Re: Information on use of Netsys.shm_open
Johan Mazel <[email protected]> Thu, 25 Mar 2010 01:28:53 +0100
| Newsgroups | gmane.comp.lang.ocaml.lib.net.devel |
|---|---|
| Message-ID | <[email protected]> |
2010/3/24 Gerd Stolpmann <[email protected]> > On Mi, 2010-03-24 at 13:25 +0100, Johan Mazel wrote: > > > > > > 2010/3/24 Gerd Stolpmann <[email protected]> > > > > 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 > > > > Hi > > I understood that. > > But is there any reason why I should use 0o666 to have the same > > behaviour as 777 for chmod ? > > The difference between 666 and 777 is that the latter also sets the x > bits, not only the r and w bits. x is used for allowing execution. You > would need it for shm if you wanted to run code there (provided the > hardware supports the control of that at all - older PC hardware does > not, and setting x would make no difference). > > > I have another question linked to the map_file command from Bigarray. > > Does this command makes mandatory the use of the rights > > Netsys.SHM_O_RDWR in the function shm_open and forbids the use of > > Netsys.SHM_O_RDONLY ? > > Yes, map_file always maps read/write, so the file descriptor needs to be > read/write, too. > > > It is a result of my tests, but I would like a confirmation. > > I see this as a way to guarantee that the function set in > > Bigarray.genarray (and others) will be available in any case, but I am > > not sure of my interpretation. > > Yes, provided there is enough memory (as usual). > > > An extra question would be what are the case when Netsys.SHM_O_RDONLY > > would be useful if I cannot the use the result of shm_open in > > map_file ? In other words are there use cases where one can use > > shm_open with Netsys.SHM_O_RDONLY ? > > I'm not sure whether there is a real-life application. Generally, > O_RDONLY would be sufficient for reading the contents of the shm object > with Unix.read, or if you only wanted to check the size of the object. > > Gerd > > > > > Thanks for your time. > > > > Johan Mazel > > > > > > > ------------------------------------------------------------------------------ > > Download Intel® Parallel Studio Eval > > 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. > > http://p.sf.net/sfu/intel-sw-dev > > _______________________________________________ Ocamlnet-devel mailing > list [email protected] > https://lists.sourceforge.net/lists/listinfo/ocamlnet-devel > > > -- > ------------------------------------------------------------ > Gerd Stolpmann, Bad Nauheimer Str.3, 64289 Darmstadt,Germany > [email protected] http://www.gerd-stolpmann.de > Phone: +49-6151-153855 Fax: +49-6151-997714 > ------------------------------------------------------------ > > Thanks for a lot for all the help, everything is perfectly working now. Johan Mazel ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval 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. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ Ocamlnet-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/ocamlnet-devel