what is .shm file extension?

[SAMI AHMAD]

I have some files with .SHM extension which I understand has something to do with installed shared images but I browsed all the vms documents and googled this but I could no where find what this file with .SHM extension do , can anyone tell me what are these files and their function ? I checked other installed images and they dont have any associated .SHM files on disk.

[RBrown_1]

I have never heard of .shm either. Perhaps it is something local to your site.

My next steps:

o dump your .shm file and see if there are any clues there.

o search your application source to see if any of them open .shm files.

Hve fun.

[SAMI AHMAD]

no its not something for our system, I found one reference on web about 'shared memory file' for open VMS but it doesnt say what function it serves.

[Robert Gezelter]

Sami,

Actually, when citing something on the www, the actual URL is helpful.

When I query "shm openvms file" using Google, one of the hits I get is http://dotwhat.net, which seems to indicate that .SHM was used as a file extension by WordPerfect.

There is also several references to MIT Shared Memory. As was suggested, DUMP can be very useful here.

Many users and third party products have been known to define their own file extensions. The trick here is to determine what it is, now what it COULD be.

- Bob Gezelter, http://www.rlgsc.com

[Steven Schweda]

> no its not something for our system,

It's certainly not something on _my_ system:

ALP $ dire sys$sysdevice:[000000...].shm
%DIRECT-W-NOFILES, no files found


> I found one reference on web about 'shared
> memory file' for open VMS but it doesnt say
> what function it serves.

And you don't seem to have said anything
about where you found this "one reference",
so I'd assume that it was not worth
mentioning.

[SAMI AHMAD]

i found a partial answer for this from the application developers , they say this file is used for sharing interprocess communications ,so it is somehow related to shared memory segments but I didnt knew that you could share segments withouts installing the image ?

[Joseph Huber_1]

So Your "application developers" are not willing to share their secrets with You , but You expect others to know why they use the extension ?
The term in VMS is usually "global section file", and the programmer is free to give it whatever name he wants.

What can be installed are shared libraries, which need not be code, but can be data sections only (Fortran common sections for example).

Global section files are not installed, but accessed by $crmpsc or $mgblsc system services at run-time.

[Joseph Huber_1]

Thinking about why the "application developers" are using the term "shared memory": they probably don't think in VMS terms, but using X/Open and Posix terms: the C "mmap" routine does the equivalent to $crmpsc/$mgblsc.

[Hoff]

We're playing "telephone" game here, and that's as frustrating for us as it is for Sami.

Personally, I'd wager the developers whispered the incantation "shm" to start this off.

$ man -k shm
...
shm_open(2) - open a shared memory object
shm_unlink(2) - remove shared memory object
shmat(2), shmdt(2) - map/unmap shared memory
shmctl(2) - shared memory control operations
shmget(2) - get shared memory area identifier
...

That's the C interface that's (if you're crossing platforms and trying to map terms and concepts) likely to trigger "installed shared image". Though an installed shared image is not where I'd land if I were using shm stuff; I tend to prefer to avoid installed shared images as they've caused me pain in the past with compatibility and upgrades.