man3/mq_open.3

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  26 .TH MQ_OPEN 3 2017-09-15 "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 .PP
  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 .PP
  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 .PP
  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 .PP
 121 The fields of the
 122 .IR "struct mq_attr"
 123 pointed to
 124 .I attr
 125 specify the maximum number of messages and
 126 the maximum size of messages that the queue will allow.
 127 This structure is defined as follows:
 128 .PP
 129 .PP
 130 .in +4n
 131 .EX
 132 struct mq_attr {
 133     long mq_flags;       /* Flags (ignored for mq_open()) */
 134     long mq_maxmsg;      /* Max. # of messages on queue */
 135     long mq_msgsize;     /* Max. message size (bytes) */
 136     long mq_curmsgs;     /* # of messages currently in queue
 137                             (ignored for mq_open()) */
 138 };
 139 .EE
 140 .in
 141 .PP
 142 Only the
 143 .I mq_maxmsg
 144 and
 145 .I mq_msgsize
 146 fields are employed when calling
 147 .BR mq_open ();
 148 the values in the remaining fields are ignored.
 149 .PP
 150 If
 151 .I attr
 152 is NULL, then the queue is created with implementation-defined
 153 default attributes.
 154 Since Linux 3.5, two
 155 .I /proc
 156 files can be used to control these defaults; see
 157 .BR mq_overview (7)
 158 for details.
 159 .SH RETURN VALUE
 160 On success,
 161 .BR mq_open ()
 162 returns a message queue descriptor for use by other
 163 message queue functions.
 164 On error,
 165 .BR mq_open ()
 166 returns
 167 .IR "(mqd_t)\ \-1",
 168 with
 169 .I errno
 170 set to indicate the error.
 171 .SH ERRORS
 172 .TP
 173 .B EACCES
 174 The queue exists, but the caller does not have permission to
 175 open it in the specified mode.
 176 .TP
 177 .B EACCES
 178 .I name
 179 contained more than one slash.
 180 .\" Note that this isn't consistent with the same case for sem_open()
 181 .TP
 182 .B EEXIST
 183 Both
 184 .B O_CREAT
 185 and
 186 .B O_EXCL
 187 were specified in
 188 .IR oflag ,
 189 but a queue with this
 190 .I name
 191 already exists.
 192 .TP
 193 .B EINVAL
 194 .\" glibc checks whether the name starts with a "/" and if not,
 195 .\" gives this error
 196 .I name
 197 doesn't follow the format in
 198 .BR mq_overview (7).
 199 .TP
 200 .B EINVAL
 201 .B O_CREAT
 202 was specified in
 203 .IR oflag ,
 204 and
 205 .I attr
 206 was not NULL, but
 207 .I attr\->mq_maxmsg
 208 or
 209 .I attr\->mq_msqsize
 210 was invalid.
 211 Both of these fields must be greater than zero.
 212 In a process that is unprivileged (does not have the
 213 .B CAP_SYS_RESOURCE
 214 capability),
 215 .I attr\->mq_maxmsg
 216 must be less than or equal to the
 217 .I msg_max
 218 limit, and
 219 .I attr\->mq_msgsize
 220 must be less than or equal to the
 221 .I msgsize_max
 222 limit.
 223 In addition, even in a privileged process,
 224 .I attr\->mq_maxmsg
 225 cannot exceed the
 226 .B HARD_MAX
 227 limit.
 228 (See
 229 .BR mq_overview (7)
 230 for details of these limits.)
 231 .TP
 232 .B EMFILE
 233 The per-process limit on the number of open file
 234 and message queue descriptors has been reached
 235 (see the description of
 236 .BR RLIMIT_NOFILE
 237 in
 238 .BR getrlimit (2)).
 239 .TP
 240 .B ENAMETOOLONG
 241 .I name
 242 was too long.
 243 .TP
 244 .B ENFILE
 245 The system-wide limit on the total number of open files
 246 and message queues has been reached.
 247 .TP
 248 .B ENOENT
 249 The
 250 .B O_CREAT
 251 flag was not specified in
 252 .IR oflag ,
 253 and no queue with this
 254 .I name
 255 exists.
 256 .TP
 257 .B ENOENT
 258 .I name
 259 was just "/" followed by no other characters.
 260 .\" Note that this isn't consistent with the same case for sem_open()
 261 .TP
 262 .B ENOMEM
 263 Insufficient memory.
 264 .TP
 265 .B ENOSPC
 266 Insufficient space for the creation of a new message queue.
 267 This probably occurred because the
 268 .I queues_max
 269 limit was encountered; see
 270 .BR mq_overview (7).
 271 .SH ATTRIBUTES
 272 For an explanation of the terms used in this section, see
 273 .BR attributes (7).
 274 .TS
 275 allbox;
 276 lb lb lb
 277 l l l.
 278 Interface       Attribute       Value
 279 T{
 280 .BR mq_open ()
 281 T}      Thread safety   MT-Safe
 282 .TE
 283 .SH CONFORMING TO
 284 POSIX.1-2001, POSIX.1-2008.
 285 .SH NOTES
 286 .SS C library/kernel differences
 287 The
 288 .BR mq_open ()
 289 library function is implemented on top of a system call of the same name.
 290 The library function performs the check that the
 291 .I name
 292 starts with a slash (/), giving the
 293 .B EINVAL
 294 error if it does not.
 295 The kernel system call expects
 296 .I name
 297 to contain no preceding slash,
 298 so the C library function passes
 299 .I name
 300 without the preceding slash (i.e.,
 301 .IR name+1 )
 302 to the system call.
 303 .SH BUGS
 304 In kernels before 2.6.14,
 305 the process umask was not applied to the permissions specified in
 306 .IR mode .
 307 .SH SEE ALSO
 308 .BR mq_close (3),
 309 .BR mq_getattr (3),
 310 .BR mq_notify (3),
 311 .BR mq_receive (3),
 312 .BR mq_send (3),
 313 .BR mq_unlink (3),
 314 .BR mq_overview (7)