man7/mq_overview.7

   1 '\" t
   2 .\" Hey Emacs! This file is -*- nroff -*- source.
   3 .\"
   4 .\" Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
   5 .\"
   6 .\" Permission is granted to make and distribute verbatim copies of this
   7 .\" manual provided the copyright notice and this permission notice are
   8 .\" preserved on all copies.
   9 .\"
  10 .\" Permission is granted to copy and distribute modified versions of this
  11 .\" manual under the conditions for verbatim copying, provided that the
  12 .\" entire resulting derived work is distributed under the terms of a
  13 .\" permission notice identical to this one.
  14 .\"
  15 .\" Since the Linux kernel and libraries are constantly changing, this
  16 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  17 .\" responsibility for errors or omissions, or for damages resulting from
  18 .\" the use of the information contained herein.
  19 .\"
  20 .\" Formatted or processed versions of this manual, if unaccompanied by
  21 .\" the source, must acknowledge the copyright and authors of this work.
  22 .\"
  23 .TH MQ_OVERVIEW 7 2006-02-25 "Linux 2.6.16" "Linux Programmer's Manual"
  24 .SH NAME
  25 mq_overview \- Overview of POSIX message queues
  26 .SH DESCRIPTION
  27 POSIX message queues allow processes to exchange data in
  28 the form of messages.
  29 This API is distinct from that provided by System V message queues
  30 .RB ( msgget (2),
  31 .BR msgsnd (2),
  32 .BR msgrcv (2),
  33 etc.), but provides similar functionality.
  34
  35 Message queues are created and opened using
  36 .BR mq_open (3);
  37 this function returns a
  38 .I message queue descriptor
  39 .RI ( mqd_t ),
  40 which is used to refer to the open message queue in later calls.
  41 Each message queue is identified by a name of the form
  42 .IR /somename .
  43 Two processes can operate on the same queue by passing the same name to
  44 .BR mq_open ().
  45
  46 Messages are transferred to and from a queue using
  47 .BR mq_send (3)
  48 and
  49 .BR mq_receive (3).
  50 When a process has finished using the queue, it closes it using
  51 .BR mq_close (3),
  52 and when the queue is no longer required, it can be deleted using
  53 .BR mq_unlink (3).
  54 Queue attributes can be retrieved and (in some cases) modified using
  55 .BR mq_getattr (3)
  56 and
  57 .BR mq_setattr (3).
  58 A process can request asynchronous notification
  59 of the arrival of a message on a previously empty queue using
  60 .BR mq_notify (3).
  61
  62 A message queue descriptor is a reference to an
  63 .IR "open message queue description"
  64 (cf.
  65 .BR open (2)).
  66 After a
  67 .BR fork (2),
  68 a child inherits copies of its parent's message queue descriptors,
  69 and these descriptors refer to the same open message queue descriptions
  70 as the corresponding descriptors in the parent.
  71 Corresponding descriptors in the two processes share the flags
  72 .RI ( mq_flags )
  73 that are associated with the open message queue description.
  74
  75 Each message has an associated
  76 .IR priority ,
  77 and messages are always delivered to the receiving process
  78 highest priority first.
  79 Message priorities range from 0 (low) to
  80 .I sysconf(_SC_MQ_PRIO_MAX)\ -\ 1
  81 (high).
  82 On Linux,
  83 .I sysconf(_SC_MQ_PRIO_MAX)
  84 returns 32768, but POSIX.1-2001 only requires
  85 an implementation to support priorities in the range 0 to 31;
  86 some implementations only provide this range.
  87 .SS Library interfaces and system calls
  88 In most cases the
  89 .B mq_*()
  90 library interfaces listed above are implemented
  91 on top of underlying system calls of the same name.
  92 Deviations from this scheme are indicated in the following table:
  93 .in +0.25i
  94 .TS
  95 lB lB
  96 l l.
  97 Library interface       System call
  98 mq_close(3)     close(2)
  99 mq_getattr(3)   mq_getsetattr(2)
 100 mq_open(3)      mq_open(2)
 101 mq_receive(3)   mq_timedreceive(2)
 102 mq_send(3)      mq_timedsend(2)
 103 mq_setattr(3)   mq_getsetattr(2)
 104 mq_timedreceive(3)      mq_timedreceive(2)
 105 mq_timedsend(3) mq_timedsend(2)
 106 mq_unlink(3)    mq_unlink(2)
 107 .TE
 108 .in -0.25i
 109 .SH LINUX SPECIFIC DETAILS
 110 .SS Versions
 111 POSIX message queues have been supported on Linux since kernel 2.6.6.
 112 Glibc support has been provided since version 2.3.4.
 113 .SS Kernel configuration
 114 Support for POSIX message queues is configurable via the
 115 .B CONFIG_POSIX_MQUEUE
 116 kernel configuration option.
 117 This option is enabled by default.
 118 .SS Persistence
 119 POSIX message queues have kernel persistence:
 120 if not removed by
 121 .BR mq_unlink (),
 122 a message queue will exist until the system is shut down.
 123 .SS Linking
 124 Programs using the POSIX message queue API must be compiled with
 125 .I cc \-lrt
 126 to link against the real-time library,
 127 .IR librt .
 128 .SS /proc interfaces
 129 The following interfaces can be used to limit the amount of
 130 kernel memory consumed by POSIX message queues:
 131 .TP
 132 .I /proc/sys/fs/mqueue/msg_max
 133 This file can be used to view and change the ceiling value for the
 134 maximum number of messages in a queue.
 135 This value acts as a ceiling on the
 136 .I attr->mq_maxmsg
 137 argument given to
 138 .BR mq_open (3).
 139 The default and minimum value for
 140 .I msg_max
 141 is 10; the upper limit is HARD_MAX:
 142 .IR "(131072\ /\ sizeof(void\ *))"
 143 (32768 on Linux/86).
 144 This limit is ignored for privileged processes
 145 .RB ( CAP_SYS_RESOURCE ),
 146 but the HARD_MAX ceiling is nevertheless imposed.
 147 .TP
 148 .I /proc/sys/fs/mqueue/msgsize_max
 149 This file can be used to view and change the ceiling on the
 150 maximum message size.
 151 This value acts as a ceiling on the
 152 .I attr->mq_msgsize
 153 argument given to
 154 .BR mq_open (3).
 155 The default and minimum value for
 156 .I msgsize_max
 157 is 8192 bytes; the upper limit is INT_MAX
 158 (2147483647 on Linux/86).
 159 This limit is ignored for privileged processes
 160 .RB ( CAP_SYS_RESOURCE ).
 161 .TP
 162 .I /proc/sys/fs/mqueue/queues_max
 163 This file can be used to view and change the system-wide limit on the
 164 number of message queues that can be created.
 165 Only privileged processes
 166 .RB ( CAP_SYS_RESOURCE )
 167 can create new message queues once this limit has been reached.
 168 The default value for
 169 .I queues_max
 170 is 256; it can be changed to any value in the range 0 to INT_MAX.
 171 .SS Resource limit
 172 The
 173 .BR RLIMIT_MSGQUEUE
 174 resource limit, which places a limit on the amount of space
 175 that can be consumed by all of the message queues
 176 belonging to a process's real user ID, is described in
 177 .BR getrlimit (2).
 178 .SS Mounting the message queue file system
 179 On Linux, message queues are created in a virtual file system.
 180 (Other implementations may also provide such a feature,
 181 but the details are likely to differ.)
 182 This file system can be mounted using the following commands:
 183 .in +0.25i
 184 .nf
 185
 186 $ mkdir /dev/mqueue
 187 $ mount -t mqueue none /dev/mqueue
 188
 189 .fi
 190 .in -0.25i
 191 The sticky bit is automatically enabled on the mount directory.
 192
 193 After the file system has been mounted, the message queues on the system
 194 can be viewed and manipulated using the commands usually used for files
 195 (e.g.,
 196 .BR ls (1)
 197 and
 198 .BR rm (1)).
 199
 200 The contents of each file in the directory consist of a single line
 201 containing information about the queue:
 202 .in +0.25i
 203 .nf
 204
 205 $ ls /dev/mqueue/mymq
 206 QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260
 207 $ mount -t mqueue none /dev/mqueue
 208
 209 .fi
 210 .in -0.25i
 211 These fields are as follows:
 212 .TP
 213 .B
 214 QSIZE
 215 Number of bytes of data in all messages in the queue.
 216 .TP
 217 .B NOTIFY_PID
 218 If this is non-zero, then the process with this PID has used
 219 .BR mq_notify (3)
 220 to register for asynchronous message notification,
 221 and the remaining fields describe how notification occurs.
 222 .TP
 223 .B NOTIFY
 224 Notification method:
 225 0 is
 226 .BR SIGEV_SIGNAL ;
 227 1 is
 228 .BR SIGEV_NONE;
 229 and
 230 2 is
 231 .BR SIGEV_THREAD .
 232 .TP
 233 .B SIGNO
 234 Signal number to be used for
 235 .BR SIGEV_SIGNAL .
 236 .SS Polling message queue descriptors
 237 On Linux, a message queue descriptor is actually a file descriptor,
 238 and can be monitored using
 239 .BR select (2),
 240 .BR poll (2),
 241 or
 242 .BR epoll (7).
 243 This is not portable.
 244 .SH "CONFORMING TO"
 245 POSIX.1-2001.
 246 .SH NOTES
 247 System V message queues
 248 .RB ( msgget (2),
 249 .BR msgsnd (2),
 250 .BR msgrcv (2),
 251 etc.) are an older API for exchanging messages between processes.
 252 POSIX message queues provide a better designed interface than
 253 System V message queues;
 254 on the other hand POSIX message queues are less widely available
 255 (especially on older systems) than System V message queues.
 256 .SH EXAMPLE
 257 An example of the use of various message queue functions is shown in
 258 .BR mq_notify (3).
 259 .SH "SEE ALSO"
 260 .BR getrlimit (2),
 261 .BR mq_getsetattr (2),
 262 .BR mq_close (3),
 263 .BR mq_getattr (3),
 264 .BR mq_notify (3),
 265 .BR mq_open (3),
 266 .BR mq_receive (3),
 267 .BR mq_send (3),
 268 .BR mq_unlink (3),
 269 .BR poll (2),
 270 .BR select (2),
 271 .BR epoll (4)