SHMOP(2)                  (2020-04-11)                   SHMOP(2)

     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 identi-
          fied by shmid to the address space of the calling process.
          The attaching address is specified by shmaddr with one of
          the following criteria:

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

          +o 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.

          +o 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 start-
               ing at shmaddr and continuing for the size of the seg-
               ment.  (Normally, an EINVAL error would result if a
               mapping already exists in this address range.)  In this
               case, shmaddr must not be NULL.

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          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:

          +o shm_atime is set to the current time.

          +o shm_lpid is set to the process-ID of the calling process.

          +o 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 cur-
          rently 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:

          +o shm_dtime is set to the current time.

          +o shm_lpid is set to the process-ID of the calling process.

          +o 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 permis-
               sions for the requested attach type, and does not have
               the CAP_IPC_OWNER capability in the user namespace that
               governs its IPC namespace.

          EIDRM

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               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 mem-
          ory 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 imple-
          mentations do not support it.

          The following system parameter affects shmat():

          SHMLBA
               Segment low boundary address multiple.  When explicitly

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               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 demonstrat-
          ing 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 cre-
          ated 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 mem-
          ory, and modifies the semaphore value.

              $ ./svshm_string_write 1114194 15 aqHello, worldaq

          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 mem-
          ory 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>

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              #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 initial-
          izes 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 seg-
          ment 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");

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                  /* 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"

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              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 aq\0aq */
                  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
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about

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

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