1mq_open(3) Library Functions Manual mq_open(3)
2
6 mq_open - open a message queue
7
9 Real-time library (librt, -lrt)
10
12 #include <fcntl.h> /* For O_* constants */
13 #include <sys/stat.h> /* For mode constants */
14 #include <mqueue.h>
15 mqd_t mq_open(const char *name, int oflag);
17 mqd_t mq_open(const char *name, int oflag, mode_t mode,
18 struct mq_attr *attr);
19
21 mq_open() creates a new POSIX message queue or opens an existing queue.
22 The queue is identified by name. For details of the construction of
23 name, see mq_overview(7).
24 The oflag argument specifies flags that control the operation of the
26 call. (Definitions of the flags values can be obtained by including
27 <fcntl.h>.) Exactly one of the following must be specified in oflag:
28 O_RDONLY
30 Open the queue to receive messages only.
31 O_WRONLY
33 Open the queue to send messages only.
34 O_RDWR Open the queue to both send and receive messages.
36 Zero or more of the following flags can additionally be ORed in oflag:
38 O_CLOEXEC (since Linux 2.6.26)
40 Set the close-on-exec flag for the message queue descriptor.
41 See open(2) for a discussion of why this flag is useful.
42 O_CREAT
44 Create the message queue if it does not exist. The owner (user
45 ID) of the message queue is set to the effective user ID of the
46 calling process. The group ownership (group ID) is set to the
47 effective group ID of the calling process.
48 O_EXCL If O_CREAT was specified in oflag, and a queue with the given
50 name already exists, then fail with the error EEXIST.
51 O_NONBLOCK
53 Open the queue in nonblocking mode. In circumstances where
54 mq_receive(3) and mq_send(3) would normally block, these func‐
55 tions instead fail with the error EAGAIN.
56 If O_CREAT is specified in oflag, then two additional arguments must be
58 supplied. The mode argument specifies the permissions to be placed on
59 the new queue, as for open(2). (Symbolic definitions for the permis‐
60 sions bits can be obtained by including <sys/stat.h>.) The permissions
61 settings are masked against the process umask.
62 The fields of the struct mq_attr pointed to attr specify the maximum
64 number of messages and the maximum size of messages that the queue will
65 allow. This structure is defined as follows:
66 struct mq_attr {
68 long mq_flags; /* Flags (ignored for mq_open()) */
69 long mq_maxmsg; /* Max. # of messages on queue */
70 long mq_msgsize; /* Max. message size (bytes) */
71 long mq_curmsgs; /* # of messages currently in queue
72 (ignored for mq_open()) */
73 };
74 Only the mq_maxmsg and mq_msgsize fields are employed when calling
76 mq_open(); the values in the remaining fields are ignored.
77 If attr is NULL, then the queue is created with implementation-defined
79 default attributes. Since Linux 3.5, two /proc files can be used to
80 control these defaults; see mq_overview(7) for details.
81
83 On success, mq_open() returns a message queue descriptor for use by
84 other message queue functions. On error, mq_open() returns (mqd_t) -1,
85 with errno set to indicate the error.
86
88 EACCES The queue exists, but the caller does not have permission to
89 open it in the specified mode.
90 EACCES name contained more than one slash.
92 EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a queue
94 with this name already exists.
95 EINVAL name doesn't follow the format in mq_overview(7).
97 EINVAL O_CREAT was specified in oflag, and attr was not NULL, but
99 attr->mq_maxmsg or attr->mq_msqsize was invalid. Both of these
100 fields must be greater than zero. In a process that is unprivi‐
101 leged (does not have the CAP_SYS_RESOURCE capability),
102 attr->mq_maxmsg must be less than or equal to the msg_max limit,
103 and attr->mq_msgsize must be less than or equal to the msg‐
104 size_max limit. In addition, even in a privileged process,
105 attr->mq_maxmsg cannot exceed the HARD_MAX limit. (See mq_over‐
106 view(7) for details of these limits.)
107 EMFILE The per-process limit on the number of open file and message
109 queue descriptors has been reached (see the description of
110 RLIMIT_NOFILE in getrlimit(2)).
111 ENAMETOOLONG
113 name was too long.
114 ENFILE The system-wide limit on the total number of open files and mes‐
116 sage queues has been reached.
117 ENOENT The O_CREAT flag was not specified in oflag, and no queue with
119 this name exists.
120 ENOENT name was just "/" followed by no other characters.
122 ENOMEM Insufficient memory.
124 ENOSPC Insufficient space for the creation of a new message queue.
126 This probably occurred because the queues_max limit was encoun‐
127 tered; see mq_overview(7).
128
130 For an explanation of the terms used in this section, see at‐
131 tributes(7).
132 ┌────────────────────────────────────────────┬───────────────┬─────────┐
134 │Interface │ Attribute │ Value │
135 ├────────────────────────────────────────────┼───────────────┼─────────┤
136 │mq_open() │ Thread safety │ MT-Safe │
137 └────────────────────────────────────────────┴───────────────┴─────────┘
138
140 C library/kernel differences
141 The mq_open() library function is implemented on top of a system call
142 of the same name. The library function performs the check that the
143 name starts with a slash (/), giving the EINVAL error if it does not.
144 The kernel system call expects name to contain no preceding slash, so
145 the C library function passes name without the preceding slash (i.e.,
146 name+1) to the system call.
147
149 POSIX.1-2008.
150
152 POSIX.1-2001.
153
155 Before Linux 2.6.14, the process umask was not applied to the permis‐
156 sions specified in mode.
157
159 mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3),
160 mq_unlink(3), mq_overview(7)
161Linux man-pages 6.04 2023-03-30 mq_open(3)