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Annotation of coherent/a/usr/man/COHERENT/shm, revision 1.1

1.1     ! root        1: 
        !             2: 
        !             3: shm                       Device Driver                       shm
        !             4: 
        !             5: 
        !             6: 
        !             7: 
        !             8: Shared memory device driver
        !             9: 
        !            10: 
        !            11: The device  /dev/shm is an interface to  the shared memory device
        !            12: driver.  It is assigned major  device 24 (minor device 0) and can
        !            13: be accessed as a character-special device.
        !            14: 
        !            15: Shared memory  access operations  are performed by  seeks, reads,
        !            16: and  writes through  the  interface /ddeevv/sshhmm.   The desired  seek
        !            17: location is (sshhmmiidd << 16L) + offset.
        !            18: 
        !            19: Shared memory control operations are performed through the system
        !            20: call ioctl.  The  operations shmctl and shmget are performed with
        !            21: an integer  parameter array.  The  first element of  the array is
        !            22: reserved for the return value (default, -1).  Subsequent elements
        !            23: represent  arguments.   ioctl passes  SHMCTL,  SHMGET, SHMAT,  or
        !            24: SHMDT  as the  second argument,  and the  parameter array  as the
        !            25: third argument.  The first argument is an open file descriptor to
        !            26: /dev/shm.  Seeks, reads, and  writes on shared memory can be per-
        !            27: formed through the file descriptor shmfd.
        !            28: 
        !            29: ***** Access *****
        !            30: 
        !            31: To access shared memory, do the following:
        !            32: 
        !            33: 1. Be  sure that /dev/shm is present  as a special-character file
        !            34:    with major number 24, minor number 0, and broad enough permis-
        !            35:    sions.  The command
        !            36: 
        !            37:            /etc/mknod /dev/shm c 24 0
        !            38: 
        !            39:    will create /dev/shm if it does not yet exist.
        !            40: 
        !            41: 2. Become the superuser root.  Execute the command
        !            42:            /etc/drvld /drv/shm
        !            43: 
        !            44:    to load the driver.
        !            45: 
        !            46: 3.  Use the  COHERENT system  call shmget()  to create  a shared-
        !            47:    memory segment and obtain shmid value for it.
        !            48: 
        !            49: 4. Use  the COHERENT system call lseek() to  position for read or
        !            50:    write of a shared-memory segment.  The first argument to lseek
        !            51:    is shmfd,  which is an external  declared in <sys/shm.h>.  The
        !            52:    second argument to lseek is a long whose high word is the seg-
        !            53:    ment identifier shmid and  whose low word is the offset within
        !            54:    the  shared-memory segment.   The third  argument to  lseek is
        !            55:    zero.
        !            56: 
        !            57: 5. Use the COHERENT system calls read() and write() to access the
        !            58:    segment.  Again, use shmfd as the file descriptor.
        !            59: 
        !            60: 6. When you are finished using shared memory, use the call
        !            61:            shmctl(shmid, IPC_RMID, 0)
        !            62: 
        !            63: 
        !            64: COHERENT Lexicon                                           Page 1
        !            65: 
        !            66: 
        !            67: 
        !            68: 
        !            69: shm                       Device Driver                       shm
        !            70: 
        !            71: 
        !            72: 
        !            73: 
        !            74:    to remove segments when you are finished.
        !            75: 
        !            76: 7. Finally,  use ps  -d to obtain  the process identifier  of the
        !            77:    shared-memory driver.  To unload the driver, become the super-
        !            78:    user root, and then type the command
        !            79: 
        !            80:            kill  -9 _x_x_x_x
        !            81: 
        !            82:    where xxxx is the process identifier for the shm driver.
        !            83: 
        !            84: Note that this manner of  proceding is not entirely in the spirit
        !            85: of  System  V  IPC  shared  memory:  COHERENT  does  not  support
        !            86: functions shmat() and shmdt().  Unfortunately, true attachment of
        !            87: shared segments is not possible in SMALL-model systems.
        !            88: 
        !            89: ***** Notes *****
        !            90: 
        !            91: If you allocate too  many shared memory identifiers, you will ex-
        !            92: haust kernel data space, and thus halt the system in its tracks.
        !            93: 
        !            94: Creating  many  large shared  memory  segments  can exhaust  main
        !            95: memory, as shared  memory segments do not currently support swap-
        !            96: ping.
        !            97: 
        !            98: The functions shmat and shmdt are not currently supported.
        !            99: 
        !           100: Private shared  memory is not supported.   Shared memory segments
        !           101: must be removed manually  when no longer required.  To remove all
        !           102: shared memory segments use the following C code:
        !           103: 
        !           104: 
        !           105: #include <sys/shm.h>
        !           106: 
        !           107: #define NSHMID 16
        !           108: 
        !           109: shmget( 0, 0, 0 );        /* must do first */
        !           110: 
        !           111: for ( id=0; id < NSHMID; ++id )
        !           112:         shmctl( id, IPC_RMID, 0 );
        !           113: 
        !           114: 
        !           115: To load shm into memory, use the command drvld.
        !           116: 
        !           117: ***** Files *****
        !           118: 
        !           119: /usr/include/sys/ipc.h
        !           120: /usr/include/sys/shm.h
        !           121: /dev/shm
        !           122: /drv/shm
        !           123: 
        !           124: ***** See Also *****
        !           125: 
        !           126: device drivers, drvld, shmctl(), shmget()
        !           127: 
        !           128: 
        !           129: 
        !           130: COHERENT Lexicon                                           Page 2
        !           131: 
        !           132: 

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