No, it is not the address (the 2nd param) passed to shmat calls. I'm passing
"(void*)0".
It is the address returned by shmat, after attaching the segment.
My code is as below:-
int id = shmget(key,size,(0777 | IPC_CREAT | IPC_EXCL));
if(errno == EEXIST){
id = shmget(key,size,(0777 | IPC_CREAT));
}
suht->_dataMemSegments[idx]._dataMemory = (pSUHT_ITEM)shmat(id, (void *)0,
0777 | SHM_SHARE_MMU);
if(NULL == suht->_dataMemSegments[idx]._dataMemory || errno != 0){
printf("shm:_dataMemory:failed: errno:%d\n",errno);
return errno;
}
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: 04 April 2007 16:49
To: Borse, Ganesh
Cc: '[email protected]'
Subject: Re: [perf-discuss] Request for guidance on shmat() call attaching at
different addre ss
Hi Genesh,
Are you specifying an address in your shmat calls? Can you post code segments
doing shmget/shmat?
max
Borse, Ganesh wrote:
>
> Hi,
> I am facing a problem of shared memory segment getting attached at
> different address, when the processes using it go down and attach
> count goes to zero.
>
> This causes the problem when the processes next time come up & try to
> access the addresses taken from the shared memory segment.
>
> The scenario in brief:-
> I've 3 processes using multiple shared memory segments. Shared memory
> segments (shm) are also used in hierarchical fashion.
>
> One shm is used as a high level data structure to hold pointers to
> other shms.
> Second shm is an array of pointers to structures (data blocks) which
> will get allocated in the third shm. For the other 2 processes to
> access these blocks, their addresses will be stored in this second shm
> (an array).
>
> The third shm actually holds the data as retrieved by the server
> process from database. This is acting as cache.
> When the third shm gets completely used, the server process creates
> new shm and starts storing data into it. Once all data gets stored,
> server process goes down - without removing any shm.
>
> Next when the client processes comes up, they attach to all these shms
> created by server process.
> When the client process calls "shmat" on the first 3 memory segments,
> shmat returns the same address which it returned to server process
> (these addresses are stored in 1st shm).
>
> But, when client process tries to attach to 4th segment (which was
> created after the third data shm filled up), the shmat returns totally
> different address than that of returned to the server process.
>
> Since, the addresses stored in array shm correspond to the different
> shm address, when the client process tries to access those addresses,
> it crashes with SIGSEGV.
>
> I am using SHM_SHARE_MMU flag with shmat to use the feature of ISM &
> to get same address.
>
> Any clue/ hint why only the 4th (last segment) shmat returns different
> address? Is this the normal behavior?
> Could I be doing something wrong only while creating this segment?
>
> Please help.
>
> Thanks and Regards,
> GaneshB
>
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