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.
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12 mq_open - open a message queue (REALTIME)
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15 #include <mqueue.h>
16 mqd_t mq_open(const char *name, int oflag, ...);
18
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 shall conform to the construction rules
30 for a pathname. If name begins with the slash character, then processes
31 calling mq_open() with the same value of name shall refer to the same
32 message queue object, as long as that name has not been removed. If
33 name does not begin with the slash character, the effect is implementa‐
34 tion-defined. The interpretation of slash characters other than the
35 leading slash character in name is implementation-defined. If the name
36 argument is not the name of an existing message queue and creation is
37 not requested, mq_open() shall fail and return an error.
38 A message queue descriptor may be implemented using a file descriptor,
40 in which case applications can open up to at least {OPEN_MAX} file and
41 message queues.
42 The oflag argument requests the desired receive and/or send access to
44 the message queue. The requested access permission to receive messages
45 or send messages shall be granted if the calling process would be
46 granted read or write access, respectively, to an equivalently pro‐
47 tected file.
48 The value of oflag is the bitwise-inclusive OR of values from the fol‐
50 lowing list. Applications shall specify exactly one of the first three
51 values (access modes) below in the value of oflag:
52 O_RDONLY
54 Open the message queue for receiving messages. The process can
55 use the returned message queue descriptor with mq_receive(), but
56 not mq_send(). A message queue may be open multiple times in the
57 same or different processes for receiving messages.
58 O_WRONLY
60 Open the queue for sending messages. The process can use the
61 returned message queue descriptor with mq_send() but not
62 mq_receive(). A message queue may be open multiple times in the
63 same or different processes for sending messages.
64 O_RDWR Open the queue for both receiving and sending messages. The
66 process can use any of the functions allowed for O_RDONLY and
67 O_WRONLY. A message queue may be open multiple times in the same
68 or different processes for sending messages.
69 Any combination of the remaining flags may be specified in the value of
72 oflag:
73 O_CREAT
75 Create a message queue. It requires two additional arguments:
76 mode, which shall be of type mode_t, and attr, which shall be a
77 pointer to an mq_attr structure. If the pathname name has
78 already been used to create a message queue that still exists,
79 then this flag shall have no effect, except as noted under
80 O_EXCL. Otherwise, a message queue shall be created without any
81 messages in it. The user ID of the message queue shall be set to
82 the effective user ID of the process, and the group ID of the
83 message queue shall be set to the effective group ID of the
84 process. The file permission bits shall be set to the value of
85 mode. When bits in mode other than file permission bits are set,
86 the effect is implementation-defined. If attr is NULL, the mes‐
87 sage queue shall be created with implementation-defined default
88 message queue attributes. If attr is non-NULL and the calling
89 process has the appropriate privilege on name, the message queue
90 mq_maxmsg and mq_msgsize attributes shall be set to the values
91 of the corresponding members in the mq_attr structure referred
92 to by attr. If attr is non-NULL, but the calling process does
93 not have the appropriate privilege on name, the mq_open() func‐
94 tion shall fail and return an error without creating the message
95 queue.
96 O_EXCL If O_EXCL and O_CREAT are set, mq_open() shall fail if the mes‐
98 sage queue name exists. The check for the existence of the mes‐
99 sage queue and the creation of the message queue if it does not
100 exist shall be atomic with respect to other threads executing
101 mq_open() naming the same name with O_EXCL and O_CREAT set. If
102 O_EXCL is set and O_CREAT is not set, the result is undefined.
103 O_NONBLOCK
105 Determines whether an mq_send() or mq_receive() waits for
106 resources or messages that are not currently available, or fails
107 with errno set to [EAGAIN]; see mq_send() and mq_receive() for
108 details.
109 The mq_open() function does not add or remove messages from the queue.
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114 Upon successful completion, the function shall return a message queue
115 descriptor; otherwise, the function shall return (mqd_t)-1 and set
116 errno to indicate the error.
117
119 The mq_open() function shall fail if:
120 EACCES The message queue exists and the permissions specified by oflag
122 are denied, or the message queue does not exist and permission
123 to create the message queue is denied.
124 EEXIST O_CREAT and O_EXCL are set and the named message queue already
126 exists.
127 EINTR The mq_open() function was interrupted by a signal.
129 EINVAL The mq_open() function is not supported for the given name.
131 EINVAL O_CREAT was specified in oflag, the value of attr is not NULL,
133 and either mq_maxmsg or mq_msgsize was less than or equal to
134 zero.
135 EMFILE Too many message queue descriptors or file descriptors are cur‐
137 rently in use by this process.
138 ENAMETOOLONG
140 The length of the name argument exceeds {PATH_MAX} or a pathname
141 component is longer than {NAME_MAX}.
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 The following sections are informative.
152
154 None.
155
157 None.
158
160 None.
161
163 None.
164
166 mq_close(), mq_getattr(), mq_receive(), mq_send(), mq_setattr(),
167 mq_timedreceive(), mq_timedsend(), mq_unlink(), msgctl(), msgget(),
168 msgrcv(), msgsnd(), the Base Definitions volume of
169 IEEE Std 1003.1-2001, <mqueue.h>
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172 Portions of this text are reprinted and reproduced in electronic form
173 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
174 -- Portable Operating System Interface (POSIX), The Open Group Base
175 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
176 Electrical and Electronics Engineers, Inc and The Open Group. In the
177 event of any discrepancy between this version and the original IEEE and
178 The Open Group Standard, the original IEEE and The Open Group Standard
179 is the referee document. The original Standard can be obtained online
180 at http://www.opengroup.org/unix/online.html .
181IEEE/The Open Group 2003 MQ_OPEN(3P)