man2/msgop.2

   1 .\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
   2 .\" and Copyright 2015 Bill Pemberton <wfp5p@worldbroken.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esr@thyrsus.com>
   7 .\" Modified Mon Jul 10 21:09:59 2000 by aeb
   8 .\" Modified 1 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
   9 .\"     Language clean-ups.
  10 .\"     Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
  11 .\"     Added note on restart behavior of msgsnd() and msgrcv()
  12 .\"     Formatting clean-ups (argument and field names marked as .I
  13 .\"             instead of .B)
  14 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  15 .\"     Added notes on capability requirements
  16 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  17 .\"     Language and formatting clean-ups
  18 .\"     Added notes on /proc files
  19 .\"
  20 .TH MSGOP 2 2021-03-22 "Linux" "Linux Programmer's Manual"
  21 .SH NAME
  22 msgrcv, msgsnd \- System V message queue operations
  23 .SH LIBRARY
  24 Standard C library
  25 .RI ( libc ", " \-lc )
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <sys/msg.h>
  29 .PP
  30 .BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \
  31 ", int " msgflg );
  32 .PP
  33 .BI "ssize_t msgrcv(int " msqid ", void *" msgp ", size_t " msgsz \
  34 ", long " msgtyp ,
  35 .BI "               int " msgflg );
  36 .fi
  37 .SH DESCRIPTION
  38 The
  39 .BR msgsnd ()
  40 and
  41 .BR msgrcv ()
  42 system calls are used to send messages to,
  43 and receive messages from, a System\ V message queue.
  44 The calling process must have write permission on the message queue
  45 in order to send a message, and read permission to receive a message.
  46 .PP
  47 The
  48 .I msgp
  49 argument is a pointer to a caller-defined structure
  50 of the following general form:
  51 .PP
  52 .in +4n
  53 .EX
  54 struct msgbuf {
  55     long mtype;       /* message type, must be > 0 */
  56     char mtext[1];    /* message data */
  57 };
  58 .EE
  59 .in
  60 .PP
  61 The
  62 .I mtext
  63 field is an array (or other structure) whose size is specified by
  64 .IR msgsz ,
  65 a nonnegative integer value.
  66 Messages of zero length (i.e., no
  67 .I mtext
  68 field) are permitted.
  69 The
  70 .I mtype
  71 field must have a strictly positive integer value.
  72 This value can be
  73 used by the receiving process for message selection
  74 (see the description of
  75 .BR msgrcv ()
  76 below).
  77 .SS msgsnd()
  78 The
  79 .BR msgsnd ()
  80 system call appends a copy of the message pointed to by
  81 .I msgp
  82 to the message queue whose identifier is specified
  83 by
  84 .IR msqid .
  85 .PP
  86 If sufficient space is available in the queue,
  87 .BR msgsnd ()
  88 succeeds immediately.
  89 The queue capacity is governed by the
  90 .I msg_qbytes
  91 field in the associated data structure for the message queue.
  92 During queue creation this field is initialized to
  93 .B MSGMNB
  94 bytes, but this limit can be modified using
  95 .BR msgctl (2).
  96 A message queue is considered to be full if either of the following
  97 conditions is true:
  98 .IP \(bu 2
  99 Adding a new message to the queue would cause the total number of bytes
 100 in the queue to exceed the queue's maximum size (the
 101 .I msg_qbytes
 102 field).
 103 .IP \(bu
 104 Adding another message to the queue would cause the total number of messages
 105 in the queue to exceed the queue's maximum size (the
 106 .I msg_qbytes
 107 field).
 108 This check is necessary to prevent an unlimited number of zero-length
 109 messages being placed on the queue.
 110 Although such messages contain no data,
 111 they nevertheless consume (locked) kernel memory.
 112 .PP
 113 If insufficient space is available in the queue, then the default
 114 behavior of
 115 .BR msgsnd ()
 116 is to block until space becomes available.
 117 If
 118 .B IPC_NOWAIT
 119 is specified in
 120 .IR msgflg ,
 121 then the call instead fails with the error
 122 .BR EAGAIN .
 123 .PP
 124 A blocked
 125 .BR msgsnd ()
 126 call may also fail if:
 127 .IP \(bu 2
 128 the queue is removed,
 129 in which case the system call fails with
 130 .I errno
 131 set to
 132 .BR EIDRM ;
 133 or
 134 .IP \(bu
 135 a signal is caught, in which case the system call fails
 136 with
 137 .I errno
 138 set to
 139 .BR EINTR ; see
 140 .BR signal (7).
 141 .RB ( msgsnd ()
 142 is never automatically restarted after being interrupted by a
 143 signal handler, regardless of the setting of the
 144 .B SA_RESTART
 145 flag when establishing a signal handler.)
 146 .PP
 147 Upon successful completion the message queue data structure is updated
 148 as follows:
 149 .IP \(bu 2
 150 .I msg_lspid
 151 is set to the process ID of the calling process.
 152 .IP \(bu
 153 .I msg_qnum
 154 is incremented by 1.
 155 .IP \(bu
 156 .I msg_stime
 157 is set to the current time.
 158 .SS msgrcv()
 159 The
 160 .BR msgrcv ()
 161 system call removes a message from the queue specified by
 162 .I msqid
 163 and places it in the buffer
 164 pointed to by
 165 .IR msgp .
 166 .PP
 167 The argument
 168 .I msgsz
 169 specifies the maximum size in bytes for the member
 170 .I mtext
 171 of the structure pointed to by the
 172 .I msgp
 173 argument.
 174 If the message text has length greater than
 175 .IR msgsz ,
 176 then the behavior depends on whether
 177 .B MSG_NOERROR
 178 is specified in
 179 .IR msgflg .
 180 If
 181 .B MSG_NOERROR
 182 is specified, then
 183 the message text will be truncated (and the truncated part will be
 184 lost); if
 185 .B MSG_NOERROR
 186 is not specified, then
 187 the message isn't removed from the queue and
 188 the system call fails returning \-1 with
 189 .I errno
 190 set to
 191 .BR E2BIG .
 192 .PP
 193 Unless
 194 .B MSG_COPY
 195 is specified in
 196 .I msgflg
 197 (see below),
 198 the
 199 .I msgtyp
 200 argument specifies the type of message requested, as follows:
 201 .IP \(bu 2
 202 If
 203 .I msgtyp
 204 is 0,
 205 then the first message in the queue is read.
 206 .IP \(bu
 207 If
 208 .I msgtyp
 209 is greater than 0,
 210 then the first message in the queue of type
 211 .I msgtyp
 212 is read, unless
 213 .B MSG_EXCEPT
 214 was specified in
 215 .IR msgflg ,
 216 in which case
 217 the first message in the queue of type not equal to
 218 .I msgtyp
 219 will be read.
 220 .IP \(bu
 221 If
 222 .I msgtyp
 223 is less than 0,
 224 then the first message in the queue with the lowest type less than or
 225 equal to the absolute value of
 226 .I msgtyp
 227 will be read.
 228 .PP
 229 The
 230 .I msgflg
 231 argument is a bit mask constructed by ORing together zero or more
 232 of the following flags:
 233 .TP
 234 .B IPC_NOWAIT
 235 Return immediately if no message of the requested type is in the queue.
 236 The system call fails with
 237 .I errno
 238 set to
 239 .BR ENOMSG .
 240 .TP
 241 .BR MSG_COPY " (since Linux 3.8)"
 242 .\" commit 4a674f34ba04a002244edaf891b5da7fc1473ae8
 243 Nondestructively fetch a copy of the message at the ordinal position
 244 in the queue specified by
 245 .I msgtyp
 246 (messages are considered to be numbered starting at 0).
 247 .IP
 248 This flag must be specified in conjunction with
 249 .BR IPC_NOWAIT ,
 250 with the result that, if there is no message available at the given position,
 251 the call fails immediately with the error
 252 .BR ENOMSG .
 253 Because they alter the meaning of
 254 .I msgtyp
 255 in orthogonal ways,
 256 .B MSG_COPY
 257 and
 258 .B MSG_EXCEPT
 259 may not both be specified in
 260 .IR msgflg .
 261 .IP
 262 The
 263 .B MSG_COPY
 264 flag was added for the implementation of
 265 the kernel checkpoint-restore facility and
 266 is available only if the kernel was built with the
 267 .B CONFIG_CHECKPOINT_RESTORE
 268 option.
 269 .TP
 270 .B MSG_EXCEPT
 271 Used with
 272 .I msgtyp
 273 greater than 0
 274 to read the first message in the queue with message type that differs
 275 from
 276 .IR msgtyp .
 277 .TP
 278 .B MSG_NOERROR
 279 To truncate the message text if longer than
 280 .I msgsz
 281 bytes.
 282 .PP
 283 If no message of the requested type is available and
 284 .B IPC_NOWAIT
 285 isn't specified in
 286 .IR msgflg ,
 287 the calling process is blocked until one of the following conditions occurs:
 288 .IP \(bu 2
 289 A message of the desired type is placed in the queue.
 290 .IP \(bu
 291 The message queue is removed from the system.
 292 In this case, the system call fails with
 293 .I errno
 294 set to
 295 .BR EIDRM .
 296 .IP \(bu
 297 The calling process catches a signal.
 298 In this case, the system call fails with
 299 .I errno
 300 set to
 301 .BR EINTR .
 302 .RB ( msgrcv ()
 303 is never automatically restarted after being interrupted by a
 304 signal handler, regardless of the setting of the
 305 .B SA_RESTART
 306 flag when establishing a signal handler.)
 307 .PP
 308 Upon successful completion the message queue data structure is updated
 309 as follows:
 310 .IP
 311 .I msg_lrpid
 312 is set to the process ID of the calling process.
 313 .IP
 314 .I msg_qnum
 315 is decremented by 1.
 316 .IP
 317 .I msg_rtime
 318 is set to the current time.
 319 .SH RETURN VALUE
 320 On success,
 321 .BR msgsnd ()
 322 returns 0
 323 and
 324 .BR msgrcv ()
 325 returns the number of bytes actually copied into the
 326 .I mtext
 327 array.
 328 On failure, both functions return \-1, and set
 329 .I errno
 330 to indicate the error.
 331 .SH ERRORS
 332 .BR msgsnd ()
 333 can fail with the following errors:
 334 .TP
 335 .B EACCES
 336 The calling process does not have write permission on the message queue,
 337 and does not have the
 338 .B CAP_IPC_OWNER
 339 capability in the user namespace that governs its IPC namespace.
 340 .TP
 341 .B EAGAIN
 342 The message can't be sent due to the
 343 .I msg_qbytes
 344 limit for the queue and
 345 .B IPC_NOWAIT
 346 was specified in
 347 .IR msgflg .
 348 .TP
 349 .B EFAULT
 350 The address pointed to by
 351 .I msgp
 352 isn't accessible.
 353 .TP
 354 .B EIDRM
 355 The message queue was removed.
 356 .TP
 357 .B EINTR
 358 Sleeping on a full message queue condition, the process caught a signal.
 359 .TP
 360 .B EINVAL
 361 Invalid
 362 .I msqid
 363 value, or nonpositive
 364 .I mtype
 365 value, or
 366 invalid
 367 .I msgsz
 368 value (less than 0 or greater than the system value
 369 .BR MSGMAX ).
 370 .TP
 371 .B ENOMEM
 372 The system does not have enough memory to make a copy of the
 373 message pointed to by
 374 .IR msgp .
 375 .PP
 376 .BR msgrcv ()
 377 can fail with the following errors:
 378 .TP
 379 .B E2BIG
 380 The message text length is greater than
 381 .I msgsz
 382 and
 383 .B MSG_NOERROR
 384 isn't specified in
 385 .IR msgflg .
 386 .TP
 387 .B EACCES
 388 The calling process does not have read permission on the message queue,
 389 and does not have the
 390 .B CAP_IPC_OWNER
 391 capability in the user namespace that governs its IPC namespace.
 392 .TP
 393 .B EFAULT
 394 The address pointed to by
 395 .I msgp
 396 isn't accessible.
 397 .TP
 398 .B EIDRM
 399 While the process was sleeping to receive a message,
 400 the message queue was removed.
 401 .TP
 402 .B EINTR
 403 While the process was sleeping to receive a message,
 404 the process caught a signal; see
 405 .BR signal (7).
 406 .TP
 407 .B EINVAL
 408 .I msqid
 409 was invalid, or
 410 .I msgsz
 411 was less than 0.
 412 .TP
 413 .BR EINVAL " (since Linux 3.14)"
 414 .I msgflg
 415 specified
 416 .BR MSG_COPY ,
 417 but not
 418 .BR IPC_NOWAIT .
 419 .TP
 420 .BR EINVAL " (since Linux 3.14)"
 421 .I msgflg
 422 specified both
 423 .B MSG_COPY
 424 and
 425 .BR MSG_EXCEPT .
 426 .TP
 427 .B ENOMSG
 428 .B IPC_NOWAIT
 429 was specified in
 430 .I msgflg
 431 and no message of the requested type existed on the message queue.
 432 .TP
 433 .B ENOMSG
 434 .B IPC_NOWAIT
 435 and
 436 .B MSG_COPY
 437 were specified in
 438 .I msgflg
 439 and the queue contains less than
 440 .I msgtyp
 441 messages.
 442 .TP
 443 .BR ENOSYS " (since Linux 3.8)"
 444 Both
 445 .B MSG_COPY
 446 and
 447 .B IPC_NOWAIT
 448 were specified in
 449 .IR msgflg ,
 450 and this kernel was configured without
 451 .BR CONFIG_CHECKPOINT_RESTORE .
 452 .SH CONFORMING TO
 453 POSIX.1-2001, POSIX.1-2008, SVr4.
 454 .PP
 455 The
 456 .B MSG_EXCEPT
 457 and
 458 .B MSG_COPY
 459 flags are Linux-specific;
 460 their definitions can be obtained by defining the
 461 .B _GNU_SOURCE
 462 .\" MSG_COPY since glibc 2.18
 463 feature test macro.
 464 .SH NOTES
 465 The
 466 .I msgp
 467 argument is declared as \fIstruct msgbuf\ *\fP in
 468 glibc 2.0 and 2.1.
 469 It is declared as \fIvoid\ *\fP
 470 in glibc 2.2 and later, as required by SUSv2 and SUSv3.
 471 .PP
 472 The following limits on message queue resources affect the
 473 .BR msgsnd ()
 474 call:
 475 .TP
 476 .B MSGMAX
 477 Maximum size of a message text, in bytes (default value: 8192 bytes).
 478 On Linux, this limit can be read and modified via
 479 .IR /proc/sys/kernel/msgmax .
 480 .TP
 481 .B MSGMNB
 482 Maximum number of bytes that can be held in a message queue
 483 (default value: 16384 bytes).
 484 On Linux, this limit can be read and modified via
 485 .IR /proc/sys/kernel/msgmnb .
 486 A privileged process
 487 (Linux: a process with the
 488 .B CAP_SYS_RESOURCE
 489 capability)
 490 can increase the size of a message queue beyond
 491 .B MSGMNB
 492 using the
 493 .BR msgctl (2)
 494 .B IPC_SET
 495 operation.
 496 .PP
 497 The implementation has no intrinsic system-wide limits on the
 498 number of message headers
 499 .RB ( MSGTQL )
 500 and the number of bytes in the message pool
 501 .RB ( MSGPOOL ).
 502 .SH BUGS
 503 In Linux 3.13 and earlier,
 504 if
 505 .BR msgrcv ()
 506 was called with the
 507 .B MSG_COPY
 508 flag, but without
 509 .BR IPC_NOWAIT ,
 510 and the message queue contained less than
 511 .I msgtyp
 512 messages, then the call would block until the next message is written
 513 to the queue.
 514 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 515 At that point, the call would return a copy of the message,
 516 .I regardless
 517 of whether that message was at the ordinal position
 518 .IR msgtyp .
 519 This bug is fixed
 520 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 521 in Linux 3.14.
 522 .PP
 523 Specifying both
 524 .B MSG_COPY
 525 and
 526 .B MSC_EXCEPT
 527 in
 528 .I msgflg
 529 is a logical error (since these flags impose different interpretations on
 530 .IR msgtyp ).
 531 In Linux 3.13 and earlier,
 532 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 533 this error was not diagnosed by
 534 .BR msgrcv ().
 535 This bug is fixed
 536 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 537 in Linux 3.14.
 538 .SH EXAMPLES
 539 The program below demonstrates the use of
 540 .BR msgsnd ()
 541 and
 542 .BR msgrcv ().
 543 .PP
 544 The example program is first run with the \fB\-s\fP option to send a
 545 message and then run again with the \fB\-r\fP option to receive a
 546 message.
 547 .PP
 548 The following shell session shows a sample run of the program:
 549 .PP
 550 .in +4n
 551 .EX
 552 .RB "$" " ./a.out \-s"
 553 sent: a message at Wed Mar  4 16:25:45 2015
 554
 555 .RB "$" " ./a.out \-r"
 556 message received: a message at Wed Mar  4 16:25:45 2015
 557 .EE
 558 .in
 559 .SS Program source
 560 \&
 561 .EX
 562 #include <stdio.h>
 563 #include <stdlib.h>
 564 #include <string.h>
 565 #include <time.h>
 566 #include <unistd.h>
 567 #include <errno.h>
 568 #include <sys/types.h>
 569 #include <sys/ipc.h>
 570 #include <sys/msg.h>
 571
 572 struct msgbuf {
 573     long mtype;
 574     char mtext[80];
 575 };
 576
 577 static void
 578 usage(char *prog_name, char *msg)
 579 {
 580     if (msg != NULL)
 581         fputs(msg, stderr);
 582
 583     fprintf(stderr, "Usage: %s [options]\en", prog_name);
 584     fprintf(stderr, "Options are:\en");
 585     fprintf(stderr, "\-s        send message using msgsnd()\en");
 586     fprintf(stderr, "\-r        read message using msgrcv()\en");
 587     fprintf(stderr, "\-t        message type (default is 1)\en");
 588     fprintf(stderr, "\-k        message queue key (default is 1234)\en");
 589     exit(EXIT_FAILURE);
 590 }
 591
 592 static void
 593 send_msg(int qid, int msgtype)
 594 {
 595     struct msgbuf msg;
 596     time_t t;
 597
 598     msg.mtype = msgtype;
 599
 600     time(&t);
 601     snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
 602             ctime(&t));
 603
 604     if (msgsnd(qid, &msg, sizeof(msg.mtext),
 605                 IPC_NOWAIT) == \-1) {
 606         perror("msgsnd error");
 607         exit(EXIT_FAILURE);
 608     }
 609     printf("sent: %s\en", msg.mtext);
 610 }
 611
 612 static void
 613 get_msg(int qid, int msgtype)
 614 {
 615     struct msgbuf msg;
 616
 617     if (msgrcv(qid, &msg, sizeof(msg.mtext), msgtype,
 618                MSG_NOERROR | IPC_NOWAIT) == \-1) {
 619         if (errno != ENOMSG) {
 620             perror("msgrcv");
 621             exit(EXIT_FAILURE);
 622         }
 623         printf("No message available for msgrcv()\en");
 624     } else
 625         printf("message received: %s\en", msg.mtext);
 626 }
 627
 628 int
 629 main(int argc, char *argv[])
 630 {
 631     int qid, opt;
 632     int mode = 0;               /* 1 = send, 2 = receive */
 633     int msgtype = 1;
 634     int msgkey = 1234;
 635
 636     while ((opt = getopt(argc, argv, "srt:k:")) != \-1) {
 637         switch (opt) {
 638         case \(aqs\(aq:
 639             mode = 1;
 640             break;
 641         case \(aqr\(aq:
 642             mode = 2;
 643             break;
 644         case \(aqt\(aq:
 645             msgtype = atoi(optarg);
 646             if (msgtype <= 0)
 647                 usage(argv[0], "\-t option must be greater than 0\en");
 648             break;
 649         case \(aqk\(aq:
 650             msgkey = atoi(optarg);
 651             break;
 652         default:
 653             usage(argv[0], "Unrecognized option\en");
 654         }
 655     }
 656
 657     if (mode == 0)
 658         usage(argv[0], "must use either \-s or \-r option\en");
 659
 660     qid = msgget(msgkey, IPC_CREAT | 0666);
 661
 662     if (qid == \-1) {
 663         perror("msgget");
 664         exit(EXIT_FAILURE);
 665     }
 666
 667     if (mode == 2)
 668         get_msg(qid, msgtype);
 669     else
 670         send_msg(qid, msgtype);
 671
 672     exit(EXIT_SUCCESS);
 673 }
 674 .EE
 675 .SH SEE ALSO
 676 .BR msgctl (2),
 677 .BR msgget (2),
 678 .BR capabilities (7),
 679 .BR mq_overview (7),
 680 .BR sysvipc (7)