man3/mq_open.3

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  26 .TH MQ_OPEN 3 2015-12-28 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 mq_open \- open a message queue
  29 .SH SYNOPSIS
  30 .nf
  31 .BR "#include <fcntl.h>" "           /* For O_* constants */"
  32 .BR "#include <sys/stat.h>" "        /* For mode constants */"
  33 .B #include <mqueue.h>
  34 .sp
  35 .BI "mqd_t mq_open(const char *" name ", int " oflag );
  36 .BI "mqd_t mq_open(const char *" name ", int " oflag ", mode_t " mode ,
  37 .BI "              struct mq_attr *" attr );
  38 .fi
  39 .sp
  40 Link with \fI\-lrt\fP.
  41 .SH DESCRIPTION
  42 .BR mq_open ()
  43 creates a new POSIX message queue or opens an existing queue.
  44 The queue is identified by
  45 .IR name .
  46 For details of the construction of
  47 .IR name ,
  48 see
  49 .BR mq_overview (7).
  50
  51 The
  52 .I oflag
  53 argument specifies flags that control the operation of the call.
  54 (Definitions of the flags values can be obtained by including
  55 .IR <fcntl.h> .)
  56 Exactly one of the following must be specified in
  57 .IR oflag :
  58 .TP
  59 .B O_RDONLY
  60 Open the queue to receive messages only.
  61 .TP
  62 .B O_WRONLY
  63 Open the queue to send messages only.
  64 .TP
  65 .B O_RDWR
  66 Open the queue to both send and receive messages.
  67 .PP
  68 Zero or more of the following flags can additionally be
  69 .IR OR ed
  70 in
  71 .IR oflag :
  72 .TP
  73 .BR O_CLOEXEC " (since Linux 2.6.26)"
  74 .\" commit 269f21344b23e552c21c9e2d7ca258479dcd7a0a
  75 Set the close-on-exec flag for the message queue descriptor.
  76 See
  77 .BR open (2)
  78 for a discussion of why this flag is useful.
  79 .TP
  80 .B O_CREAT
  81 Create the message queue if it does not exist.
  82 The owner (user ID) of the message queue is set to the effective
  83 user ID of the calling process.
  84 The group ownership (group ID) is set to the effective group ID
  85 of the calling process.
  86 .\" In reality the filesystem IDs are used on Linux.
  87 .TP
  88 .B O_EXCL
  89 If
  90 .B O_CREAT
  91 was specified in
  92 .IR oflag ,
  93 and a queue with the given
  94 .I name
  95 already exists, then fail with the error
  96 .BR EEXIST .
  97 .TP
  98 .B O_NONBLOCK
  99 Open the queue in nonblocking mode.
 100 In circumstances where
 101 .BR mq_receive (3)
 102 and
 103 .BR mq_send (3)
 104 would normally block, these functions instead fail with the error
 105 .BR EAGAIN .
 106 .PP
 107 If
 108 .B O_CREAT
 109 is specified in
 110 .IR oflag ,
 111 then two additional arguments must be supplied.
 112 The
 113 .I mode
 114 argument specifies the permissions to be placed on the new queue,
 115 as for
 116 .BR open (2).
 117 (Symbolic definitions for the permissions bits can be obtained by including
 118 .IR <sys/stat.h> .)
 119 The permissions settings are masked against the process umask.
 120
 121 The
 122 .I attr
 123 argument specifies attributes for the queue.
 124 See
 125 .BR mq_getattr (3)
 126 for details.
 127 If
 128 .I attr
 129 is NULL, then the queue is created with implementation-defined
 130 default attributes.
 131 Since Linux 3.5, two
 132 .I /proc
 133 files can be used to control these defaults; see
 134 .BR mq_overview (7)
 135 for details.
 136 .SH RETURN VALUE
 137 On success,
 138 .BR mq_open ()
 139 returns a message queue descriptor for use by other
 140 message queue functions.
 141 On error,
 142 .BR mq_open ()
 143 returns
 144 .IR "(mqd_t)\ \-1",
 145 with
 146 .I errno
 147 set to indicate the error.
 148 .SH ERRORS
 149 .TP
 150 .B EACCES
 151 The queue exists, but the caller does not have permission to
 152 open it in the specified mode.
 153 .TP
 154 .B EACCES
 155 .I name
 156 contained more than one slash.
 157 .\" Note that this isn't consistent with the same case for sem_open()
 158 .TP
 159 .B EEXIST
 160 Both
 161 .B O_CREAT
 162 and
 163 .B O_EXCL
 164 were specified in
 165 .IR oflag ,
 166 but a queue with this
 167 .I name
 168 already exists.
 169 .TP
 170 .B EINVAL
 171 .\" glibc checks whether the name starts with a "/" and if not,
 172 .\" gives this error
 173 .I name
 174 doesn't follow the format in
 175 .BR mq_overview (7).
 176 .TP
 177 .B EINVAL
 178 .B O_CREAT
 179 was specified in
 180 .IR oflag ,
 181 and
 182 .I attr
 183 was not NULL, but
 184 .I attr\->mq_maxmsg
 185 or
 186 .I attr\->mq_msqsize
 187 was invalid.
 188 Both of these fields must be greater than zero.
 189 In a process that is unprivileged (does not have the
 190 .B CAP_SYS_RESOURCE
 191 capability),
 192 .I attr\->mq_maxmsg
 193 must be less than or equal to the
 194 .I msg_max
 195 limit, and
 196 .I attr\->mq_msgsize
 197 must be less than or equal to the
 198 .I msgsize_max
 199 limit.
 200 In addition, even in a privileged process,
 201 .I attr\->mq_maxmsg
 202 cannot exceed the
 203 .B HARD_MAX
 204 limit.
 205 (See
 206 .BR mq_overview (7)
 207 for details of these limits.)
 208 .TP
 209 .B EMFILE
 210 The per-process limit on the number of open file
 211 and message queue descriptors has been reached
 212 (see the description of
 213 .BR RLIMIT_NOFILE
 214 in
 215 .BR getrlimit (2)).
 216 .TP
 217 .B ENAMETOOLONG
 218 .I name
 219 was too long.
 220 .TP
 221 .B ENFILE
 222 The system-wide limit on the total number of open files
 223 and message queues has been reached.
 224 .TP
 225 .B ENOENT
 226 The
 227 .B O_CREAT
 228 flag was not specified in
 229 .IR oflag ,
 230 and no queue with this
 231 .I name
 232 exists.
 233 .TP
 234 .B ENOENT
 235 .I name
 236 was just "/" followed by no other characters.
 237 .\" Note that this isn't consistent with the same case for sem_open()
 238 .TP
 239 .B ENOMEM
 240 Insufficient memory.
 241 .TP
 242 .B ENOSPC
 243 Insufficient space for the creation of a new message queue.
 244 This probably occurred because the
 245 .I queues_max
 246 limit was encountered; see
 247 .BR mq_overview (7).
 248 .SH ATTRIBUTES
 249 For an explanation of the terms used in this section, see
 250 .BR attributes (7).
 251 .TS
 252 allbox;
 253 lb lb lb
 254 l l l.
 255 Interface       Attribute       Value
 256 T{
 257 .BR mq_open ()
 258 T}      Thread safety   MT-Safe
 259 .TE
 260 .SH CONFORMING TO
 261 POSIX.1-2001, POSIX.1-2008.
 262 .SH NOTES
 263 .SS C library/kernel differences
 264 The
 265 .BR mq_open ()
 266 library function is implemented on top of a system call of the same name.
 267 The library function performs the check that the
 268 .I name
 269 starts with a slash (/), giving the
 270 .B EINVAL
 271 error if it does not.
 272 The kernel system call expects
 273 .I name
 274 to contain no preceding slash,
 275 so the C library function passes
 276 .I name
 277 without the preceding slash (i.e.,
 278 .IR name+1 )
 279 to the system call.
 280 .SH BUGS
 281 In kernels before 2.6.14,
 282 the process umask was not applied to the permissions specified in
 283 .IR mode .
 284 .SH SEE ALSO
 285 .BR mq_close (3),
 286 .BR mq_getattr (3),
 287 .BR mq_notify (3),
 288 .BR mq_receive (3),
 289 .BR mq_send (3),
 290 .BR mq_unlink (3),
 291 .BR mq_overview (7)