Re: Information on use of Netsys.shm_open

On Mi, 2010-03-24 at 01:32 +0100, Johan Mazel wrote:
> 
> 
> 2010/3/23 Gerd Stolpmann <[email protected]>
>         
>         On Di, 2010-03-23 at 19:15 +0100, Johan Mazel wrote:
>         > Hi
>         > I'm trying this little piece of code to open a shared memory
>         and
>         > modify it.
>         > The code looks like this :
>         >
>         > open Unix
>         > open Bigarray.Array1
>         >
>         > let _ =
>         >
>         >     Printf.printf("Test shared memory\n");
>         >
>         >     if (Netsys.have_posix_shm () = false) then
>         >
>         >         Printf.printf "No POSIX SHM\n"
>         >     else
>         >         (
>         >
>         >             Printf.printf "POSIX SHM OK\n";
>         >
>         >             let shm_name = "/shm_test" in
>         >
>         >             let right_list = ref [] in
>         >
>         >             right_list := Netsys.SHM_O_CREAT :: !right_list;
>         >             right_list := Netsys.SHM_O_RDWR :: !right_list;
>         >
>         >             ignore(
>         >                     try
>         >                         (
>         >                             Printf.printf "Netsys.shm_open
>         call\n";
>         >
>         >                             let sd = Netsys.shm_open
>         shm_name !
>         > right_list 1 in
>         >
>         >                             Printf.printf "Netsys.shm_open
>         ok\n";
>         >
>         >                             let my_big_array =
>         > Bigarray.Array1.map_file sd Bigarray.char Bigarray.c_layout
>         true 100
>         > in
>         >
>         >                             let big_array_size =
>         Bigarray.Array1.dim
>         > my_big_array in
>         >
>         >                             Printf.printf "Size of BigArray:
>         %d\n"
>         > big_array_size;
>         >
>         >                             Bigarray.Array1.set my_big_array
>         0 'a';
>         >                         )
>         >                     with
>         >                     | Unix_error (_ , invoked_function ,
>         > shm_name_failure) ->
>         >                             (
>         >                                 Printf.printf
>         >                                     "Opening failed through
>         using
>         > function %s on SHM %s\n"
>         >                                     invoked_function
>         >                                     shm_name_failure
>         >                                 ;
>         >                             )
>         >                 );
>         >
>         >             Printf.printf "Shared memory tested\n";
>         >         )
>         >
>         > I can create the shared memory file descriptor, but only
>         once.
>         > When I try to access it again (y rzlaunching my program,
>         there is an
>         > error on shm_open.
>         >
>         > I have to guess about a cause to this problem: first, I did
>         not set
>         > the right in the right manner and second, I do not use
>         unlink.
>         > The doc about the permissions is very light in this page
>         >
>         (http://projects.camlcity.org/projects/dl/ocamlnet-2.2.9/doc/html-main/Netsys.html). I tried with with 0, 1, 777 and none of these are working.
>         > For the unlink function, my understanding is that it
>         destroys the
>         > file. But maybe, it has the same behviour as "close" in C
>         for a file
>         > descriptor.
>         
>         
>         Well, shm_open just calls the C function with the same name,
>         so for any
>         detailed documentation see there.
> 
> Which function does unlink call ? Because there is no unlink function
> in the posix C shared memory API.

shm_unlink
 
>         Your guess is right, the error in your code is the bad
>         permissions
>         bitfield. 0o666 instead of 1 should be just ok to give
>         everybody
>         read/write access. The bitfield is interpreted as for any
>         other file.
> 
> Is there any doc available to know how to choose this code ?

Your favourite Unix book. It is the same bitstring you can pass to the
chmod command.

Gerd

> 
> Thanks for your time.
> 
> Johan
>  
>         
>         Note that the shared memory file may even pop up in the real
>         file
>         hierarchy (depending on your OS). E.g. for Linux it
>         is /dev/shm/shm_test. Another popular location is somewhere
>         in /tmp.
>         
>         Gerd
>         
>         >
>         > Thanks in advance for your time.
>         >
>         > Johan Mazel
>         
>         
>         >
>         ------------------------------------------------------------------------------
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>         
>         --
>         ------------------------------------------------------------
>         Gerd Stolpmann, Bad Nauheimer Str.3, 64289 Darmstadt,Germany
>         [email protected]          http://www.gerd-stolpmann.de
>         Phone: +49-6151-153855                  Fax: +49-6151-997714
>         ------------------------------------------------------------
>         
>         
> 


-- 
------------------------------------------------------------
Gerd Stolpmann, Bad Nauheimer Str.3, 64289 Darmstadt,Germany 
[email protected]          http://www.gerd-stolpmann.de
Phone: +49-6151-153855                  Fax: +49-6151-997714
------------------------------------------------------------


------------------------------------------------------------------------------
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Try the new software tools for yourself. Speed compiling, find bugs
proactively, and fine-tune applications for parallel performance.
See why Intel Parallel Studio got high marks during beta.
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