a question about share memory

[William Cai_1]

I am developing a software system on HP9000. Now I encounted a problem. I create a block of share memory, store some important information into it. but sometimes the information is changed automaticly. My program do not do that . Is there some system process change the contain in the share memory. Anyone can answer my question?

Thank you very much!

[U.SivaKumar_2]

Hi,

Ensure that shared memory is sufficient for other programs.
Some running program may be already accessing the shard memory address segments which your program is using. Try using higher order of shared memory address in your program which can be free.
regards,
U.SivaKumar

[Stefan Farrelly]

According to the manpage for shmget any process can also access this shared memory segment if it has the proper permissions - so yes, other processes can use it/overwrite it.

For a test trying running it as root and locking it into memory (shmctl) and see if it still gets overwritten.

[William Cai_1]

thank U.SivaKumar and Stefan Farrelly.

I get the key use ftok function,I think it's unique in the whole system.

I'll try to use shmctl function , But I have to run the program as a common user.

what can I do?

Is it system process overwrite the share memory?

[William Cai_1]

how to use shmctl?
key = ftok("/usr/caiw",'r');
shmid = shmget(key,SHMSIZE,IPC_CREAT|IPC_EXCL|0666);
if (shmid == -1){...}
shmptr = shmat(shmid,0,0);

shmctl(shmid,IPC_STAT?IPC_SET,buff);

[U.SivaKumar_2]

Hi,

Use this command to see the information of current active shared memory segments like memory address , owner and permissions in the system

#ipcs -m

Avoid using those segments shown by the above command in your program.

regards,
U.SivaKumar

[William Cai_1]

[/home/caiwl]ipcs -m
IPC status from /dev/kmem as of Wed Dec 11 17:18:29 2002
T ID KEY MODE OWNER GROUP
Shared Memory:
m 0 0x41200629 --rw-rw-rw- root root
m 1 0x4e0c0002 --rw-rw-rw- root root
m 2 0x4124037b --rw-rw-rw- root root
m 44003 0x2ee8af94 --rw-r----- oracle dba
m 41004 0x0001e243 --rw-rw-rw- tuxdman app
m 8005 0x00000000 --rw------- tuxdman app
m 12006 0x64203cec --rw-rw-rw- root sys
m 2007 0x6b203cec --rw-rw-rw- root sys
m 2008 0x000003e8 --rw-rw-rw- tuxdman app
m 21009 0x7a203cec --rw-rw-rw- caiwl users
m 4010 0x78203cec --rw-rw-rw- tuxdman app

my key is 0x64203cec and 0x6b203cec,I think is unique in the system.
:'(

[Deepak Extross]

It's rather improbable that an unrelated process will use the same key to get a handle to your shared memory segment. Most likely, an old instance of your process is running and messing up your shared memory.

[U.SivaKumar_2]

Hi,

Iam not a expert in calls.

try changing the permission of your program shared memory segment to rw-----

regards,
U.SivaKumar