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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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