shmdt

SHMOP(2)                    Linux Programmer's Manual                   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 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 error.

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

ERRORS
       shmat() can fail with one of the following errors:

       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.

       shmdt() errno can fail with one of the following errors:

       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
       This page is part of release 5.11 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.




Linux                              2021-03-22                           SHMOP(2)