man2/msgop.2

   1 .\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
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  24 .\"
  25 .\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esr@thyrsus.com>
  26 .\" Modified Mon Jul 10 21:09:59 2000 by aeb
  27 .\" Modified 1 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  28 .\"     Language clean-ups.
  29 .\"     Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
  30 .\"     Added note on restart behavior of msgsnd() and msgrcv()
  31 .\"     Formatting clean-ups (argument and field names marked as .I
  32 .\"             instead of .B)
  33 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Added notes on capability requirements
  35 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  36 .\"     Language and formatting clean-ups
  37 .\"     Added notes on /proc files
  38 .\"
  39 .TH MSGOP 2 2019-08-02 "Linux" "Linux Programmer's Manual"
  40 .SH NAME
  41 msgrcv, msgsnd \- System V message queue operations
  42 .SH SYNOPSIS
  43 .nf
  44 .B #include <sys/types.h>
  45 .B #include <sys/ipc.h>
  46 .B #include <sys/msg.h>
  47 .PP
  48 .BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \
  49 ", int " msgflg );
  50 .PP
  51 .BI "ssize_t msgrcv(int " msqid ", void *" msgp ", size_t " msgsz \
  52 ", long " msgtyp ,
  53 .BI "               int " msgflg );
  54 .fi
  55 .SH DESCRIPTION
  56 The
  57 .BR msgsnd ()
  58 and
  59 .BR msgrcv ()
  60 system calls are used to send messages to,
  61 and receive messages from, a System\ V message queue.
  62 The calling process must have write permission on the message queue
  63 in order to send a message, and read permission to receive a message.
  64 .PP
  65 The
  66 .I msgp
  67 argument is a pointer to a caller-defined structure
  68 of the following general form:
  69 .PP
  70 .in +4n
  71 .EX
  72 struct msgbuf {
  73     long mtype;       /* message type, must be > 0 */
  74     char mtext[1];    /* message data */
  75 };
  76 .EE
  77 .in
  78 .PP
  79 The
  80 .I mtext
  81 field is an array (or other structure) whose size is specified by
  82 .IR msgsz ,
  83 a nonnegative integer value.
  84 Messages of zero length (i.e., no
  85 .I mtext
  86 field) are permitted.
  87 The
  88 .I mtype
  89 field must have a strictly positive integer value.
  90 This value can be
  91 used by the receiving process for message selection
  92 (see the description of
  93 .BR msgrcv ()
  94 below).
  95 .SS msgsnd()
  96 The
  97 .BR msgsnd ()
  98 system call appends a copy of the message pointed to by
  99 .I msgp
 100 to the message queue whose identifier is specified
 101 by
 102 .IR msqid .
 103 .PP
 104 If sufficient space is available in the queue,
 105 .BR msgsnd ()
 106 succeeds immediately.
 107 The queue capacity is governed by the
 108 .I msg_qbytes
 109 field in the associated data structure for the message queue.
 110 During queue creation this field is initialized to
 111 .B MSGMNB
 112 bytes, but this limit can be modified using
 113 .BR msgctl (2).
 114 A message queue is considered to be full if either of the following
 115 conditions is true:
 116 .IP * 2
 117 Adding a new message to the queue would cause the total number of bytes
 118 in the queue to exceed the queue's maximum size (the
 119 .I msg_qbytes
 120 field).
 121 .IP *
 122 Adding another message to the queue would cause the total number of messages
 123 in the queue to exceed the queue's maximum size (the
 124 .I msg_qbytes
 125 field).
 126 This check is necessary to prevent an unlimited number of zero-length
 127 messages being placed on the queue.
 128 Although such messages contain no data,
 129 they nevertheless consume (locked) kernel memory.
 130 .PP
 131 If insufficient space is available in the queue, then the default
 132 behavior of
 133 .BR msgsnd ()
 134 is to block until space becomes available.
 135 If
 136 .B IPC_NOWAIT
 137 is specified in
 138 .IR msgflg ,
 139 then the call instead fails with the error
 140 .BR EAGAIN .
 141 .PP
 142 A blocked
 143 .BR msgsnd ()
 144 call may also fail if:
 145 .IP * 2
 146 the queue is removed,
 147 in which case the system call fails with
 148 .I errno
 149 set to
 150 .BR EIDRM ;
 151 or
 152 .IP *
 153 a signal is caught, in which case the system call fails
 154 with
 155 .I errno
 156 set to
 157 .BR EINTR ; see
 158 .BR signal (7).
 159 .RB ( msgsnd ()
 160 is never automatically restarted after being interrupted by a
 161 signal handler, regardless of the setting of the
 162 .B SA_RESTART
 163 flag when establishing a signal handler.)
 164 .PP
 165 Upon successful completion the message queue data structure is updated
 166 as follows:
 167 .IP
 168 .I msg_lspid
 169 is set to the process ID of the calling process.
 170 .IP
 171 .I msg_qnum
 172 is incremented by 1.
 173 .IP
 174 .I msg_stime
 175 is set to the current time.
 176 .SS msgrcv()
 177 The
 178 .BR msgrcv ()
 179 system call removes a message from the queue specified by
 180 .I msqid
 181 and places it in the buffer
 182 pointed to by
 183 .IR msgp .
 184 .PP
 185 The argument
 186 .I msgsz
 187 specifies the maximum size in bytes for the member
 188 .I mtext
 189 of the structure pointed to by the
 190 .I msgp
 191 argument.
 192 If the message text has length greater than
 193 .IR msgsz ,
 194 then the behavior depends on whether
 195 .B MSG_NOERROR
 196 is specified in
 197 .IR msgflg .
 198 If
 199 .B MSG_NOERROR
 200 is specified, then
 201 the message text will be truncated (and the truncated part will be
 202 lost); if
 203 .B MSG_NOERROR
 204 is not specified, then
 205 the message isn't removed from the queue and
 206 the system call fails returning \-1 with
 207 .I errno
 208 set to
 209 .BR E2BIG .
 210 .PP
 211 Unless
 212 .B MSG_COPY
 213 is specified in
 214 .IR msgflg
 215 (see below),
 216 the
 217 .I msgtyp
 218 argument specifies the type of message requested, as follows:
 219 .IP * 2
 220 If
 221 .I msgtyp
 222 is 0,
 223 then the first message in the queue is read.
 224 .IP *
 225 If
 226 .I msgtyp
 227 is greater than 0,
 228 then the first message in the queue of type
 229 .I msgtyp
 230 is read, unless
 231 .B MSG_EXCEPT
 232 was specified in
 233 .IR msgflg ,
 234 in which case
 235 the first message in the queue of type not equal to
 236 .I msgtyp
 237 will be read.
 238 .IP *
 239 If
 240 .I msgtyp
 241 is less than 0,
 242 then the first message in the queue with the lowest type less than or
 243 equal to the absolute value of
 244 .I msgtyp
 245 will be read.
 246 .PP
 247 The
 248 .I msgflg
 249 argument is a bit mask constructed by ORing together zero or more
 250 of the following flags:
 251 .TP
 252 .B IPC_NOWAIT
 253 Return immediately if no message of the requested type is in the queue.
 254 The system call fails with
 255 .I errno
 256 set to
 257 .BR ENOMSG .
 258 .TP
 259 .BR MSG_COPY " (since Linux 3.8)"
 260 .\" commit 4a674f34ba04a002244edaf891b5da7fc1473ae8
 261 Nondestructively fetch a copy of the message at the ordinal position
 262 in the queue specified by
 263 .I msgtyp
 264 (messages are considered to be numbered starting at 0).
 265 .IP
 266 This flag must be specified in conjunction with
 267 .BR IPC_NOWAIT ,
 268 with the result that, if there is no message available at the given position,
 269 the call fails immediately with the error
 270 .BR ENOMSG .
 271 Because they alter the meaning of
 272 .I msgtyp
 273 in orthogonal ways,
 274 .BR MSG_COPY
 275 and
 276 .BR MSG_EXCEPT
 277 may not both be specified in
 278 .IR msgflg .
 279 .IP
 280 The
 281 .BR MSG_COPY
 282 flag was added for the implementation of
 283 the kernel checkpoint-restore facility and
 284 is available only if the kernel was built with the
 285 .B CONFIG_CHECKPOINT_RESTORE
 286 option.
 287 .TP
 288 .B MSG_EXCEPT
 289 Used with
 290 .I msgtyp
 291 greater than 0
 292 to read the first message in the queue with message type that differs
 293 from
 294 .IR msgtyp .
 295 .TP
 296 .B MSG_NOERROR
 297 To truncate the message text if longer than
 298 .I msgsz
 299 bytes.
 300 .PP
 301 If no message of the requested type is available and
 302 .B IPC_NOWAIT
 303 isn't specified in
 304 .IR msgflg ,
 305 the calling process is blocked until one of the following conditions occurs:
 306 .IP * 2
 307 A message of the desired type is placed in the queue.
 308 .IP *
 309 The message queue is removed from the system.
 310 In this case, the system call fails with
 311 .I errno
 312 set to
 313 .BR EIDRM .
 314 .IP *
 315 The calling process catches a signal.
 316 In this case, the system call fails with
 317 .I errno
 318 set to
 319 .BR EINTR .
 320 .RB ( msgrcv ()
 321 is never automatically restarted after being interrupted by a
 322 signal handler, regardless of the setting of the
 323 .B SA_RESTART
 324 flag when establishing a signal handler.)
 325 .PP
 326 Upon successful completion the message queue data structure is updated
 327 as follows:
 328 .IP
 329 .I msg_lrpid
 330 is set to the process ID of the calling process.
 331 .IP
 332 .I msg_qnum
 333 is decremented by 1.
 334 .IP
 335 .I msg_rtime
 336 is set to the current time.
 337 .SH RETURN VALUE
 338 On failure both functions return \-1
 339 with
 340 .I errno
 341 indicating the error,
 342 otherwise
 343 .BR msgsnd ()
 344 returns 0
 345 and
 346 .BR msgrcv ()
 347 returns the number of bytes actually copied into the
 348 .I mtext
 349 array.
 350 .SH ERRORS
 351 When
 352 .BR msgsnd ()
 353 fails,
 354 .I errno
 355 will be set to one among the following values:
 356 .TP
 357 .B EACCES
 358 The calling process does not have write permission on the message queue,
 359 and does not have the
 360 .B CAP_IPC_OWNER
 361 capability in the user namespace that governs its IPC namespace.
 362 .TP
 363 .B EAGAIN
 364 The message can't be sent due to the
 365 .I msg_qbytes
 366 limit for the queue and
 367 .B IPC_NOWAIT
 368 was specified in
 369 .IR msgflg .
 370 .TP
 371 .B EFAULT
 372 The address pointed to by
 373 .I msgp
 374 isn't accessible.
 375 .TP
 376 .B EIDRM
 377 The message queue was removed.
 378 .TP
 379 .B EINTR
 380 Sleeping on a full message queue condition, the process caught a signal.
 381 .TP
 382 .B EINVAL
 383 Invalid
 384 .I msqid
 385 value, or nonpositive
 386 .I mtype
 387 value, or
 388 invalid
 389 .I msgsz
 390 value (less than 0 or greater than the system value
 391 .BR MSGMAX ).
 392 .TP
 393 .B ENOMEM
 394 The system does not have enough memory to make a copy of the
 395 message pointed to by
 396 .IR msgp .
 397 .PP
 398 When
 399 .BR msgrcv ()
 400 fails,
 401 .I errno
 402 will be set to one among the following values:
 403 .TP
 404 .B E2BIG
 405 The message text length is greater than
 406 .I msgsz
 407 and
 408 .B MSG_NOERROR
 409 isn't specified in
 410 .IR msgflg .
 411 .TP
 412 .B EACCES
 413 The calling process does not have read permission on the message queue,
 414 and does not have the
 415 .B CAP_IPC_OWNER
 416 capability in the user namespace that governs its IPC namespace.
 417 .TP
 418 .B EFAULT
 419 The address pointed to by
 420 .I msgp
 421 isn't accessible.
 422 .TP
 423 .B EIDRM
 424 While the process was sleeping to receive a message,
 425 the message queue was removed.
 426 .TP
 427 .B EINTR
 428 While the process was sleeping to receive a message,
 429 the process caught a signal; see
 430 .BR signal (7).
 431 .TP
 432 .B EINVAL
 433 .I msqid
 434 was invalid, or
 435 .I msgsz
 436 was less than 0.
 437 .TP
 438 .BR EINVAL " (since Linux 3.14)"
 439 .I msgflg
 440 specified
 441 .BR MSG_COPY ,
 442 but not
 443 .BR IPC_NOWAIT .
 444 .TP
 445 .BR EINVAL " (since Linux 3.14)"
 446 .I msgflg
 447 specified both
 448 .BR MSG_COPY
 449 and
 450 .BR MSG_EXCEPT .
 451 .TP
 452 .B ENOMSG
 453 .B IPC_NOWAIT
 454 was specified in
 455 .I msgflg
 456 and no message of the requested type existed on the message queue.
 457 .TP
 458 .B ENOMSG
 459 .B IPC_NOWAIT
 460 and
 461 .B MSG_COPY
 462 were specified in
 463 .I msgflg
 464 and the queue contains less than
 465 .I msgtyp
 466 messages.
 467 .TP
 468 .BR ENOSYS " (since Linux 3.8)"
 469 .I MSG_COPY
 470 was specified in
 471 .IR msgflg ,
 472 and this kernel was configured without
 473 .BR CONFIG_CHECKPOINT_RESTORE .
 474 .SH CONFORMING TO
 475 POSIX.1-2001, POSIX.1-2008, SVr4.
 476 .PP
 477 The
 478 .B MSG_EXCEPT
 479 and
 480 .B MSG_COPY
 481 flags are Linux-specific;
 482 their definitions can be obtained by defining the
 483 .B _GNU_SOURCE
 484 .\" MSG_COPY since glibc 2.18
 485 feature test macro.
 486 .SH NOTES
 487 The inclusion of
 488 .I <sys/types.h>
 489 and
 490 .I <sys/ipc.h>
 491 isn't required on Linux or by any version of POSIX.
 492 However,
 493 some old implementations required the inclusion of these header files,
 494 and the SVID also documented their inclusion.
 495 Applications intended to be portable to such old systems may need
 496 to include these header files.
 497 .\" Like Linux, the FreeBSD man pages still document
 498 .\" the inclusion of these header files.
 499 .PP
 500 The
 501 .I msgp
 502 argument is declared as \fIstruct msgbuf\ *\fP in
 503 glibc 2.0 and 2.1.
 504 It is declared as \fIvoid\ *\fP
 505 in glibc 2.2 and later, as required by SUSv2 and SUSv3.
 506 .PP
 507 The following limits on message queue resources affect the
 508 .BR msgsnd ()
 509 call:
 510 .TP
 511 .B MSGMAX
 512 Maximum size of a message text, in bytes (default value: 8192 bytes).
 513 On Linux, this limit can be read and modified via
 514 .IR /proc/sys/kernel/msgmax .
 515 .TP
 516 .B MSGMNB
 517 Maximum number of bytes that can be held in a message queue
 518 (default value: 16384 bytes).
 519 On Linux, this limit can be read and modified via
 520 .IR /proc/sys/kernel/msgmnb .
 521 A privileged process
 522 (Linux: a process with the
 523 .B CAP_SYS_RESOURCE
 524 capability)
 525 can increase the size of a message queue beyond
 526 .B MSGMNB
 527 using the
 528 .BR msgctl (2)
 529 .B IPC_SET
 530 operation.
 531 .PP
 532 The implementation has no intrinsic system-wide limits on the
 533 number of message headers
 534 .RB ( MSGTQL )
 535 and the number of bytes in the message pool
 536 .RB ( MSGPOOL ).
 537 .SH BUGS
 538 In Linux 3.13 and earlier,
 539 if
 540 .BR msgrcv ()
 541 was called with the
 542 .BR MSG_COPY
 543 flag, but without
 544 .BR IPC_NOWAIT ,
 545 and the message queue contained less than
 546 .I msgtyp
 547 messages, then the call would block until the next message is written
 548 to the queue.
 549 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 550 At that point, the call would return a copy of the message,
 551 .I regardless
 552 of whether that message was at the ordinal position
 553 .IR msgtyp .
 554 This bug is fixed
 555 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 556 in Linux 3.14.
 557 .PP
 558 Specifying both
 559 .B MSG_COPY
 560 and
 561 .B MSC_EXCEPT
 562 in
 563 .I msgflg
 564 is a logical error (since these flags impose different interpretations on
 565 .IR msgtyp ).
 566 In Linux 3.13 and earlier,
 567 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 568 this error was not diagnosed by
 569 .BR msgrcv ().
 570 This bug is fixed
 571 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 572 in Linux 3.14.
 573 .SH EXAMPLE
 574 The program below demonstrates the use of
 575 .BR msgsnd ()
 576 and
 577 .BR msgrcv ().
 578 .PP
 579 The example program is first run with the \fB\-s\fP option to send a
 580 message and then run again with the \fB\-r\fP option to receive a
 581 message.
 582 .PP
 583 The following shell session shows a sample run of the program:
 584 .PP
 585 .in +4n
 586 .EX
 587 .RB "$" " ./a.out \-s"
 588 sent: a message at Wed Mar  4 16:25:45 2015
 589
 590 .RB "$" " ./a.out \-r"
 591 message received: a message at Wed Mar  4 16:25:45 2015
 592 .EE
 593 .in
 594 .SS Program source
 595 \&
 596 .EX
 597 #include <stdio.h>
 598 #include <stdlib.h>
 599 #include <string.h>
 600 #include <time.h>
 601 #include <unistd.h>
 602 #include <errno.h>
 603 #include <sys/types.h>
 604 #include <sys/ipc.h>
 605 #include <sys/msg.h>
 606
 607 struct msgbuf {
 608     long mtype;
 609     char mtext[80];
 610 };
 611
 612 static void
 613 usage(char *prog_name, char *msg)
 614 {
 615     if (msg != NULL)
 616         fputs(msg, stderr);
 617
 618     fprintf(stderr, "Usage: %s [options]\en", prog_name);
 619     fprintf(stderr, "Options are:\en");
 620     fprintf(stderr, "\-s        send message using msgsnd()\en");
 621     fprintf(stderr, "\-r        read message using msgrcv()\en");
 622     fprintf(stderr, "\-t        message type (default is 1)\en");
 623     fprintf(stderr, "\-k        message queue key (default is 1234)\en");
 624     exit(EXIT_FAILURE);
 625 }
 626
 627 static void
 628 send_msg(int qid, int msgtype)
 629 {
 630     struct msgbuf msg;
 631     time_t t;
 632
 633     msg.mtype = msgtype;
 634
 635     time(&t);
 636     snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
 637             ctime(&t));
 638
 639     if (msgsnd(qid, (void *) &msg, sizeof(msg.mtext),
 640                 IPC_NOWAIT) == \-1) {
 641         perror("msgsnd error");
 642         exit(EXIT_FAILURE);
 643     }
 644     printf("sent: %s\en", msg.mtext);
 645 }
 646
 647 static void
 648 get_msg(int qid, int msgtype)
 649 {
 650     struct msgbuf msg;
 651
 652     if (msgrcv(qid, (void *) &msg, sizeof(msg.mtext), msgtype,
 653                MSG_NOERROR | IPC_NOWAIT) == \-1) {
 654         if (errno != ENOMSG) {
 655             perror("msgrcv");
 656             exit(EXIT_FAILURE);
 657         }
 658         printf("No message available for msgrcv()\en");
 659     } else
 660         printf("message received: %s\en", msg.mtext);
 661 }
 662
 663 int
 664 main(int argc, char *argv[])
 665 {
 666     int qid, opt;
 667     int mode = 0;               /* 1 = send, 2 = receive */
 668     int msgtype = 1;
 669     int msgkey = 1234;
 670
 671     while ((opt = getopt(argc, argv, "srt:k:")) != \-1) {
 672         switch (opt) {
 673         case \(aqs\(aq:
 674             mode = 1;
 675             break;
 676         case \(aqr\(aq:
 677             mode = 2;
 678             break;
 679         case \(aqt\(aq:
 680             msgtype = atoi(optarg);
 681             if (msgtype <= 0)
 682                 usage(argv[0], "\-t option must be greater than 0\en");
 683             break;
 684         case \(aqk\(aq:
 685             msgkey = atoi(optarg);
 686             break;
 687         default:
 688             usage(argv[0], "Unrecognized option\en");
 689         }
 690     }
 691
 692     if (mode == 0)
 693         usage(argv[0], "must use either \-s or \-r option\en");
 694
 695     qid = msgget(msgkey, IPC_CREAT | 0666);
 696
 697     if (qid == \-1) {
 698         perror("msgget");
 699         exit(EXIT_FAILURE);
 700     }
 701
 702     if (mode == 2)
 703         get_msg(qid, msgtype);
 704     else
 705         send_msg(qid, msgtype);
 706
 707     exit(EXIT_SUCCESS);
 708 }
 709 .EE
 710 .SH SEE ALSO
 711 .BR msgctl (2),
 712 .BR msgget (2),
 713 .BR capabilities (7),
 714 .BR mq_overview (7),
 715 .BR sysvipc (7)