 #include <mqueue.h>
10 mqd_t mq_open(const char *name, int oflag, ...);
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15 The mq_open() function shall establish the connection between a process
16 and a message queue with a message queue descriptor. It shall create an
17 open message queue description that refers to the message queue, and a
18 message queue descriptor that refers to that open message queue
19 description. The message queue descriptor is used by other functions to
20 refer to that message queue. The name argument points to a string nam‐
21 ing a message queue. It is unspecified whether the name appears in the
22 file system and is visible to other functions that take pathnames as
23 arguments. The name argument shall conform to the construction rules
24 for a pathname. If name begins with the slash character, then processes
25 calling mq_open() with the same value of name shall refer to the same
26 message queue object, as long as that name has not been removed. If
27 name does not begin with the slash character, the effect is implementa‐
28 tion-defined. The interpretation of slash characters other than the
29 leading slash character in name is implementation-defined. If the name
30 argument is not the name of an existing message queue and creation is
31 not requested, mq_open() shall fail and return an error.
32 A message queue descriptor may be implemented using a file descriptor,
34 in which case applications can open up to at least {OPEN_MAX} file and
35 message queues.
36 The oflag argument requests the desired receive and/or send access to
38 the message queue. The requested access permission to receive messages
39 or send messages shall be granted if the calling process would be
40 granted read or write access, respectively, to an equivalently pro‐
41 tected file.
42 The value of oflag is the bitwise-inclusive OR of values from the fol‐
44 lowing list. Applications shall specify exactly one of the first three
45 values (access modes) below in the value of oflag:
46 O_RDONLY
48 Open the message queue for receiving messages. The process can
49 use the returned message queue descriptor with mq_receive(), but
50 not mq_send(). A message queue may be open multiple times in the
51 same or different processes for receiving messages.
52 O_WRONLY
54 Open the queue for sending messages. The process can use the
55 returned message queue descriptor with mq_send() but not
56 mq_receive(). A message queue may be open multiple times in the
57 same or different processes for sending messages.
58 O_RDWR Open the queue for both receiving and sending messages. The
60 process can use any of the functions allowed for O_RDONLY and
61 O_WRONLY. A message queue may be open multiple times in the same
62 or different processes for sending messages.
63 Any combination of the remaining flags may be specified in the value of
66 oflag:
67 O_CREAT
69 Create a message queue. It requires two additional arguments:
70 mode, which shall be of type mode_t, and attr, which shall be a
71 pointer to an mq_attr structure. If the pathname name has
72 already been used to create a message queue that still exists,
73 then this flag shall have no effect, except as noted under
74 O_EXCL. Otherwise, a message queue shall be created without any
75 messages in it. The user ID of the message queue shall be set to
76 the effective user ID of the process, and the group ID of the
77 message queue shall be set to the effective group ID of the
78 process. The file permission bits shall be set to the value of
79 mode. When bits in mode other than file permission bits are set,
80 the effect is implementation-defined. If attr is NULL, the mes‐
81 sage queue shall be created with implementation-defined default
82 message queue attributes. If attr is non-NULL and the calling
83 process has the appropriate privilege on name, the message queue
84 mq_maxmsg and mq_msgsize attributes shall be set to the values
85 of the corresponding members in the mq_attr structure referred
86 to by attr. If attr is non-NULL, but the calling process does
87 not have the appropriate privilege on name, the mq_open() func‐
88 tion shall fail and return an error without creating the message
89 queue.
90 O_EXCL If O_EXCL and O_CREAT are set, mq_open() shall fail if the mes‐
92 sage queue name exists. The check for the existence of the mes‐
93 sage queue and the creation of the message queue if it does not
94 exist shall be atomic with respect to other threads executing
95 mq_open() naming the same name with O_EXCL and O_CREAT set. If
96 O_EXCL is set and O_CREAT is not set, the result is undefined.
97 O_NONBLOCK
99 Determines whether an mq_send() or mq_receive() waits for
100 resources or messages that are not currently available, or fails
101 with errno set to [EAGAIN]; see mq_send() and mq_receive() for
102 details.
103 The mq_open() function does not add or remove messages from the queue.
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108 Upon successful completion, the function shall return a message queue
109 descriptor; otherwise, the function shall return (mqd_t)-1 and set
110 errno to indicate the error.
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113 The mq_open() function shall fail if:
114 EACCES The message queue exists and the permissions specified by oflag
116 are denied, or the message queue does not exist and permission
117 to create the message queue is denied.
118 EEXIST O_CREAT and O_EXCL are set and the named message queue already
120 exists.
121 EINTR The mq_open() function was interrupted by a signal.
123 EINVAL The mq_open() function is not supported for the given name.
125 EINVAL O_CREAT was specified in oflag, the value of attr is not NULL,
127 and either mq_maxmsg or mq_msgsize was less than or equal to
128 zero.
129 EMFILE Too many message queue descriptors or file descriptors are cur‐
131 rently in use by this process.
132 ENAMETOOLONG
134 The length of the name argument exceeds {PATH_MAX} or a pathname
135 component is longer than {NAME_MAX}.
136 ENFILE Too many message queues are currently open in the system.
138 ENOENT O_CREAT is not set and the named message queue does not exist.
140 ENOSPC There is insufficient space for the creation of the new message
142 queue.
143 The following sections are informative.
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148 None.
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151 None.
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154 None.
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157 None.
158
160 mq_close() , mq_getattr() , mq_receive() , mq_send() , mq_setattr() ,
161 mq_timedreceive() , mq_timedsend() , mq_unlink() , msgctl() , msgget()
162 , msgrcv() , msgsnd() , the Base Definitions volume of
163 IEEE Std 1003.1-2001, <mqueue.h>
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166 Portions of this text are reprinted and reproduced in electronic form
167 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
168 -- Portable Operating System Interface (POSIX), The Open Group Base
169 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
170 Electrical and Electronics Engineers, Inc and The Open Group. In the
171 event of any discrepancy between this version and the original IEEE and
172 The Open Group Standard, the original IEEE and The Open Group Standard
173 is the referee document. The original Standard can be obtained online
174 at http://www.opengroup.org/unix/online.html .
175IEEE/The Open Group 2003 MQ_OPEN(P)