1MQ_OPEN(3P) POSIX Programmer's Manual MQ_OPEN(3P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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13 mq_open — open a message queue (REALTIME)
14
16 #include <mqueue.h>
17 mqd_t mq_open(const char *name, int oflag, ...);
19
21 The mq_open() function shall establish the connection between a process
22 and a message queue with a message queue descriptor. It shall create an
23 open message queue description that refers to the message queue, and a
24 message queue descriptor that refers to that open message queue
25 description. The message queue descriptor is used by other functions to
26 refer to that message queue. The name argument points to a string nam‐
27 ing a message queue. It is unspecified whether the name appears in the
28 file system and is visible to other functions that take pathnames as
29 arguments. The name argument conforms to the construction rules for a
30 pathname, except that the interpretation of <slash> characters other
31 than the leading <slash> character in name is implementation-defined,
32 and that the length limits for the name argument are implementation-
33 defined and need not be the same as the pathname limits {PATH_MAX} and
34 {NAME_MAX}. If name begins with the <slash> character, then processes
35 calling mq_open() with the same value of name shall refer to the same
36 message queue object, as long as that name has not been removed. If
37 name does not begin with the <slash> character, the effect is implemen‐
38 tation-defined. If the name argument is not the name of an existing
39 message queue and creation is not requested, mq_open() shall fail and
40 return an error.
41 A message queue descriptor may be implemented using a file descriptor,
43 in which case applications can open up to at least {OPEN_MAX} file and
44 message queues.
45 The oflag argument requests the desired receive and/or send access to
47 the message queue. The requested access permission to receive messages
48 or send messages shall be granted if the calling process would be
49 granted read or write access, respectively, to an equivalently pro‐
50 tected file.
51 The value of oflag is the bitwise-inclusive OR of values from the fol‐
53 lowing list. Applications shall specify exactly one of the first three
54 values (access modes) below in the value of oflag:
55 O_RDONLY Open the message queue for receiving messages. The process
57 can use the returned message queue descriptor with
58 mq_receive(), but not mq_send(). A message queue may be
59 open multiple times in the same or different processes for
60 receiving messages.
61 O_WRONLY Open the queue for sending messages. The process can use
63 the returned message queue descriptor with mq_send() but
64 not mq_receive(). A message queue may be open multiple
65 times in the same or different processes for sending mes‐
66 sages.
67 O_RDWR Open the queue for both receiving and sending messages. The
69 process can use any of the functions allowed for O_RDONLY
70 and O_WRONLY. A message queue may be open multiple times in
71 the same or different processes for sending messages.
72 Any combination of the remaining flags may be specified in the value of
74 oflag:
75 O_CREAT Create a message queue. It requires two additional argu‐
77 ments: mode, which shall be of type mode_t, and attr, which
78 shall be a pointer to an mq_attr structure. If the pathname
79 name has already been used to create a message queue that
80 still exists, then this flag shall have no effect, except
81 as noted under O_EXCL. Otherwise, a message queue shall be
82 created without any messages in it. The user ID of the mes‐
83 sage queue shall be set to the effective user ID of the
84 process. The group ID of the message queue shall be set to
85 the effective group ID of the process; however, if the name
86 argument is visible in the file system, the group ID may be
87 set to the group ID of the containing directory. When bits
88 in mode other than the file permission bits are specified,
89 the effect is unspecified. If attr is NULL, the message
90 queue shall be created with implementation-defined default
91 message queue attributes. If attr is non-NULL and the call‐
92 ing process has appropriate privileges on name, the message
93 queue mq_maxmsg and mq_msgsize attributes shall be set to
94 the values of the corresponding members in the mq_attr
95 structure referred to by attr. The values of the mq_flags
96 and mq_curmsgs members of the mq_attr structure shall be
97 ignored. If attr is non-NULL, but the calling process does
98 not have appropriate privileges on name, the mq_open()
99 function shall fail and return an error without creating
100 the message queue.
101 O_EXCL If O_EXCL and O_CREAT are set, mq_open() shall fail if the
103 message queue name exists. The check for the existence of
104 the message queue and the creation of the message queue if
105 it does not exist shall be atomic with respect to other
106 threads executing mq_open() naming the same name with
107 O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not
108 set, the result is undefined.
109 O_NONBLOCK Determines whether an mq_send() or mq_receive() waits for
111 resources or messages that are not currently available, or
112 fails with errno set to [EAGAIN]; see mq_send() and
113 mq_receive() for details.
114 The mq_open() function does not add or remove messages from the queue.
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118 Upon successful completion, the function shall return a message queue
119 descriptor; otherwise, the function shall return (mqd_t)−1 and set
120 errno to indicate the error.
121
123 The mq_open() function shall fail if:
124 EACCES The message queue exists and the permissions specified by oflag
126 are denied, or the message queue does not exist and permission
127 to create the message queue is denied.
128 EEXIST O_CREAT and O_EXCL are set and the named message queue already
130 exists.
131 EINTR The mq_open() function was interrupted by a signal.
133 EINVAL The mq_open() function is not supported for the given name.
135 EINVAL O_CREAT was specified in oflag, the value of attr is not NULL,
137 and either mq_maxmsg or mq_msgsize was less than or equal to
138 zero.
139 EMFILE Too many message queue descriptors or file descriptors are cur‐
141 rently in use by this process.
142 ENFILE Too many message queues are currently open in the system.
144 ENOENT O_CREAT is not set and the named message queue does not exist.
146 ENOSPC There is insufficient space for the creation of the new message
148 queue.
149 If any of the following conditions occur, the mq_open() function may
151 return (mqd_t)−1 and set errno to the corresponding value.
152 ENAMETOOLONG
154 The length of the name argument exceeds {_POSIX_PATH_MAX} on
155 systems that do not support the XSI option or exceeds
156 {_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
157 that is longer than {_POSIX_NAME_MAX} on systems that do not
158 support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI
159 systems.
160 The following sections are informative.
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164 None.
165
167 None.
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170 None.
171
173 A future version might require the mq_open() and mq_unlink() functions
174 to have semantics similar to normal file system operations.
175
177 mq_close(), mq_getattr(), mq_receive(), mq_send(), mq_setattr(),
178 mq_unlink(), msgctl(), msgget(), msgrcv(), msgsnd()
179 The Base Definitions volume of POSIX.1‐2008, <mqueue.h>
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183 Portions of this text are reprinted and reproduced in electronic form
184 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
185 -- Portable Operating System Interface (POSIX), The Open Group Base
186 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
187 cal and Electronics Engineers, Inc and The Open Group. (This is
188 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
189 event of any discrepancy between this version and the original IEEE and
190 The Open Group Standard, the original IEEE and The Open Group Standard
191 is the referee document. The original Standard can be obtained online
192 at http://www.unix.org/online.html .
193 Any typographical or formatting errors that appear in this page are
195 most likely to have been introduced during the conversion of the source
196 files to man page format. To report such errors, see https://www.ker‐
197 nel.org/doc/man-pages/reporting_bugs.html .
198IEEE/The Open Group 2013 MQ_OPEN(3P)