man2/msgop.2

   1 .\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
   2 .\" and Copyright 2015 Bill Pemberton <wfp5p@worldbroken.com>
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  25 .\"
  26 .\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esr@thyrsus.com>
  27 .\" Modified Mon Jul 10 21:09:59 2000 by aeb
  28 .\" Modified 1 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Language clean-ups.
  30 .\"     Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
  31 .\"     Added note on restart behavior of msgsnd() and msgrcv()
  32 .\"     Formatting clean-ups (argument and field names marked as .I
  33 .\"             instead of .B)
  34 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  35 .\"     Added notes on capability requirements
  36 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  37 .\"     Language and formatting clean-ups
  38 .\"     Added notes on /proc files
  39 .\"
  40 .TH MSGOP 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  41 .SH NAME
  42 msgrcv, msgsnd \- System V message queue operations
  43 .SH SYNOPSIS
  44 .nf
  45 .B #include <sys/types.h>
  46 .B #include <sys/ipc.h>
  47 .B #include <sys/msg.h>
  48 .PP
  49 .BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \
  50 ", int " msgflg );
  51 .PP
  52 .BI "ssize_t msgrcv(int " msqid ", void *" msgp ", size_t " msgsz \
  53 ", long " msgtyp ,
  54 .BI "               int " msgflg );
  55 .fi
  56 .SH DESCRIPTION
  57 The
  58 .BR msgsnd ()
  59 and
  60 .BR msgrcv ()
  61 system calls are used to send messages to,
  62 and receive messages from, a System\ V message queue.
  63 The calling process must have write permission on the message queue
  64 in order to send a message, and read permission to receive a message.
  65 .PP
  66 The
  67 .I msgp
  68 argument is a pointer to a caller-defined structure
  69 of the following general form:
  70 .PP
  71 .in +4n
  72 .EX
  73 struct msgbuf {
  74     long mtype;       /* message type, must be > 0 */
  75     char mtext[1];    /* message data */
  76 };
  77 .EE
  78 .in
  79 .PP
  80 The
  81 .I mtext
  82 field is an array (or other structure) whose size is specified by
  83 .IR msgsz ,
  84 a nonnegative integer value.
  85 Messages of zero length (i.e., no
  86 .I mtext
  87 field) are permitted.
  88 The
  89 .I mtype
  90 field must have a strictly positive integer value.
  91 This value can be
  92 used by the receiving process for message selection
  93 (see the description of
  94 .BR msgrcv ()
  95 below).
  96 .SS msgsnd()
  97 The
  98 .BR msgsnd ()
  99 system call appends a copy of the message pointed to by
 100 .I msgp
 101 to the message queue whose identifier is specified
 102 by
 103 .IR msqid .
 104 .PP
 105 If sufficient space is available in the queue,
 106 .BR msgsnd ()
 107 succeeds immediately.
 108 The queue capacity is governed by the
 109 .I msg_qbytes
 110 field in the associated data structure for the message queue.
 111 During queue creation this field is initialized to
 112 .B MSGMNB
 113 bytes, but this limit can be modified using
 114 .BR msgctl (2).
 115 A message queue is considered to be full if either of the following
 116 conditions is true:
 117 .IP \(bu 2
 118 Adding a new message to the queue would cause the total number of bytes
 119 in the queue to exceed the queue's maximum size (the
 120 .I msg_qbytes
 121 field).
 122 .IP \(bu
 123 Adding another message to the queue would cause the total number of messages
 124 in the queue to exceed the queue's maximum size (the
 125 .I msg_qbytes
 126 field).
 127 This check is necessary to prevent an unlimited number of zero-length
 128 messages being placed on the queue.
 129 Although such messages contain no data,
 130 they nevertheless consume (locked) kernel memory.
 131 .PP
 132 If insufficient space is available in the queue, then the default
 133 behavior of
 134 .BR msgsnd ()
 135 is to block until space becomes available.
 136 If
 137 .B IPC_NOWAIT
 138 is specified in
 139 .IR msgflg ,
 140 then the call instead fails with the error
 141 .BR EAGAIN .
 142 .PP
 143 A blocked
 144 .BR msgsnd ()
 145 call may also fail if:
 146 .IP \(bu 2
 147 the queue is removed,
 148 in which case the system call fails with
 149 .I errno
 150 set to
 151 .BR EIDRM ;
 152 or
 153 .IP \(bu
 154 a signal is caught, in which case the system call fails
 155 with
 156 .I errno
 157 set to
 158 .BR EINTR ; see
 159 .BR signal (7).
 160 .RB ( msgsnd ()
 161 is never automatically restarted after being interrupted by a
 162 signal handler, regardless of the setting of the
 163 .B SA_RESTART
 164 flag when establishing a signal handler.)
 165 .PP
 166 Upon successful completion the message queue data structure is updated
 167 as follows:
 168 .IP \(bu 2
 169 .I msg_lspid
 170 is set to the process ID of the calling process.
 171 .IP \(bu
 172 .I msg_qnum
 173 is incremented by 1.
 174 .IP \(bu
 175 .I msg_stime
 176 is set to the current time.
 177 .SS msgrcv()
 178 The
 179 .BR msgrcv ()
 180 system call removes a message from the queue specified by
 181 .I msqid
 182 and places it in the buffer
 183 pointed to by
 184 .IR msgp .
 185 .PP
 186 The argument
 187 .I msgsz
 188 specifies the maximum size in bytes for the member
 189 .I mtext
 190 of the structure pointed to by the
 191 .I msgp
 192 argument.
 193 If the message text has length greater than
 194 .IR msgsz ,
 195 then the behavior depends on whether
 196 .B MSG_NOERROR
 197 is specified in
 198 .IR msgflg .
 199 If
 200 .B MSG_NOERROR
 201 is specified, then
 202 the message text will be truncated (and the truncated part will be
 203 lost); if
 204 .B MSG_NOERROR
 205 is not specified, then
 206 the message isn't removed from the queue and
 207 the system call fails returning \-1 with
 208 .I errno
 209 set to
 210 .BR E2BIG .
 211 .PP
 212 Unless
 213 .B MSG_COPY
 214 is specified in
 215 .IR msgflg
 216 (see below),
 217 the
 218 .I msgtyp
 219 argument specifies the type of message requested, as follows:
 220 .IP \(bu 2
 221 If
 222 .I msgtyp
 223 is 0,
 224 then the first message in the queue is read.
 225 .IP \(bu
 226 If
 227 .I msgtyp
 228 is greater than 0,
 229 then the first message in the queue of type
 230 .I msgtyp
 231 is read, unless
 232 .B MSG_EXCEPT
 233 was specified in
 234 .IR msgflg ,
 235 in which case
 236 the first message in the queue of type not equal to
 237 .I msgtyp
 238 will be read.
 239 .IP \(bu
 240 If
 241 .I msgtyp
 242 is less than 0,
 243 then the first message in the queue with the lowest type less than or
 244 equal to the absolute value of
 245 .I msgtyp
 246 will be read.
 247 .PP
 248 The
 249 .I msgflg
 250 argument is a bit mask constructed by ORing together zero or more
 251 of the following flags:
 252 .TP
 253 .B IPC_NOWAIT
 254 Return immediately if no message of the requested type is in the queue.
 255 The system call fails with
 256 .I errno
 257 set to
 258 .BR ENOMSG .
 259 .TP
 260 .BR MSG_COPY " (since Linux 3.8)"
 261 .\" commit 4a674f34ba04a002244edaf891b5da7fc1473ae8
 262 Nondestructively fetch a copy of the message at the ordinal position
 263 in the queue specified by
 264 .I msgtyp
 265 (messages are considered to be numbered starting at 0).
 266 .IP
 267 This flag must be specified in conjunction with
 268 .BR IPC_NOWAIT ,
 269 with the result that, if there is no message available at the given position,
 270 the call fails immediately with the error
 271 .BR ENOMSG .
 272 Because they alter the meaning of
 273 .I msgtyp
 274 in orthogonal ways,
 275 .BR MSG_COPY
 276 and
 277 .BR MSG_EXCEPT
 278 may not both be specified in
 279 .IR msgflg .
 280 .IP
 281 The
 282 .BR MSG_COPY
 283 flag was added for the implementation of
 284 the kernel checkpoint-restore facility and
 285 is available only if the kernel was built with the
 286 .B CONFIG_CHECKPOINT_RESTORE
 287 option.
 288 .TP
 289 .B MSG_EXCEPT
 290 Used with
 291 .I msgtyp
 292 greater than 0
 293 to read the first message in the queue with message type that differs
 294 from
 295 .IR msgtyp .
 296 .TP
 297 .B MSG_NOERROR
 298 To truncate the message text if longer than
 299 .I msgsz
 300 bytes.
 301 .PP
 302 If no message of the requested type is available and
 303 .B IPC_NOWAIT
 304 isn't specified in
 305 .IR msgflg ,
 306 the calling process is blocked until one of the following conditions occurs:
 307 .IP \(bu 2
 308 A message of the desired type is placed in the queue.
 309 .IP \(bu
 310 The message queue is removed from the system.
 311 In this case, the system call fails with
 312 .I errno
 313 set to
 314 .BR EIDRM .
 315 .IP \(bu
 316 The calling process catches a signal.
 317 In this case, the system call fails with
 318 .I errno
 319 set to
 320 .BR EINTR .
 321 .RB ( msgrcv ()
 322 is never automatically restarted after being interrupted by a
 323 signal handler, regardless of the setting of the
 324 .B SA_RESTART
 325 flag when establishing a signal handler.)
 326 .PP
 327 Upon successful completion the message queue data structure is updated
 328 as follows:
 329 .IP
 330 .I msg_lrpid
 331 is set to the process ID of the calling process.
 332 .IP
 333 .I msg_qnum
 334 is decremented by 1.
 335 .IP
 336 .I msg_rtime
 337 is set to the current time.
 338 .SH RETURN VALUE
 339 On failure both functions return \-1
 340 with
 341 .I errno
 342 indicating the error,
 343 otherwise
 344 .BR msgsnd ()
 345 returns 0
 346 and
 347 .BR msgrcv ()
 348 returns the number of bytes actually copied into the
 349 .I mtext
 350 array.
 351 .SH ERRORS
 352 When
 353 .BR msgsnd ()
 354 fails,
 355 .I errno
 356 will be set to one among the following values:
 357 .TP
 358 .B EACCES
 359 The calling process does not have write permission on the message queue,
 360 and does not have the
 361 .B CAP_IPC_OWNER
 362 capability in the user namespace that governs its IPC namespace.
 363 .TP
 364 .B EAGAIN
 365 The message can't be sent due to the
 366 .I msg_qbytes
 367 limit for the queue and
 368 .B IPC_NOWAIT
 369 was specified in
 370 .IR msgflg .
 371 .TP
 372 .B EFAULT
 373 The address pointed to by
 374 .I msgp
 375 isn't accessible.
 376 .TP
 377 .B EIDRM
 378 The message queue was removed.
 379 .TP
 380 .B EINTR
 381 Sleeping on a full message queue condition, the process caught a signal.
 382 .TP
 383 .B EINVAL
 384 Invalid
 385 .I msqid
 386 value, or nonpositive
 387 .I mtype
 388 value, or
 389 invalid
 390 .I msgsz
 391 value (less than 0 or greater than the system value
 392 .BR MSGMAX ).
 393 .TP
 394 .B ENOMEM
 395 The system does not have enough memory to make a copy of the
 396 message pointed to by
 397 .IR msgp .
 398 .PP
 399 When
 400 .BR msgrcv ()
 401 fails,
 402 .I errno
 403 will be set to one among the following values:
 404 .TP
 405 .B E2BIG
 406 The message text length is greater than
 407 .I msgsz
 408 and
 409 .B MSG_NOERROR
 410 isn't specified in
 411 .IR msgflg .
 412 .TP
 413 .B EACCES
 414 The calling process does not have read permission on the message queue,
 415 and does not have the
 416 .B CAP_IPC_OWNER
 417 capability in the user namespace that governs its IPC namespace.
 418 .TP
 419 .B EFAULT
 420 The address pointed to by
 421 .I msgp
 422 isn't accessible.
 423 .TP
 424 .B EIDRM
 425 While the process was sleeping to receive a message,
 426 the message queue was removed.
 427 .TP
 428 .B EINTR
 429 While the process was sleeping to receive a message,
 430 the process caught a signal; see
 431 .BR signal (7).
 432 .TP
 433 .B EINVAL
 434 .I msqid
 435 was invalid, or
 436 .I msgsz
 437 was less than 0.
 438 .TP
 439 .BR EINVAL " (since Linux 3.14)"
 440 .I msgflg
 441 specified
 442 .BR MSG_COPY ,
 443 but not
 444 .BR IPC_NOWAIT .
 445 .TP
 446 .BR EINVAL " (since Linux 3.14)"
 447 .I msgflg
 448 specified both
 449 .BR MSG_COPY
 450 and
 451 .BR MSG_EXCEPT .
 452 .TP
 453 .B ENOMSG
 454 .B IPC_NOWAIT
 455 was specified in
 456 .I msgflg
 457 and no message of the requested type existed on the message queue.
 458 .TP
 459 .B ENOMSG
 460 .B IPC_NOWAIT
 461 and
 462 .B MSG_COPY
 463 were specified in
 464 .I msgflg
 465 and the queue contains less than
 466 .I msgtyp
 467 messages.
 468 .TP
 469 .BR ENOSYS " (since Linux 3.8)"
 470 .I MSG_COPY
 471 was specified in
 472 .IR msgflg ,
 473 and this kernel was configured without
 474 .BR CONFIG_CHECKPOINT_RESTORE .
 475 .SH CONFORMING TO
 476 POSIX.1-2001, POSIX.1-2008, SVr4.
 477 .PP
 478 The
 479 .B MSG_EXCEPT
 480 and
 481 .B MSG_COPY
 482 flags are Linux-specific;
 483 their definitions can be obtained by defining the
 484 .B _GNU_SOURCE
 485 .\" MSG_COPY since glibc 2.18
 486 feature test macro.
 487 .SH NOTES
 488 The inclusion of
 489 .I <sys/types.h>
 490 and
 491 .I <sys/ipc.h>
 492 isn't required on Linux or by any version of POSIX.
 493 However,
 494 some old implementations required the inclusion of these header files,
 495 and the SVID also documented their inclusion.
 496 Applications intended to be portable to such old systems may need
 497 to include these header files.
 498 .\" Like Linux, the FreeBSD man pages still document
 499 .\" the inclusion of these header files.
 500 .PP
 501 The
 502 .I msgp
 503 argument is declared as \fIstruct msgbuf\ *\fP in
 504 glibc 2.0 and 2.1.
 505 It is declared as \fIvoid\ *\fP
 506 in glibc 2.2 and later, as required by SUSv2 and SUSv3.
 507 .PP
 508 The following limits on message queue resources affect the
 509 .BR msgsnd ()
 510 call:
 511 .TP
 512 .B MSGMAX
 513 Maximum size of a message text, in bytes (default value: 8192 bytes).
 514 On Linux, this limit can be read and modified via
 515 .IR /proc/sys/kernel/msgmax .
 516 .TP
 517 .B MSGMNB
 518 Maximum number of bytes that can be held in a message queue
 519 (default value: 16384 bytes).
 520 On Linux, this limit can be read and modified via
 521 .IR /proc/sys/kernel/msgmnb .
 522 A privileged process
 523 (Linux: a process with the
 524 .B CAP_SYS_RESOURCE
 525 capability)
 526 can increase the size of a message queue beyond
 527 .B MSGMNB
 528 using the
 529 .BR msgctl (2)
 530 .B IPC_SET
 531 operation.
 532 .PP
 533 The implementation has no intrinsic system-wide limits on the
 534 number of message headers
 535 .RB ( MSGTQL )
 536 and the number of bytes in the message pool
 537 .RB ( MSGPOOL ).
 538 .SH BUGS
 539 In Linux 3.13 and earlier,
 540 if
 541 .BR msgrcv ()
 542 was called with the
 543 .BR MSG_COPY
 544 flag, but without
 545 .BR IPC_NOWAIT ,
 546 and the message queue contained less than
 547 .I msgtyp
 548 messages, then the call would block until the next message is written
 549 to the queue.
 550 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 551 At that point, the call would return a copy of the message,
 552 .I regardless
 553 of whether that message was at the ordinal position
 554 .IR msgtyp .
 555 This bug is fixed
 556 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 557 in Linux 3.14.
 558 .PP
 559 Specifying both
 560 .B MSG_COPY
 561 and
 562 .B MSC_EXCEPT
 563 in
 564 .I msgflg
 565 is a logical error (since these flags impose different interpretations on
 566 .IR msgtyp ).
 567 In Linux 3.13 and earlier,
 568 .\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
 569 this error was not diagnosed by
 570 .BR msgrcv ().
 571 This bug is fixed
 572 .\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
 573 in Linux 3.14.
 574 .SH EXAMPLE
 575 The program below demonstrates the use of
 576 .BR msgsnd ()
 577 and
 578 .BR msgrcv ().
 579 .PP
 580 The example program is first run with the \fB\-s\fP option to send a
 581 message and then run again with the \fB\-r\fP option to receive a
 582 message.
 583 .PP
 584 The following shell session shows a sample run of the program:
 585 .PP
 586 .in +4n
 587 .EX
 588 .RB "$" " ./a.out \-s"
 589 sent: a message at Wed Mar  4 16:25:45 2015
 590
 591 .RB "$" " ./a.out \-r"
 592 message received: a message at Wed Mar  4 16:25:45 2015
 593 .EE
 594 .in
 595 .SS Program source
 596 \&
 597 .EX
 598 #include <stdio.h>
 599 #include <stdlib.h>
 600 #include <string.h>
 601 #include <time.h>
 602 #include <unistd.h>
 603 #include <errno.h>
 604 #include <sys/types.h>
 605 #include <sys/ipc.h>
 606 #include <sys/msg.h>
 607
 608 struct msgbuf {
 609     long mtype;
 610     char mtext[80];
 611 };
 612
 613 static void
 614 usage(char *prog_name, char *msg)
 615 {
 616     if (msg != NULL)
 617         fputs(msg, stderr);
 618
 619     fprintf(stderr, "Usage: %s [options]\en", prog_name);
 620     fprintf(stderr, "Options are:\en");
 621     fprintf(stderr, "\-s        send message using msgsnd()\en");
 622     fprintf(stderr, "\-r        read message using msgrcv()\en");
 623     fprintf(stderr, "\-t        message type (default is 1)\en");
 624     fprintf(stderr, "\-k        message queue key (default is 1234)\en");
 625     exit(EXIT_FAILURE);
 626 }
 627
 628 static void
 629 send_msg(int qid, int msgtype)
 630 {
 631     struct msgbuf msg;
 632     time_t t;
 633
 634     msg.mtype = msgtype;
 635
 636     time(&t);
 637     snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
 638             ctime(&t));
 639
 640     if (msgsnd(qid, (void *) &msg, sizeof(msg.mtext),
 641                 IPC_NOWAIT) == \-1) {
 642         perror("msgsnd error");
 643         exit(EXIT_FAILURE);
 644     }
 645     printf("sent: %s\en", msg.mtext);
 646 }
 647
 648 static void
 649 get_msg(int qid, int msgtype)
 650 {
 651     struct msgbuf msg;
 652
 653     if (msgrcv(qid, (void *) &msg, sizeof(msg.mtext), msgtype,
 654                MSG_NOERROR | IPC_NOWAIT) == \-1) {
 655         if (errno != ENOMSG) {
 656             perror("msgrcv");
 657             exit(EXIT_FAILURE);
 658         }
 659         printf("No message available for msgrcv()\en");
 660     } else
 661         printf("message received: %s\en", msg.mtext);
 662 }
 663
 664 int
 665 main(int argc, char *argv[])
 666 {
 667     int qid, opt;
 668     int mode = 0;               /* 1 = send, 2 = receive */
 669     int msgtype = 1;
 670     int msgkey = 1234;
 671
 672     while ((opt = getopt(argc, argv, "srt:k:")) != \-1) {
 673         switch (opt) {
 674         case \(aqs\(aq:
 675             mode = 1;
 676             break;
 677         case \(aqr\(aq:
 678             mode = 2;
 679             break;
 680         case \(aqt\(aq:
 681             msgtype = atoi(optarg);
 682             if (msgtype <= 0)
 683                 usage(argv[0], "\-t option must be greater than 0\en");
 684             break;
 685         case \(aqk\(aq:
 686             msgkey = atoi(optarg);
 687             break;
 688         default:
 689             usage(argv[0], "Unrecognized option\en");
 690         }
 691     }
 692
 693     if (mode == 0)
 694         usage(argv[0], "must use either \-s or \-r option\en");
 695
 696     qid = msgget(msgkey, IPC_CREAT | 0666);
 697
 698     if (qid == \-1) {
 699         perror("msgget");
 700         exit(EXIT_FAILURE);
 701     }
 702
 703     if (mode == 2)
 704         get_msg(qid, msgtype);
 705     else
 706         send_msg(qid, msgtype);
 707
 708     exit(EXIT_SUCCESS);
 709 }
 710 .EE
 711 .SH SEE ALSO
 712 .BR msgctl (2),
 713 .BR msgget (2),
 714 .BR capabilities (7),
 715 .BR mq_overview (7),
 716 .BR sysvipc (7)