man2/shmop.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright 2020 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu)
   7 .\"          with material from Luigi P. Bai (lpb@softint.com)
   8 .\" Portions Copyright 1993 Luigi P. Bai
   9 .\" Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond <esr@thyrsus.com>
  10 .\" Modified, 5 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  11 .\" Modified, 19 Sep 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  12 .\"     Added SHM_REMAP flag description
  13 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  14 .\"     Added notes on capability requirements
  15 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  16 .\"     Language and formatting clean-ups
  17 .\"     Changed wording and placement of sentence regarding attachment
  18 .\"             of segments marked for destruction
  19 .\"
  20 .TH SHMOP 2 (date) "Linux man-pages (unreleased)"
  21 .SH NAME
  22 shmat, shmdt \- System V shared memory operations
  23 .SH LIBRARY
  24 Standard C library
  25 .RI ( libc ", " \-lc )
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <sys/shm.h>
  29 .PP
  30 .BI "void *shmat(int " shmid ", const void *" shmaddr ", int " shmflg );
  31 .BI "int shmdt(const void *" shmaddr );
  32 .fi
  33 .SH DESCRIPTION
  34 .SS shmat()
  35 .BR shmat ()
  36 attaches the System\ V shared memory segment identified by
  37 .I shmid
  38 to the address space of the calling process.
  39 The attaching address is specified by
  40 .I shmaddr
  41 with one of the following criteria:
  42 .IP \(bu 2
  43 If
  44 .I shmaddr
  45 is NULL,
  46 the system chooses a suitable (unused) page-aligned address to attach
  47 the segment.
  48 .IP \(bu
  49 If
  50 .I shmaddr
  51 isn't NULL
  52 and
  53 .B SHM_RND
  54 is specified in
  55 .IR shmflg ,
  56 the attach occurs at the address equal to
  57 .I shmaddr
  58 rounded down to the nearest multiple of
  59 .BR SHMLBA .
  60 .IP \(bu
  61 Otherwise,
  62 .I shmaddr
  63 must be a page-aligned address at which the attach occurs.
  64 .PP
  65 In addition to
  66 .BR SHM_RND ,
  67 the following flags may be specified in the
  68 .I shmflg
  69 bit-mask argument:
  70 .TP
  71 .BR SHM_EXEC " (Linux-specific; since Linux 2.6.9)"
  72 Allow the contents of the segment to be executed.
  73 The caller must have execute permission on the segment.
  74 .TP
  75 .B SHM_RDONLY
  76 Attach the segment for read-only access.
  77 The process must have read permission for the segment.
  78 If this flag is not specified,
  79 the segment is attached for read and write access,
  80 and the process must have read and write permission for the segment.
  81 There is no notion of a write-only shared memory segment.
  82 .TP
  83 .BR SHM_REMAP " (Linux-specific)"
  84 This flag specifies
  85 that the mapping of the segment should replace
  86 any existing mapping in the range starting at
  87 .I shmaddr
  88 and continuing for the size of the segment.
  89 (Normally, an
  90 .B EINVAL
  91 error would result if a mapping already exists in this address range.)
  92 In this case,
  93 .I shmaddr
  94 must not be NULL.
  95 .PP
  96 The
  97 .BR brk (2)
  98 value of the calling process is not altered by the attach.
  99 The segment will automatically be detached at process exit.
 100 The same segment may be attached as a read and as a read-write
 101 one, and more than once, in the process's address space.
 102 .PP
 103 A successful
 104 .BR shmat ()
 105 call updates the members of the
 106 .I shmid_ds
 107 structure (see
 108 .BR shmctl (2))
 109 associated with the shared memory segment as follows:
 110 .IP \(bu 2
 111 .I shm_atime
 112 is set to the current time.
 113 .IP \(bu
 114 .I shm_lpid
 115 is set to the process-ID of the calling process.
 116 .IP \(bu
 117 .I shm_nattch
 118 is incremented by one.
 119 .\"
 120 .SS shmdt()
 121 .BR shmdt ()
 122 detaches the shared memory segment located at the address specified by
 123 .I shmaddr
 124 from the address space of the calling process.
 125 The to-be-detached segment must be currently
 126 attached with
 127 .I shmaddr
 128 equal to the value returned by the attaching
 129 .BR shmat ()
 130 call.
 131 .PP
 132 On a successful
 133 .BR shmdt ()
 134 call, the system updates the members of the
 135 .I shmid_ds
 136 structure associated with the shared memory segment as follows:
 137 .IP \(bu 2
 138 .I shm_dtime
 139 is set to the current time.
 140 .IP \(bu
 141 .I shm_lpid
 142 is set to the process-ID of the calling process.
 143 .IP \(bu
 144 .I shm_nattch
 145 is decremented by one.
 146 If it becomes 0 and the segment is marked for deletion,
 147 the segment is deleted.
 148 .SH RETURN VALUE
 149 On success,
 150 .BR shmat ()
 151 returns the address of the attached shared memory segment; on error,
 152 .I (void\ *)\ \-1
 153 is returned, and
 154 .I errno
 155 is set to indicate the error.
 156 .PP
 157 On success,
 158 .BR shmdt ()
 159 returns 0; on error \-1 is returned, and
 160 .I errno
 161 is set to indicate the error.
 162 .SH ERRORS
 163 .BR shmat ()
 164 can fail with one of the following errors:
 165 .TP
 166 .B EACCES
 167 The calling process does not have the required permissions for
 168 the requested attach type, and does not have the
 169 .B CAP_IPC_OWNER
 170 capability in the user namespace that governs its IPC namespace.
 171 .TP
 172 .B EIDRM
 173 \fIshmid\fP points to a removed identifier.
 174 .TP
 175 .B EINVAL
 176 Invalid
 177 .I shmid
 178 value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not
 179 specified) or invalid
 180 .I shmaddr
 181 value, or can't attach segment at
 182 .IR shmaddr ,
 183 or
 184 .B SHM_REMAP
 185 was specified and
 186 .I shmaddr
 187 was NULL.
 188 .TP
 189 .B ENOMEM
 190 Could not allocate memory for the descriptor or for the page tables.
 191 .PP
 192 .BR shmdt ()
 193 can fail with one of the following errors:
 194 .TP
 195 .B EINVAL
 196 There is no shared memory segment attached at
 197 .IR shmaddr ;
 198 or,
 199 .\" The following since 2.6.17-rc1:
 200 .I shmaddr
 201 is not aligned on a page boundary.
 202 .SH STANDARDS
 203 POSIX.1-2001, POSIX.1-2008, SVr4.
 204 .\" SVr4 documents an additional error condition EMFILE.
 205 .PP
 206 In SVID 3 (or perhaps earlier),
 207 the type of the \fIshmaddr\fP argument was changed from
 208 .I "char\ *"
 209 into
 210 .IR "const void\ *" ,
 211 and the returned type of
 212 .BR shmat ()
 213 from
 214 .I "char\ *"
 215 into
 216 .IR "void\ *" .
 217 .SH NOTES
 218 After a
 219 .BR fork (2),
 220 the child inherits the attached shared memory segments.
 221 .PP
 222 After an
 223 .BR execve (2),
 224 all attached shared memory segments are detached from the process.
 225 .PP
 226 Upon
 227 .BR _exit (2),
 228 all attached shared memory segments are detached from the process.
 229 .PP
 230 Using
 231 .BR shmat ()
 232 with
 233 .I shmaddr
 234 equal to NULL
 235 is the preferred, portable way of attaching a shared memory segment.
 236 Be aware that the shared memory segment attached in this way
 237 may be attached at different addresses in different processes.
 238 Therefore, any pointers maintained within the shared memory must be
 239 made relative (typically to the starting address of the segment),
 240 rather than absolute.
 241 .PP
 242 On Linux, it is possible to attach a shared memory segment even if it
 243 is already marked to be deleted.
 244 However, POSIX.1 does not specify this behavior and
 245 many other implementations do not support it.
 246 .PP
 247 The following system parameter affects
 248 .BR shmat ():
 249 .TP
 250 .B SHMLBA
 251 Segment low boundary address multiple.
 252 When explicitly specifying an attach address in a call to
 253 .BR shmat (),
 254 the caller should ensure that the address is a multiple of this value.
 255 This is necessary on some architectures,
 256 in order either to ensure good CPU cache performance or to ensure that
 257 different attaches of the same segment have consistent views
 258 within the CPU cache.
 259 .B SHMLBA
 260 is normally some multiple of the system page size.
 261 (On many Linux architectures,
 262 .B SHMLBA
 263 is the same as the system page size.)
 264 .PP
 265 The implementation places no intrinsic per-process limit on the
 266 number of shared memory segments
 267 .RB ( SHMSEG ).
 268 .SH EXAMPLES
 269 The two programs shown below exchange a string using a shared memory segment.
 270 Further details about the programs are given below.
 271 First, we show a shell session demonstrating their use.
 272 .PP
 273 In one terminal window, we run the "reader" program,
 274 which creates a System V shared memory segment and a System V semaphore set.
 275 The program prints out the IDs of the created objects,
 276 and then waits for the semaphore to change value.
 277 .PP
 278 .in +4n
 279 .EX
 280 $ \fB./svshm_string_read\fP
 281 shmid = 1114194; semid = 15
 282 .EE
 283 .in
 284 .PP
 285 In another terminal window, we run the "writer" program.
 286 The "writer" program takes three command-line arguments:
 287 the IDs of the shared memory segment and semaphore set created
 288 by the "reader", and a string.
 289 It attaches the existing shared memory segment,
 290 copies the string to the shared memory, and modifies the semaphore value.
 291 .PP
 292 .in +4n
 293 .EX
 294 $ \fB./svshm_string_write 1114194 15 \(aqHello, world\(aq\fP
 295 .EE
 296 .in
 297 .PP
 298 Returning to the terminal where the "reader" is running,
 299 we see that the program has ceased waiting on the semaphore
 300 and has printed the string that was copied into the
 301 shared memory segment by the writer:
 302 .PP
 303 .in +4n
 304 .EX
 305 Hello, world
 306 .EE
 307 .in
 308 .\"
 309 .SS Program source: svshm_string.h
 310 The following header file is included by the "reader" and "writer" programs:
 311 .PP
 312 .in +4n
 313 .\" SRC BEGIN (svshm_string.h)
 314 .EX
 315 /* svshm_string.h
 316
 317    Licensed under GNU General Public License v2 or later.
 318 */
 319 #include <sys/types.h>
 320 #include <sys/ipc.h>
 321 #include <sys/shm.h>
 322 #include <sys/sem.h>
 323 #include <stdio.h>
 324 #include <stdlib.h>
 325 #include <string.h>
 326
 327 #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
 328                         } while (0)
 329
 330 union semun {                   /* Used in calls to semctl() */
 331     int                 val;
 332     struct semid_ds *   buf;
 333     unsigned short *    array;
 334 #if defined(__linux__)
 335     struct seminfo *    __buf;
 336 #endif
 337 };
 338
 339 #define MEM_SIZE 4096
 340 .EE
 341 .\" SRC END
 342 .in
 343 .\"
 344 .SS Program source: svshm_string_read.c
 345 The "reader" program creates a shared memory segment and a semaphore set
 346 containing one semaphore.
 347 It then attaches the shared memory object into its address space
 348 and initializes the semaphore value to 1.
 349 Finally, the program waits for the semaphore value to become 0,
 350 and afterwards prints the string that has been copied into the
 351 shared memory segment by the "writer".
 352 .PP
 353 .in +4n
 354 .\" SRC BEGIN (svshm_string_read.c)
 355 .EX
 356 /* svshm_string_read.c
 357
 358    Licensed under GNU General Public License v2 or later.
 359 */
 360 #include <stdio.h>
 361 #include <stdlib.h>
 362 #include <sys/ipc.h>
 363 #include <sys/sem.h>
 364 #include <sys/shm.h>
 365
 366 #include "svshm_string.h"
 367
 368 int
 369 main(void)
 370 {
 371     int            semid, shmid;
 372     char           *addr;
 373     union semun    arg, dummy;
 374     struct sembuf  sop;
 375
 376     /* Create shared memory and semaphore set containing one
 377        semaphore. */
 378
 379     shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
 380     if (shmid == \-1)
 381         errExit("shmget");
 382
 383     semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
 384     if (semid == \-1)
 385         errExit("semget");
 386
 387     /* Attach shared memory into our address space. */
 388
 389     addr = shmat(shmid, NULL, SHM_RDONLY);
 390     if (addr == (void *) \-1)
 391         errExit("shmat");
 392
 393     /* Initialize semaphore 0 in set with value 1. */
 394
 395     arg.val = 1;
 396     if (semctl(semid, 0, SETVAL, arg) == \-1)
 397         errExit("semctl");
 398
 399     printf("shmid = %d; semid = %d\en", shmid, semid);
 400
 401     /* Wait for semaphore value to become 0. */
 402
 403     sop.sem_num = 0;
 404     sop.sem_op = 0;
 405     sop.sem_flg = 0;
 406
 407     if (semop(semid, &sop, 1) == \-1)
 408         errExit("semop");
 409
 410     /* Print the string from shared memory. */
 411
 412     printf("%s\en", addr);
 413
 414     /* Remove shared memory and semaphore set. */
 415
 416     if (shmctl(shmid, IPC_RMID, NULL) == \-1)
 417         errExit("shmctl");
 418     if (semctl(semid, 0, IPC_RMID, dummy) == \-1)
 419         errExit("semctl");
 420
 421     exit(EXIT_SUCCESS);
 422 }
 423 .EE
 424 .\" SRC END
 425 .in
 426 .\"
 427 .SS Program source: svshm_string_write.c
 428 The writer program takes three command-line arguments:
 429 the IDs of the shared memory segment and semaphore set
 430 that have already been created by the "reader", and a string.
 431 It attaches the shared memory segment into its address space,
 432 and then decrements the semaphore value to 0 in order to inform the
 433 "reader" that it can now examine the contents of the shared memory.
 434 .PP
 435 .in +4n
 436 .\" SRC BEGIN (svshm_string_write.c)
 437 .EX
 438 /* svshm_string_write.c
 439
 440    Licensed under GNU General Public License v2 or later.
 441 */
 442 #include <stdio.h>
 443 #include <stdlib.h>
 444 #include <string.h>
 445 #include <sys/sem.h>
 446 #include <sys/shm.h>
 447
 448 #include "svshm_string.h"
 449
 450 int
 451 main(int argc, char *argv[])
 452 {
 453     int            semid, shmid;
 454     char           *addr;
 455     size_t         len;
 456     struct sembuf  sop;
 457
 458     if (argc != 4) {
 459         fprintf(stderr, "Usage: %s shmid semid string\en", argv[0]);
 460         exit(EXIT_FAILURE);
 461     }
 462
 463     len = strlen(argv[3]) + 1;  /* +1 to include trailing \(aq\e0\(aq */
 464     if (len > MEM_SIZE) {
 465         fprintf(stderr, "String is too big!\en");
 466         exit(EXIT_FAILURE);
 467     }
 468
 469     /* Get object IDs from command\-line. */
 470
 471     shmid = atoi(argv[1]);
 472     semid = atoi(argv[2]);
 473
 474     /* Attach shared memory into our address space and copy string
 475        (including trailing null byte) into memory. */
 476
 477     addr = shmat(shmid, NULL, 0);
 478     if (addr == (void *) \-1)
 479         errExit("shmat");
 480
 481     memcpy(addr, argv[3], len);
 482
 483     /* Decrement semaphore to 0. */
 484
 485     sop.sem_num = 0;
 486     sop.sem_op = \-1;
 487     sop.sem_flg = 0;
 488
 489     if (semop(semid, &sop, 1) == \-1)
 490         errExit("semop");
 491
 492     exit(EXIT_SUCCESS);
 493 }
 494 .EE
 495 .\" SRC END
 496 .in
 497 .SH SEE ALSO
 498 .BR brk (2),
 499 .BR mmap (2),
 500 .BR shmctl (2),
 501 .BR shmget (2),
 502 .BR capabilities (7),
 503 .BR shm_overview (7),
 504 .BR sysvipc (7)