1MQ_OPEN(3) Linux Programmer's Manual MQ_OPEN(3)
2
6 mq_open - open a message queue
7
9 #include <fcntl.h> /* For O_* constants */
10 #include <sys/stat.h> /* For mode constants */
11 #include <mqueue.h>
12 mqd_t mq_open(const char *name, int oflag);
14 mqd_t mq_open(const char *name, int oflag, mode_t mode,
15 struct mq_attr *attr);
16 Link with -lrt.
18
20 mq_open() creates a new POSIX message queue or opens an existing queue.
21 The queue is identified by name. For details of the construction of
22 name, see mq_overview(7).
23 The oflag argument specifies flags that control the operation of the
25 call. (Definitions of the flags values can be obtained by including
26 <fcntl.h>.) Exactly one of the following must be specified in oflag:
27 O_RDONLY
29 Open the queue to receive messages only.
30 O_WRONLY
32 Open the queue to send messages only.
33 O_RDWR Open the queue to both send and receive messages.
35 Zero or more of the following flags can additionally be ORed in oflag:
37 O_CLOEXEC (since Linux 2.6.26)
39 Set the close-on-exec flag for the message queue descriptor.
40 See open(2) for a discussion of why this flag is useful.
41 O_CREAT
43 Create the message queue if it does not exist. The owner (user
44 ID) of the message queue is set to the effective user ID of the
45 calling process. The group ownership (group ID) is set to the
46 effective group ID of the calling process.
47 O_EXCL If O_CREAT was specified in oflag, and a queue with the given
49 name already exists, then fail with the error EEXIST.
50 O_NONBLOCK
52 Open the queue in nonblocking mode. In circumstances where
53 mq_receive(3) and mq_send(3) would normally block, these func‐
54 tions instead fail with the error EAGAIN.
55 If O_CREAT is specified in oflag, then two additional arguments must be
57 supplied. The mode argument specifies the permissions to be placed on
58 the new queue, as for open(2). (Symbolic definitions for the permis‐
59 sions bits can be obtained by including <sys/stat.h>.) The permissions
60 settings are masked against the process umask.
61 The fields of the struct mq_attr pointed to attr specify the maximum
63 number of messages and the maximum size of messages that the queue will
64 allow. This structure is defined as follows:
65 struct mq_attr {
67 long mq_flags; /* Flags (ignored for mq_open()) */
68 long mq_maxmsg; /* Max. # of messages on queue */
69 long mq_msgsize; /* Max. message size (bytes) */
70 long mq_curmsgs; /* # of messages currently in queue
71 (ignored for mq_open()) */
72 };
73 Only the mq_maxmsg and mq_msgsize fields are employed when calling
75 mq_open(); the values in the remaining fields are ignored.
76 If attr is NULL, then the queue is created with implementation-defined
78 default attributes. Since Linux 3.5, two /proc files can be used to
79 control these defaults; see mq_overview(7) for details.
80
82 On success, mq_open() returns a message queue descriptor for use by
83 other message queue functions. On error, mq_open() returns (mqd_t) -1,
84 with errno set to indicate the error.
85
87 EACCES The queue exists, but the caller does not have permission to
88 open it in the specified mode.
89 EACCES name contained more than one slash.
91 EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a queue
93 with this name already exists.
94 EINVAL name doesn't follow the format in mq_overview(7).
96 EINVAL O_CREAT was specified in oflag, and attr was not NULL, but
98 attr->mq_maxmsg or attr->mq_msqsize was invalid. Both of these
99 fields must be greater than zero. In a process that is unprivi‐
100 leged (does not have the CAP_SYS_RESOURCE capability),
101 attr->mq_maxmsg must be less than or equal to the msg_max limit,
102 and attr->mq_msgsize must be less than or equal to the msg‐
103 size_max limit. In addition, even in a privileged process,
104 attr->mq_maxmsg cannot exceed the HARD_MAX limit. (See mq_over‐
105 view(7) for details of these limits.)
106 EMFILE The per-process limit on the number of open file and message
108 queue descriptors has been reached (see the description of
109 RLIMIT_NOFILE in getrlimit(2)).
110 ENAMETOOLONG
112 name was too long.
113 ENFILE The system-wide limit on the total number of open files and mes‐
115 sage queues has been reached.
116 ENOENT The O_CREAT flag was not specified in oflag, and no queue with
118 this name exists.
119 ENOENT name was just "/" followed by no other characters.
121 ENOMEM Insufficient memory.
123 ENOSPC Insufficient space for the creation of a new message queue.
125 This probably occurred because the queues_max limit was encoun‐
126 tered; see mq_overview(7).
127
129 For an explanation of the terms used in this section, see at‐
130 tributes(7).
131 ┌────────────────────────────────────────────┬───────────────┬─────────┐
133 │Interface │ Attribute │ Value │
134 ├────────────────────────────────────────────┼───────────────┼─────────┤
135 │mq_open() │ Thread safety │ MT-Safe │
136 └────────────────────────────────────────────┴───────────────┴─────────┘
137
139 POSIX.1-2001, POSIX.1-2008.
140
142 C library/kernel differences
143 The mq_open() library function is implemented on top of a system call
144 of the same name. The library function performs the check that the
145 name starts with a slash (/), giving the EINVAL error if it does not.
146 The kernel system call expects name to contain no preceding slash, so
147 the C library function passes name without the preceding slash (i.e.,
148 name+1) to the system call.
149
151 In kernels before 2.6.14, the process umask was not applied to the per‐
152 missions specified in mode.
153
155 mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3),
156 mq_unlink(3), mq_overview(7)
157
159 This page is part of release 5.13 of the Linux man-pages project. A
160 description of the project, information about reporting bugs, and the
161 latest version of this page, can be found at
162 https://www.kernel.org/doc/man-pages/.
163Linux 2021-03-22 MQ_OPEN(3)