How to keep a pointer on a shared data in a boost shared memory segment
Hi everyone ! How to keep a pointer on a shared data in a boost shared memory segment ? I have a function which returns shm.construct<SharedData>(_nameSeg.c_str())(innerDataAllocator); But outside of the function, the object pointed by SharedData is inaccessible My general problem is that I want to have way to get, and set data in shared memory, but without, each time, having to : -get shm -find object in shm based on its name -construct allocators. So I would like to store 1) allocators and 2)a pointer to the shared object How could I do this ? I made a SO question about my problem, can you have a look ? It should be clearer :) https://stackoverflow.com/questions/55195149/how-to-keep-a-pointer-on-a-shar...
On 21/03/2019 01:03, jucin loic wrote:
How to keep a pointer on a shared data in a boost shared memory segment ? I have a function which returns shm.construct<SharedData>(_nameSeg.c_str())(innerDataAllocator); But outside of the function, the object pointed by SharedData is inaccessible My general problem is that I want to have way to get, and set data in shared memory, but without, each time, having to : -get shm -find object in shm based on its name -construct allocators. So I would like to store 1) allocators and 2)a pointer to the shared object How could I do this ?
In general it makes no sense to store pointers inside shared memory, because it is very likely that each process maps the shared memory to different addresses, so a pointer would point to invalid memory in all but one process. To solve this, you store indexes or offsets relative to the base of the shared memory itself (or some other known field within the shared memory) instead of a "real" pointer. Some compilers support a so-called "based pointer", which let you use offsets as if they were pointers, but this is not portable. You can accomplish something similar in portable code by using a smart pointer wrapper type.
Hi,
To solve this, you store indexes or offsets relative to the base of the shared memory itself (or some other known field within the shared memory) instead of a "real" pointer.
I once saw a system, where pointers in shared memory were self-relative. It should be possible to wrap them in a pointer-like class. 73, Mario
Yes, this is a good idea, but you got wrong on my real problem: I agree, and but I don't want to share the pointers between different process. I want to get the pointer to the shared data using managed_shared_memory::construct() or managed_shared_memory::find() and then store this pointer. And do this for of course all different processes. Please, read the SO question, I described there more precisely my problem, and even provided a READY-TO-USE example ( https://stackoverflow.com/questions/55195149/how-to-keep-a-pointer-on-a-shar... ) Thank you in advance ! Le jeu. 21 mars 2019 à 14:10, Klebsch, Mario via Boost-users < boost-users@lists.boost.org> a écrit :
Hi,
To solve this, you store indexes or offsets relative to the base of the shared memory itself (or some other known field within the shared memory) instead of a "real" pointer.
I once saw a system, where pointers in shared memory were self-relative. It should be possible to wrap them in a pointer-like class.
73, Mario
_______________________________________________ Boost-users mailing list Boost-users@lists.boost.org https://lists.boost.org/mailman/listinfo.cgi/boost-users
On 20/03/2019 13:03, jucin loic via Boost-users wrote:
Hi everyone ! How to keep a pointer on a shared data in a boost shared memory segment ? I have a function which returns shm.construct<SharedData>(_nameSeg.c_str())(innerDataAllocator); But outside of the function, the object pointed by SharedData is inaccessible My general problem is that I want to have way to get, and set data in shared memory, but without, each time, having to : -get shm -find object in shm based on its name -construct allocators. So I would like to store 1) allocators and 2)a pointer to the shared object How could I do this ? I made a SO question about my problem, can you have a look ? It should be clearer :) https://stackoverflow.com/questions/55195149/how-to-keep-a-pointer-on-a-shar...
I'm afraid I don't understand the problem, and your minimal example needs password. Could you send a single cpp file with the minimal code to understand your question? Best, Ion
Yes, forgot to give the password, it's simply: stackoverflow In the minimal example is only a signle cpp, that you can also find attached. Thanks :) Jucin Loic Le dim. 24 mars 2019 à 01:49, Ion Gaztañaga via Boost-users < boost-users@lists.boost.org> a écrit :
On 20/03/2019 13:03, jucin loic via Boost-users wrote:
Hi everyone ! How to keep a pointer on a shared data in a boost shared memory segment ? I have a function which returns shm.construct<SharedData>(_nameSeg.c_str())(innerDataAllocator); But outside of the function, the object pointed by SharedData is inaccessible My general problem is that I want to have way to get, and set data in shared memory, but without, each time, having to : -get shm -find object in shm based on its name -construct allocators. So I would like to store 1) allocators and 2)a pointer to the shared object How could I do this ? I made a SO question about my problem, can you have a look ? It should be clearer :)
https://stackoverflow.com/questions/55195149/how-to-keep-a-pointer-on-a-shar...
I'm afraid I don't understand the problem, and your minimal example needs password. Could you send a single cpp file with the minimal code to understand your question?
Best,
Ion _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org https://lists.boost.org/mailman/listinfo.cgi/boost-users
participants (3)
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Gavin Lambert -
Ion Gaztañaga -
jucin loic -
Klebsch, Mario