shmop(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 error.

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

ERRORS         top

       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         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 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         top

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

COLOPHON         top

       This page is part of release 5.11 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)

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