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 2015-02-21 "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 .sp
  48 .BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \
  49 ", int " msgflg );
  50 .sp
  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, respectively, 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 .in +4n
  70 .nf
  71
  72 struct msgbuf {
  73     long mtype;       /* message type, must be > 0 */
  74     char mtext[1];    /* message data */
  75 };
  76 .fi
  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
 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
 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
 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.
 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.
 417 .TP
 418 .B EAGAIN
 419 No message was available in the queue and
 420 .B IPC_NOWAIT
 421 was specified in
 422 .IR msgflg .
 423 .TP
 424 .B EFAULT
 425 The address pointed to by
 426 .I msgp
 427 isn't accessible.
 428 .TP
 429 .B EIDRM
 430 While the process was sleeping to receive a message,
 431 the message queue was removed.
 432 .TP
 433 .B EINTR
 434 While the process was sleeping to receive a message,
 435 the process caught a signal; see
 436 .BR signal (7).
 437 .TP
 438 .B EINVAL
 439 .I msgqid
 440 was invalid, or
 441 .I msgsz
 442 was less than 0.
 443 .TP
 444 .BR EINVAL " (since Linux 3.14)"
 445 .I msgflg
 446 specified
 447 .BR MSG_COPY ,
 448 but not
 449 .BR IPC_NOWAIT .
 450 .TP
 451 .BR EINVAL " (since Linux 3.14)"
 452 .I msgflg
 453 specified both
 454 .BR MSG_COPY
 455 and
 456 .BR MSG_EXCEPT .
 457 .TP
 458 .B ENOMSG
 459 .B IPC_NOWAIT
 460 was specified in
 461 .I msgflg
 462 and no message of the requested type existed on the message queue.
 463 .TP
 464 .B ENOMSG
 465 .B IPC_NOWAIT
 466 and
 467 .B MSG_COPY
 468 were specified in
 469 .I msgflg
 470 and the queue contains less than
 471 .I msgtyp
 472 messages.
 473 .TP
 474 .BR ENOSYS " (since Linux 3.8)"
 475 .I MSG_COPY
 476 was specified in
 477 .IR msgflg ,
 478 and this kernel was configured without
 479 .BR CONFIG_CHECKPOINT_RESTORE .
 480 .SH CONFORMING TO
 481 SVr4, POSIX.1-2001.
 482
 483 The
 484 .B MSG_EXCEPT
 485 and
 486 .B MSG_COPY
 487 flags are Linux-specific;
 488 their definitions can be obtained by defining the
 489 .B _GNU_SOURCE
 490 .\" MSG_COPY since glibc 2.18
 491 feature test macro.
 492 .SH NOTES
 493 The inclusion of
 494 .I <sys/types.h>
 495 and
 496 .I <sys/ipc.h>
 497 isn't required on Linux or by any version of POSIX.
 498 However,
 499 some old implementations required the inclusion of these header files,
 500 and the SVID also documented their inclusion.
 501 Applications intended to be portable to such old systems may need
 502 to include these header files.
 503 .\" Like Linux, the FreeBSD man pages still document
 504 .\" the inclusion of these header files.
 505
 506 The
 507 .I msgp
 508 argument is declared as \fIstruct msgbuf\ *\fP in
 509 glibc 2.0 and 2.1.
 510 It is declared as \fIvoid\ *\fP
 511 in glibc 2.2 and later, as required by SUSv2 and SUSv3.
 512
 513 The following limits on message queue resources affect the
 514 .BR msgsnd ()
 515 call:
 516 .TP
 517 .B MSGMAX
 518 Maximum size of a message text, in bytes (default value: 8192 bytes).
 519 On Linux, this limit can be read and modified via
 520 .IR /proc/sys/kernel/msgmax .
 521 .TP
 522 .B MSGMNB
 523 Maximum number of bytes that can be held in a message queue
 524 (default value: 16384 bytes).
 525 On Linux, this limit can be read and modified via
 526 .IR /proc/sys/kernel/msgmnb .
 527 A privileged process
 528 (Linux: a process with the
 529 .B CAP_SYS_RESOURCE
 530 capability)
 531 can increase the size of a message queue beyond
 532 .B MSGMNB
 533 using the
 534 .BR msgctl (2)
 535 .B IPC_SET
 536 operation.
 537 .PP
 538 The implementation has no intrinsic system-wide limits on the
 539 number of message headers
 540 .RB ( MSGTQL )
 541 and the number of bytes in the message pool
 542 .RB ( MSGPOOL ).
 543 .SH BUGS
 544 In Linux 3.13 and earlier,
 545 if
 546 .BR msgrcv ()
 547 was called with the
 548 .BR MSG_COPY
 549 flag, but without
 550 .BR IPC_NOWAIT ,
 551 and the message queue contained less than
 552 .I msgtyp
 553 messages, then the call would block until the next message is written
 554 to the queue.
 555 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 556 At that point, the call would return a copy of the message,
 557 .I regardless
 558 of whether that message was at the ordinal position
 559 .IR msgtyp .
 560 This bug is fixed
 561 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 562 in Linux 3.14.
 563
 564 Specifying both
 565 .B MSG_COPY
 566 and
 567 .B MSC_EXCEPT
 568 in
 569 .I msgflg
 570 is a logical error (since these flags impose different interpretations on
 571 .IR msgtyp ).
 572 In Linux 3.13 and earlier,
 573 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 574 this error was not diagnosed by
 575 .BR msgrcv ().
 576 This bug is fixed
 577 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 578 in Linux 3.14.
 579 .SH EXAMPLE
 580 The program below demonstrates the use of
 581 .BR msgsnd ()
 582 and
 583 .BR msgrcv ().
 584
 585 The example program is first run with the \fB\-s\fP option to send a
 586 message and then run again with the \fB\-r\fP option to receive a
 587 message.
 588
 589 The following shell session shows a sample run of the program:
 590 .in +4n
 591 .nf
 592
 593 .RB "$" " ./a.out \-s"
 594 sent: a message at Wed Mar  4 16:25:45 2015
 595
 596 .RB "$" " ./a.out \-r"
 597 message received: a message at Wed Mar  4 16:25:45 2015
 598
 599 .fi
 600 .in
 601 .SS Program source
 602 \&
 603 .nf
 604 #include <stdio.h>
 605 #include <stdlib.h>
 606 #include <string.h>
 607 #include <time.h>
 608 #include <unistd.h>
 609 #include <errno.h>
 610 #include <sys/types.h>
 611 #include <sys/ipc.h>
 612 #include <sys/msg.h>
 613
 614 struct msgbuf {
 615     long mtype;
 616     char mtext[80];
 617 };
 618
 619 static void
 620 usage(char *prog_name, char *msg)
 621 {
 622     if (msg != NULL)
 623         fputs(msg, stderr);
 624
 625     fprintf(stderr, "Usage: %s [options]\\n", prog_name);
 626     fprintf(stderr, "Options are:\\n");
 627     fprintf(stderr, "\-s        send message using msgsnd()\\n");
 628     fprintf(stderr, "\-r        read message using msgrcv()\\n");
 629     fprintf(stderr, "\-t        message type (default is 1)\\n");
 630     fprintf(stderr, "\-k        message queue key (default is 1234)\\n");
 631     exit(EXIT_FAILURE);
 632 }
 633
 634 static void
 635 send_msg(int qid, int msgtype)
 636 {
 637     struct msgbuf msg;
 638     time_t t;
 639
 640     msg.mtype = msgtype;
 641
 642     time(&t);
 643     snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
 644             ctime(&t));
 645
 646     if (msgsnd(qid, (void *) &msg, sizeof(msg.mtext),
 647                 IPC_NOWAIT) == \-1) {
 648         perror("msgsnd error");
 649         exit(EXIT_FAILURE);
 650     }
 651     printf("sent: %s\\n", msg.mtext);
 652 }
 653
 654 static void
 655 get_msg(int qid, int msgtype)
 656 {
 657     struct msgbuf msg;
 658
 659     if (msgrcv(qid, (void *) &msg, sizeof(msg.mtext), msgtype,
 660                MSG_NOERROR | IPC_NOWAIT) == \-1) {
 661         if (errno != ENOMSG) {
 662             perror("msgrcv");
 663             exit(EXIT_FAILURE);
 664         }
 665         printf("No message available for msgrcv()\\n");
 666     } else
 667         printf("message received: %s\\n", msg.mtext);
 668 }
 669
 670 int
 671 main(int argc, char *argv[])
 672 {
 673     int qid, opt;
 674     int mode = 0;               /* 1 = send, 2 = receive */
 675     int msgtype = 1;
 676     int msgkey = 1234;
 677
 678     while ((opt = getopt(argc, argv, "srt:k:")) != \-1) {
 679         switch (opt) {
 680         case \(aqs\(aq:
 681             mode = 1;
 682             break;
 683         case \(aqr\(aq:
 684             mode = 2;
 685             break;
 686         case \(aqt\(aq:
 687             msgtype = atoi(optarg);
 688             if (msgtype <= 0)
 689                 usage(argv[0], "\-t option must be greater than 0\\n");
 690             break;
 691         case \(aqk\(aq:
 692             msgkey = atoi(optarg);
 693             break;
 694         default:
 695             usage(argv[0], "Unrecognized option\\n");
 696         }
 697     }
 698
 699     if (mode == 0)
 700         usage(argv[0], "must use either \-s or \-r option\\n");
 701
 702     qid = msgget(msgkey, IPC_CREAT | 0666);
 703
 704     if (qid == \-1) {
 705         perror("msgget");
 706         exit(EXIT_FAILURE);
 707     }
 708
 709     if (mode == 2)
 710         get_msg(qid, msgtype);
 711     else
 712         send_msg(qid, msgtype);
 713
 714     exit(EXIT_SUCCESS);
 715 }
 716 .fi
 717 .SH SEE ALSO
 718 .BR msgctl (2),
 719 .BR msgget (2),
 720 .BR capabilities (7),
 721 .BR mq_overview (7),
 722 .BR svipc (7)