1MQ_OPEN(3P) POSIX Programmer's Manual MQ_OPEN(3P)
2
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.
10
12 mq_open — open a message queue (REALTIME)
13
15 #include <mqueue.h>
16 mqd_t mq_open(const char *name, int oflag, ...);
18
20 The mq_open() function shall establish the connection between a process
21 and a message queue with a message queue descriptor. It shall create an
22 open message queue description that refers to the message queue, and a
23 message queue descriptor that refers to that open message queue
24 description. The message queue descriptor is used by other functions to
25 refer to that message queue. The name argument points to a string nam‐
26 ing a message queue. It is unspecified whether the name appears in the
27 file system and is visible to other functions that take pathnames as
28 arguments. The name argument conforms to the construction rules for a
29 pathname, except that the interpretation of <slash> characters other
30 than the leading <slash> character in name is implementation-defined,
31 and that the length limits for the name argument are implementation-
32 defined and need not be the same as the pathname limits {PATH_MAX} and
33 {NAME_MAX}. If name begins with the <slash> character, then processes
34 calling mq_open() with the same value of name shall refer to the same
35 message queue object, as long as that name has not been removed. If
36 name does not begin with the <slash> character, the effect is implemen‐
37 tation-defined. If the name argument is not the name of an existing
38 message queue and creation is not requested, mq_open() shall fail and
39 return an error.
40 A message queue descriptor may be implemented using a file descriptor,
42 in which case applications can open up to at least {OPEN_MAX} file and
43 message queues.
44 The oflag argument requests the desired receive and/or send access to
46 the message queue. The requested access permission to receive messages
47 or send messages shall be granted if the calling process would be
48 granted read or write access, respectively, to an equivalently pro‐
49 tected file.
50 The value of oflag is the bitwise-inclusive OR of values from the fol‐
52 lowing list. Applications shall specify exactly one of the first three
53 values (access modes) below in the value of oflag:
54 O_RDONLY Open the message queue for receiving messages. The process
56 can use the returned message queue descriptor with
57 mq_receive(), but not mq_send(). A message queue may be
58 open multiple times in the same or different processes for
59 receiving messages.
60 O_WRONLY Open the queue for sending messages. The process can use
62 the returned message queue descriptor with mq_send() but
63 not mq_receive(). A message queue may be open multiple
64 times in the same or different processes for sending mes‐
65 sages.
66 O_RDWR Open the queue for both receiving and sending messages. The
68 process can use any of the functions allowed for O_RDONLY
69 and O_WRONLY. A message queue may be open multiple times in
70 the same or different processes for sending messages.
71 Any combination of the remaining flags may be specified in the value of
73 oflag:
74 O_CREAT Create a message queue. It requires two additional argu‐
76 ments: mode, which shall be of type mode_t, and attr, which
77 shall be a pointer to an mq_attr structure. If the pathname
78 name has already been used to create a message queue that
79 still exists, then this flag shall have no effect, except
80 as noted under O_EXCL. Otherwise, a message queue shall be
81 created without any messages in it. The user ID of the mes‐
82 sage queue shall be set to the effective user ID of the
83 process. The group ID of the message queue shall be set to
84 the effective group ID of the process; however, if the name
85 argument is visible in the file system, the group ID may be
86 set to the group ID of the containing directory. When bits
87 in mode other than the file permission bits are specified,
88 the effect is unspecified. If attr is NULL, the message
89 queue shall be created with implementation-defined default
90 message queue attributes. If attr is non-NULL and the call‐
91 ing process has appropriate privileges on name, the message
92 queue mq_maxmsg and mq_msgsize attributes shall be set to
93 the values of the corresponding members in the mq_attr
94 structure referred to by attr. The values of the mq_flags
95 and mq_curmsgs members of the mq_attr structure shall be
96 ignored. If attr is non-NULL, but the calling process does
97 not have appropriate privileges on name, the mq_open()
98 function shall fail and return an error without creating
99 the message queue.
100 O_EXCL If O_EXCL and O_CREAT are set, mq_open() shall fail if the
102 message queue name exists. The check for the existence of
103 the message queue and the creation of the message queue if
104 it does not exist shall be atomic with respect to other
105 threads executing mq_open() naming the same name with
106 O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not
107 set, the result is undefined.
108 O_NONBLOCK Determines whether an mq_send() or mq_receive() waits for
110 resources or messages that are not currently available, or
111 fails with errno set to [EAGAIN]; see mq_send() and
112 mq_receive() for details.
113 The mq_open() function does not add or remove messages from the queue.
115
117 Upon successful completion, the function shall return a message queue
118 descriptor; otherwise, the function shall return (mqd_t)-1 and set
119 errno to indicate the error.
120
122 The mq_open() function shall fail if:
123 EACCES The message queue exists and the permissions specified by oflag
125 are denied, or the message queue does not exist and permission
126 to create the message queue is denied.
127 EEXIST O_CREAT and O_EXCL are set and the named message queue already
129 exists.
130 EINTR The mq_open() function was interrupted by a signal.
132 EINVAL The mq_open() function is not supported for the given name.
134 EINVAL O_CREAT was specified in oflag, the value of attr is not NULL,
136 and either mq_maxmsg or mq_msgsize was less than or equal to
137 zero.
138 EMFILE Too many message queue descriptors or file descriptors are cur‐
140 rently in use by this process.
141 ENFILE Too many message queues are currently open in the system.
143 ENOENT O_CREAT is not set and the named message queue does not exist.
145 ENOSPC There is insufficient space for the creation of the new message
147 queue.
148 If any of the following conditions occur, the mq_open() function may
150 return (mqd_t)-1 and set errno to the corresponding value.
151 ENAMETOOLONG
153 The length of the name argument exceeds {_POSIX_PATH_MAX} on
154 systems that do not support the XSI option or exceeds
155 {_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
156 that is longer than {_POSIX_NAME_MAX} on systems that do not
157 support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI
158 systems.
159 The following sections are informative.
161
163 None.
164
166 None.
167
169 None.
170
172 A future version might require the mq_open() and mq_unlink() functions
173 to have semantics similar to normal file system operations.
174
176 mq_close(), mq_getattr(), mq_receive(), mq_send(), mq_setattr(),
177 mq_unlink(), msgctl(), msgget(), msgrcv(), msgsnd()
178 The Base Definitions volume of POSIX.1‐2017, <mqueue.h>
180
182 Portions of this text are reprinted and reproduced in electronic form
183 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
184 table Operating System Interface (POSIX), The Open Group Base Specifi‐
185 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
186 Electrical and Electronics Engineers, Inc and The Open Group. In the
187 event of any discrepancy between this version and the original IEEE and
188 The Open Group Standard, the original IEEE and The Open Group Standard
189 is the referee document. The original Standard can be obtained online
190 at http://www.opengroup.org/unix/online.html .
191 Any typographical or formatting errors that appear in this page are
193 most likely to have been introduced during the conversion of the source
194 files to man page format. To report such errors, see https://www.ker‐
195 nel.org/doc/man-pages/reporting_bugs.html .
196IEEE/The Open Group 2017 MQ_OPEN(3P)