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NAME

       shmat, shmdt - System V shared memory operations

SYNOPSIS

       #include <sys/types.h>
       #include <sys/shm.h>

       void *shmat(int shmid, const void *shmaddr, int shmflg);

       int shmdt(const void *shmaddr);

DESCRIPTION

   shmat()
       shmat()  attaches  the  System V  shared memory segment identified by shmid to the address
       space of the calling process.  The attaching address is specified by shmaddr with  one  of
       the following criteria:

       · If  shmaddr  is  NULL,  the  system  chooses a suitable (unused) page-aligned address to
         attach the segment.

       · If shmaddr isn't NULL and SHM_RND is specified in  shmflg,  the  attach  occurs  at  the
         address equal to shmaddr rounded down to the nearest multiple of SHMLBA.

       · Otherwise, shmaddr must be a page-aligned address at which the attach occurs.

       In  addition  to  SHM_RND,  the  following  flags  may be specified in the shmflg bit-mask
       argument:

       SHM_EXEC (Linux-specific; since Linux 2.6.9)
              Allow the contents of the segment to be executed.  The  caller  must  have  execute
              permission on the segment.

       SHM_RDONLY
              Attach the segment for read-only access.  The process must have read permission for
              the segment.  If this flag is not specified, the segment is attached for  read  and
              write  access, and the process must have read and write permission for the segment.
              There is no notion of a write-only shared memory segment.

       SHM_REMAP (Linux-specific)
              This flag specifies that the mapping of the segment  should  replace  any  existing
              mapping  in  the  range  starting  at  shmaddr  and  continuing for the size of the
              segment.  (Normally, an EINVAL error would result if a mapping  already  exists  in
              this address range.)  In this case, shmaddr must not be NULL.

       The  brk(2)  value  of the calling process is not altered by the attach.  The segment will
       automatically be detached at process exit.  The same segment may be attached as a read and
       as a read-write one, and more than once, in the process's address space.

       A  successful  shmat()  call updates the members of the shmid_ds structure (see shmctl(2))
       associated with the shared memory segment as follows:

       · shm_atime is set to the current time.

       · shm_lpid is set to the process-ID of the calling process.

       · shm_nattch is incremented by one.

   shmdt()
       shmdt() detaches the shared memory segment located at the  address  specified  by  shmaddr
       from  the  address  space  of  the  calling  process.   The to-be-detached segment must be
       currently attached with shmaddr equal to the value returned by the attaching shmat() call.

       On a successful shmdt() call, the system updates the members  of  the  shmid_ds  structure
       associated with the shared memory segment as follows:

       · shm_dtime is set to the current time.

       · shm_lpid is set to the process-ID of the calling process.

       · shm_nattch  is  decremented  by  one.   If  it  becomes  0 and the segment is marked for
         deletion, the segment is deleted.

RETURN VALUE

       On success, shmat() returns the address of the attached shared memory segment;  on  error,
       (void *) -1 is returned, and errno is set to indicate the cause of the error.

       On  success,  shmdt() returns 0; on error -1 is returned, and errno is set to indicate the
       cause of the error.

ERRORS

       When shmat() fails, errno is set to one of the following:

       EACCES The calling process does not have the required permissions for the requested attach
              type,  and  does  not  have the CAP_IPC_OWNER capability in the user namespace that
              governs its IPC namespace.

       EIDRM  shmid points to a removed identifier.

       EINVAL Invalid shmid  value,  unaligned  (i.e.,  not  page-aligned  and  SHM_RND  was  not
              specified)  or  invalid  shmaddr  value,  or  can't  attach  segment at shmaddr, or
              SHM_REMAP was specified and shmaddr was NULL.

       ENOMEM Could not allocate memory for the descriptor or for the page tables.

       When shmdt() fails, errno is set as follows:

       EINVAL There is no shared memory segment attached at shmaddr; or, shmaddr is  not  aligned
              on a page boundary.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4.

       In  SVID  3 (or perhaps earlier), the type of the shmaddr argument was changed from char *
       into const void *, and the returned type of shmat() from char * into void *.

NOTES

       After a fork(2), the child inherits the attached shared memory segments.

       After an execve(2), all attached shared memory segments are detached from the process.

       Upon _exit(2), all attached shared memory segments are detached from the process.

       Using shmat() with shmaddr equal to NULL is the preferred, portable  way  of  attaching  a
       shared  memory  segment.  Be aware that the shared memory segment attached in this way may
       be attached at different  addresses  in  different  processes.   Therefore,  any  pointers
       maintained  within  the  shared  memory  must  be made relative (typically to the starting
       address of the segment), rather than absolute.

       On Linux, it is possible to attach a shared memory segment even if it is already marked to
       be   deleted.    However,   POSIX.1   does  not  specify  this  behavior  and  many  other
       implementations do not support it.

       The following system parameter affects shmat():

       SHMLBA Segment low boundary  address  multiple.   When  explicitly  specifying  an  attach
              address  in  a  call  to  shmat(),  the  caller should ensure that the address is a
              multiple of this value.  This is necessary on some architectures, in  order  either
              to  ensure  good  CPU cache performance or to ensure that different attaches of the
              same segment have consistent views within the CPU cache.  SHMLBA is  normally  some
              multiple of the system page size.  (On many Linux architectures, SHMLBA is the same
              as the system page size.)

       The implementation places no intrinsic per-process limit on the number  of  shared  memory
       segments (SHMSEG).

EXAMPLES

       The  two  programs  shown  below exchange a string using a shared memory segment.  Further
       details about the programs are given below.  First, we show a shell session  demonstrating
       their use.

       In  one  terminal  window,  we  run  the "reader" program, which creates a System V shared
       memory segment and a System V semaphore set.  The  program  prints  out  the  IDs  of  the
       created objects, and then waits for the semaphore to change value.

           $ ./svshm_string_read
           shmid = 1114194; semid = 15

       In another terminal window, we run the "writer" program.  The "writer" program takes three
       command-line arguments: the IDs of the shared memory segment and semaphore set created  by
       the  "reader",  and  a string.  It attaches the existing shared memory segment, copies the
       string to the shared memory, and modifies the semaphore value.

           $ ./svshm_string_write 1114194 15 'Hello, world'

       Returning to the terminal where the "reader" is running,  we  see  that  the  program  has
       ceased waiting on the semaphore and has printed the string that was copied into the shared
       memory segment by the writer:

           Hello, world

   Program source: svshm_string.h
       The following header file is included by the "reader" and "writer" programs.

           #include <sys/types.h>
           #include <sys/ipc.h>
           #include <sys/shm.h>
           #include <sys/sem.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>

           #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                                   } while (0)

           union semun {                   /* Used in calls to semctl() */
               int                 val;
               struct semid_ds *   buf;
               unsigned short *    array;
           #if defined(__linux__)
               struct seminfo *    __buf;
           #endif
           };

           #define MEM_SIZE 4096

   Program source: svshm_string_read.c
       The "reader" program creates a shared memory segment and a semaphore  set  containing  one
       semaphore.   It  then  attaches  the  shared  memory  object  into  its  address space and
       initializes the semaphore value to 1.  Finally, the program waits for the semaphore  value
       to  become 0, and afterwards prints the string that has been copied into the shared memory
       segment by the "writer".

           /* svshm_string_read.c

              Licensed under GNU General Public License v2 or later.
           */
           #include "svshm_string.h"

           int
           main(int argc, char *argv[])
           {
               int semid, shmid;
               union semun arg, dummy;
               struct sembuf sop;
               char *addr;

               /* Create shared memory and semaphore set containing one
                  semaphore */

               shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
               if (shmid == -1)
                   errExit("shmget");

               semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
               if (shmid == -1)
                   errExit("shmget");

               /* Attach shared memory into our address space */

               addr = shmat(shmid, NULL, SHM_RDONLY);
               if (addr == (void *) -1)
                   errExit("shmat");

               /* Initialize semaphore 0 in set with value 1 */

               arg.val = 1;
               if (semctl(semid, 0, SETVAL, arg) == -1)
                   errExit("semctl");

               printf("shmid = %d; semid = %d\n", shmid, semid);

               /* Wait for semaphore value to become 0 */

               sop.sem_num = 0;
               sop.sem_op = 0;
               sop.sem_flg = 0;

               if (semop(semid, &sop, 1) == -1)
                   errExit("semop");

               /* Print the string from shared memory */

               printf("%s\n", addr);

               /* Remove shared memory and semaphore set */

               if (shmctl(shmid, IPC_RMID, NULL) == -1)
                   errExit("shmctl");
               if (semctl(semid, 0, IPC_RMID, dummy) == -1)
                   errExit("semctl");

               exit(EXIT_SUCCESS);
           }

   Program source: svshm_string_write.c
       The writer program takes three command-line  arguments:  the  IDs  of  the  shared  memory
       segment  and  semaphore  set that have already been created by the "reader", and a string.
       It attaches the shared memory segment into its address  space,  and  then  decrements  the
       semaphore  value to 0 in order to inform the "reader" that it can now examine the contents
       of the shared memory.

           /* svshm_string_write.c

              Licensed under GNU General Public License v2 or later.
           */
           #include "svshm_string.h"

           int
           main(int argc, char *argv[])
           {
               int semid, shmid;
               struct sembuf sop;
               char *addr;
               size_t len;

               if (argc != 4) {
                   fprintf(stderr, "Usage: %s shmid semid string\n", argv[0]);
                   exit(EXIT_FAILURE);
               }

               len = strlen(argv[3]) + 1;  /* +1 to include trailing '\0' */
               if (len > MEM_SIZE) {
                   fprintf(stderr, "String is too big!\n");
                   exit(EXIT_FAILURE);
               }

               /* Get object IDs from command-line */

               shmid = atoi(argv[1]);
               semid = atoi(argv[2]);

               /* Attach shared memory into our address space and copy string
                  (including trailing null byte) into memory. */

               addr = shmat(shmid, NULL, 0);
               if (addr == (void *) -1)
                   errExit("shmat");

               memcpy(addr, argv[3], len);

               /* Decrement semaphore to 0 */

               sop.sem_num = 0;
               sop.sem_op = -1;
               sop.sem_flg = 0;

               if (semop(semid, &sop, 1) == -1)
                   errExit("semop");

               exit(EXIT_SUCCESS);
           }

SEE ALSO

       brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), sysvipc(7)

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.