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shm                       Device Driver                       shm




Shared memory device driver


The device  /dev/shm is an interface to  the shared memory device
driver.  It is assigned major  device 24 (minor device 0) and can
be accessed as a character-special device.

Shared memory  access operations  are performed by  seeks, reads,
and  writes through  the  interface /ddeevv/sshhmm.   The desired  seek
location is (sshhmmiidd << 16L) + offset.

Shared memory control operations are performed through the system
call ioctl.  The  operations shmctl and shmget are performed with
an integer  parameter array.  The  first element of  the array is
reserved for the return value (default, -1).  Subsequent elements
represent  arguments.   ioctl passes  SHMCTL,  SHMGET, SHMAT,  or
SHMDT  as the  second argument,  and the  parameter array  as the
third argument.  The first argument is an open file descriptor to
/dev/shm.  Seeks, reads, and  writes on shared memory can be per-
formed through the file descriptor shmfd.

***** Access *****

To access shared memory, do the following:

1. Be  sure that /dev/shm is present  as a special-character file
   with major number 24, minor number 0, and broad enough permis-
   sions.  The command

           /etc/mknod /dev/shm c 24 0

   will create /dev/shm if it does not yet exist.

2. Become the superuser root.  Execute the command
           /etc/drvld /drv/shm

   to load the driver.

3.  Use the  COHERENT system  call shmget()  to create  a shared-
   memory segment and obtain shmid value for it.

4. Use  the COHERENT system call lseek() to  position for read or
   write of a shared-memory segment.  The first argument to lseek
   is shmfd,  which is an external  declared in <sys/shm.h>.  The
   second argument to lseek is a long whose high word is the seg-
   ment identifier shmid and  whose low word is the offset within
   the  shared-memory segment.   The third  argument to  lseek is
   zero.

5. Use the COHERENT system calls read() and write() to access the
   segment.  Again, use shmfd as the file descriptor.

6. When you are finished using shared memory, use the call
           shmctl(shmid, IPC_RMID, 0)


COHERENT Lexicon                                           Page 1




shm                       Device Driver                       shm




   to remove segments when you are finished.

7. Finally,  use ps  -d to obtain  the process identifier  of the
   shared-memory driver.  To unload the driver, become the super-
   user root, and then type the command

           kill  -9 _x_x_x_x

   where xxxx is the process identifier for the shm driver.

Note that this manner of  proceding is not entirely in the spirit
of  System  V  IPC  shared  memory:  COHERENT  does  not  support
functions shmat() and shmdt().  Unfortunately, true attachment of
shared segments is not possible in SMALL-model systems.

***** Notes *****

If you allocate too  many shared memory identifiers, you will ex-
haust kernel data space, and thus halt the system in its tracks.

Creating  many  large shared  memory  segments  can exhaust  main
memory, as shared  memory segments do not currently support swap-
ping.

The functions shmat and shmdt are not currently supported.

Private shared  memory is not supported.   Shared memory segments
must be removed manually  when no longer required.  To remove all
shared memory segments use the following C code:


#include <sys/shm.h>

#define NSHMID 16

shmget( 0, 0, 0 );        /* must do first */

for ( id=0; id < NSHMID; ++id )
        shmctl( id, IPC_RMID, 0 );


To load shm into memory, use the command drvld.

***** Files *****

/usr/include/sys/ipc.h
/usr/include/sys/shm.h
/dev/shm
/drv/shm

***** See Also *****

device drivers, drvld, shmctl(), shmget()



COHERENT Lexicon                                           Page 2

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