shmdt(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

SHMOP(2)                  Linux Programmer's Manual                 SHMOP(2)

NAME         top

       shmat, shmdt - System V shared memory operations

SYNOPSIS         top

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

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

       int shmdt(const void *shmaddr);

DESCRIPTION         top

   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         top

       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         top

       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         top

       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         top

       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         top

       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 cre‐
       ated 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 t0 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         top

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

COLOPHON         top

       This page is part of release 5.07 of the Linux man-pages project.  A
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Linux                            2020-04-11                         SHMOP(2)

Pages that refer to this page: ipcrm(1)ipcs(1)lsipc(1)pcp-ipcs(1)ipc(2)shmctl(2)shmget(2)syscalls(2)svipc(7)sysvipc(7)