man2/shmop.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright 2020 Michael Kerrisk <mtk.manpages@gmail.com>
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  25 .\"
  26 .\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu)
  27 .\"          with material from Luigi P. Bai (lpb@softint.com)
  28 .\" Portions Copyright 1993 Luigi P. Bai
  29 .\" Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond <esr@thyrsus.com>
  30 .\" Modified, 5 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  31 .\" Modified, 19 Sep 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\"     Added SHM_REMAP flag description
  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 .\"     Changed wording and placement of sentence regarding attachment
  38 .\"             of segments marked for destruction
  39 .\"
  40 .TH SHMOP 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  41 .SH NAME
  42 shmat, shmdt \- System V shared memory operations
  43 .SH SYNOPSIS
  44 .nf
  45 .B #include <sys/types.h>
  46 .B #include <sys/shm.h>
  47 .PP
  48 .BI "void *shmat(int " shmid ", const void *" shmaddr ", int " shmflg );
  49 .PP
  50 .BI "int shmdt(const void *" shmaddr );
  51 .fi
  52 .SH DESCRIPTION
  53 .SS shmat()
  54 .BR shmat ()
  55 attaches the System\ V shared memory segment identified by
  56 .I shmid
  57 to the address space of the calling process.
  58 The attaching address is specified by
  59 .I shmaddr
  60 with one of the following criteria:
  61 .IP \(bu 2
  62 If
  63 .I shmaddr
  64 is NULL,
  65 the system chooses a suitable (unused) page-aligned address to attach
  66 the segment.
  67 .IP \(bu
  68 If
  69 .I shmaddr
  70 isn't NULL
  71 and
  72 .B SHM_RND
  73 is specified in
  74 .IR shmflg ,
  75 the attach occurs at the address equal to
  76 .I shmaddr
  77 rounded down to the nearest multiple of
  78 .BR SHMLBA .
  79 .IP \(bu
  80 Otherwise,
  81 .I shmaddr
  82 must be a page-aligned address at which the attach occurs.
  83 .PP
  84 In addition to
  85 .BR SHM_RND ,
  86 the following flags may be specified in the
  87 .I shmflg
  88 bit-mask argument:
  89 .TP
  90 .BR SHM_EXEC " (Linux-specific; since Linux 2.6.9)"
  91 Allow the contents of the segment to be executed.
  92 The caller must have execute permission on the segment.
  93 .TP
  94 .BR SHM_RDONLY
  95 Attach the segment for read-only access.
  96 The process must have read permission for the segment.
  97 If this flag is not specified,
  98 the segment is attached for read and write access,
  99 and the process must have read and write permission for the segment.
 100 There is no notion of a write-only shared memory segment.
 101 .TP
 102 .BR SHM_REMAP " (Linux-specific)"
 103 This flag specifies
 104 that the mapping of the segment should replace
 105 any existing mapping in the range starting at
 106 .I shmaddr
 107 and continuing for the size of the segment.
 108 (Normally, an
 109 .B EINVAL
 110 error would result if a mapping already exists in this address range.)
 111 In this case,
 112 .I shmaddr
 113 must not be NULL.
 114 .PP
 115 The
 116 .BR brk (2)
 117 value of the calling process is not altered by the attach.
 118 The segment will automatically be detached at process exit.
 119 The same segment may be attached as a read and as a read-write
 120 one, and more than once, in the process's address space.
 121 .PP
 122 A successful
 123 .BR shmat ()
 124 call updates the members of the
 125 .I shmid_ds
 126 structure (see
 127 .BR shmctl (2))
 128 associated with the shared memory segment as follows:
 129 .IP \(bu 2
 130 .I shm_atime
 131 is set to the current time.
 132 .IP \(bu
 133 .I shm_lpid
 134 is set to the process-ID of the calling process.
 135 .IP \(bu
 136 .I shm_nattch
 137 is incremented by one.
 138 .\"
 139 .SS shmdt()
 140 .BR shmdt ()
 141 detaches the shared memory segment located at the address specified by
 142 .I shmaddr
 143 from the address space of the calling process.
 144 The to-be-detached segment must be currently
 145 attached with
 146 .I shmaddr
 147 equal to the value returned by the attaching
 148 .BR shmat ()
 149 call.
 150 .PP
 151 On a successful
 152 .BR shmdt ()
 153 call, the system updates the members of the
 154 .I shmid_ds
 155 structure associated with the shared memory segment as follows:
 156 .IP \(bu 2
 157 .I shm_dtime
 158 is set to the current time.
 159 .IP \(bu
 160 .I shm_lpid
 161 is set to the process-ID of the calling process.
 162 .IP \(bu
 163 .I shm_nattch
 164 is decremented by one.
 165 If it becomes 0 and the segment is marked for deletion,
 166 the segment is deleted.
 167 .SH RETURN VALUE
 168 On success,
 169 .BR shmat ()
 170 returns the address of the attached shared memory segment; on error,
 171 .I (void\ *)\ \-1
 172 is returned, and
 173 .I errno
 174 is set to indicate the cause of the error.
 175 .PP
 176 On success,
 177 .BR shmdt ()
 178 returns 0; on error \-1 is returned, and
 179 .I errno
 180 is set to indicate the cause of the error.
 181 .SH ERRORS
 182 When
 183 .BR shmat ()
 184 fails,
 185 .I errno
 186 is set to one of the following:
 187 .TP
 188 .B EACCES
 189 The calling process does not have the required permissions for
 190 the requested attach type, and does not have the
 191 .B CAP_IPC_OWNER
 192 capability in the user namespace that governs its IPC namespace.
 193 .TP
 194 .B EIDRM
 195 \fIshmid\fP points to a removed identifier.
 196 .TP
 197 .B EINVAL
 198 Invalid
 199 .I shmid
 200 value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not
 201 specified) or invalid
 202 .I shmaddr
 203 value, or can't attach segment at
 204 .IR shmaddr ,
 205 or
 206 .B SHM_REMAP
 207 was specified and
 208 .I shmaddr
 209 was NULL.
 210 .TP
 211 .B ENOMEM
 212 Could not allocate memory for the descriptor or for the page tables.
 213 .PP
 214 When
 215 .BR shmdt ()
 216 fails,
 217 .I errno
 218 is set as follows:
 219 .TP
 220 .B EINVAL
 221 There is no shared memory segment attached at
 222 .IR shmaddr ;
 223 or,
 224 .\" The following since 2.6.17-rc1:
 225 .I shmaddr
 226 is not aligned on a page boundary.
 227 .SH CONFORMING TO
 228 POSIX.1-2001, POSIX.1-2008, SVr4.
 229 .\" SVr4 documents an additional error condition EMFILE.
 230 .PP
 231 In SVID 3 (or perhaps earlier),
 232 the type of the \fIshmaddr\fP argument was changed from
 233 .I "char\ *"
 234 into
 235 .IR "const void\ *" ,
 236 and the returned type of
 237 .BR shmat ()
 238 from
 239 .I "char\ *"
 240 into
 241 .IR "void\ *" .
 242 .SH NOTES
 243 .PP
 244 After a
 245 .BR fork (2),
 246 the child inherits the attached shared memory segments.
 247 .PP
 248 After an
 249 .BR execve (2),
 250 all attached shared memory segments are detached from the process.
 251 .PP
 252 Upon
 253 .BR _exit (2),
 254 all attached shared memory segments are detached from the process.
 255 .PP
 256 Using
 257 .BR shmat ()
 258 with
 259 .I shmaddr
 260 equal to NULL
 261 is the preferred, portable way of attaching a shared memory segment.
 262 Be aware that the shared memory segment attached in this way
 263 may be attached at different addresses in different processes.
 264 Therefore, any pointers maintained within the shared memory must be
 265 made relative (typically to the starting address of the segment),
 266 rather than absolute.
 267 .PP
 268 On Linux, it is possible to attach a shared memory segment even if it
 269 is already marked to be deleted.
 270 However, POSIX.1 does not specify this behavior and
 271 many other implementations do not support it.
 272 .PP
 273 The following system parameter affects
 274 .BR shmat ():
 275 .TP
 276 .B SHMLBA
 277 Segment low boundary address multiple.
 278 When explicitly specifying an attach address in a call to
 279 .BR shmat (),
 280 the caller should ensure that the address is a multiple of this value.
 281 This is necessary on some architectures,
 282 in order either to ensure good CPU cache performance or to ensure that
 283 different attaches of the same segment have consistent views
 284 within the CPU cache.
 285 .B SHMLBA
 286 is normally some multiple of the system page size.
 287 (On many Linux architectures,
 288 .B SHMLBA
 289 is the same as the system page size.)
 290 .PP
 291 The implementation places no intrinsic per-process limit on the
 292 number of shared memory segments
 293 .RB ( SHMSEG ).
 294 .SH EXAMPLE
 295 .PP
 296 The two programs shown below exchange a string using a shared memory segment.
 297 Further details about the programs are given below.
 298 First, we show a shell session demonstrating their use.
 299 .PP
 300 In one terminal window, we run the "reader" program,
 301 which creates a System V shared memory segment and a System V semaphore set.
 302 The program prints out the IDs of the created objects,
 303 and then waits for the semaphore to change value.
 304 .PP
 305 .in +4n
 306 .EX
 307 $ \fB./svshm_string_read \fP
 308 shmid = 1114194; semid = 15
 309 .EE
 310 .in
 311 .PP
 312 In another terminal window, we run the "writer" program.
 313 The "writer" program takes three command-line arguments:
 314 the IDs of the shared memory segment and semaphore set created
 315 by the "reader", and a string.
 316 It attaches the existing shared memory segment,
 317 copies the string to the shared memory, and modifies the semaphore value.
 318 .PP
 319 .in +4n
 320 .EX
 321 $ \fB./svshm_string_write 1114194 15 \(aqHello, world\(aq\fP
 322 .EE
 323 .in
 324 .PP
 325 Returning to the terminal where the "reader" is running,
 326 we see that the program has ceased waiting on the semaphore
 327 and has printed the string that was copied into the
 328 shared memory segment by the writer:
 329 .PP
 330 .in +4n
 331 .EX
 332 Hello, world
 333 .EE
 334 .in
 335 .PP
 336 .\"
 337 .SS Program source: svshm_string.h
 338 The following header file is included by the "reader" and "writer" programs.
 339 .PP
 340 .in +4n
 341 .EX
 342 #include <sys/types.h>
 343 #include <sys/ipc.h>
 344 #include <sys/shm.h>
 345 #include <sys/sem.h>
 346 #include <stdio.h>
 347 #include <stdlib.h>
 348 #include <string.h>
 349
 350 #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
 351                         } while (0)
 352
 353 union semun {                   /* Used in calls to semctl() */
 354     int                 val;
 355     struct semid_ds *   buf;
 356     unsigned short *    array;
 357 #if defined(__linux__)
 358     struct seminfo *    __buf;
 359 #endif
 360 };
 361
 362 #define MEM_SIZE 4096
 363 .EE
 364 .in
 365 .\"
 366 .SS Program source: svshm_string_read.c
 367 The "reader" program creates a shared memory segment and a semaphore set
 368 containing one semaphore.
 369 It then attaches the shared memory object into its address space
 370 and initializes the semaphore value to 1.
 371 Finally, the program waits for the semaphore value to become 0,
 372 and afterwards prints the string that has been copied into the
 373 shared memory segment by the "writer".
 374 .PP
 375 .in +4n
 376 .EX
 377 /* svshm_string_read.c
 378
 379    Licensed under GNU General Public License v2 or later.
 380 */
 381 #include "svshm_string.h"
 382
 383 int
 384 main(int argc, char *argv[])
 385 {
 386     int semid, shmid;
 387     union semun arg, dummy;
 388     struct sembuf sop;
 389     char *addr;
 390
 391     /* Create shared memory and semaphore set containing one
 392        semaphore */
 393
 394     shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
 395     if (shmid == \-1)
 396         errExit("shmget");
 397
 398     semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
 399     if (shmid == \-1)
 400         errExit("shmget");
 401
 402     /* Attach shared memory into our address space */
 403
 404     addr = shmat(shmid, NULL, SHM_RDONLY);
 405     if (addr == (void *) \-1)
 406         errExit("shmat");
 407
 408     /* Initialize semaphore 0 in set with value 1 */
 409
 410     arg.val = 1;
 411     if (semctl(semid, 0, SETVAL, arg) == \-1)
 412         errExit("semctl");
 413
 414     printf("shmid = %d; semid = %d\en", shmid, semid);
 415
 416     /* Wait for semaphore value to become 0 */
 417
 418     sop.sem_num = 0;
 419     sop.sem_op = 0;
 420     sop.sem_flg = 0;
 421
 422     if (semop(semid, &sop, 1) == \-1)
 423         errExit("semop");
 424
 425     /* Print the string from shared memory */
 426
 427     printf("%s\en", addr);
 428
 429     /* Remove shared memory and semaphore set */
 430
 431     if (shmctl(shmid, IPC_RMID, NULL) == \-1)
 432         errExit("shmctl");
 433     if (semctl(semid, 0, IPC_RMID, dummy) == \-1)
 434         errExit("semctl");
 435
 436     exit(EXIT_SUCCESS);
 437 }
 438 .EE
 439 .in
 440 .\"
 441 .SS Program source: svshm_string_write.c
 442 The writer program takes three command-line arguments:
 443 the IDs of the shared memory segment and semaphore set
 444 that have already been created by the "reader", and a string.
 445 It attaches the shared memory segment into its address space,
 446 and then decrements the semaphore value to 0 in order to inform the
 447 "reader" that it can now examine the contents of the shared memory.
 448 .PP
 449 .in +4n
 450 .EX
 451 /* svshm_string_write.c
 452
 453    Licensed under GNU General Public License v2 or later.
 454 */
 455 #include "svshm_string.h"
 456
 457 int
 458 main(int argc, char *argv[])
 459 {
 460     int semid, shmid;
 461     struct sembuf sop;
 462     char *addr;
 463     size_t len;
 464
 465     if (argc != 4) {
 466         fprintf(stderr, "Usage: %s shmid semid string\en", argv[0]);
 467         exit(EXIT_FAILURE);
 468     }
 469
 470     len = strlen(argv[3]) + 1;  /* +1 to include trailing \(aq\e0\(aq */
 471     if (len > MEM_SIZE) {
 472         fprintf(stderr, "String is too big!\en");
 473         exit(EXIT_FAILURE);
 474     }
 475
 476     /* Get object IDs from command\-line */
 477
 478     shmid = atoi(argv[1]);
 479     semid = atoi(argv[2]);
 480
 481     /* Attach shared memory into our address space and copy string
 482        (including trailing null byte) into memory. */
 483
 484     addr = shmat(shmid, NULL, 0);
 485     if (addr == (void *) \-1)
 486         errExit("shmat");
 487
 488     memcpy(addr, argv[3], len);
 489
 490     /* Decrement semaphore t0 0 */
 491
 492     sop.sem_num = 0;
 493     sop.sem_op = \-1;
 494     sop.sem_flg = 0;
 495
 496     if (semop(semid, &sop, 1) == \-1)
 497         errExit("semop");
 498
 499     exit(EXIT_SUCCESS);
 500 }
 501 .EE
 502 .in
 503 .SH SEE ALSO
 504 .BR brk (2),
 505 .BR mmap (2),
 506 .BR shmctl (2),
 507 .BR shmget (2),
 508 .BR capabilities (7),
 509 .BR shm_overview (7),
 510 .BR sysvipc (7)