How to share an existing dynamic array within Linux POSIX model in c language?

Question

I have very big quickly growing (realloc, tcmalloc) dynamic array (about 2-4 billion double). After the growth ends I would like to share this array between two different applications. I know how to prepare shared memory region and copy my full-grown array into it, but this is too prodigally for memory, because I must to keep source and shared destination array at the same moment. Is it possible to share already existing dynamic array within POSIX model without copying?

EDITED:

Little bit explanation.

I am able to use memory allocation within POSIX model (shm_open() and others) but if I do it, I have to reallocate already shared memory segment many times (reading digits row by row from database to memory). It is much more overhead in comparison with simple realloc().

I have one producer, who reads from database and writes into shared memory.

I can't know beforehand how many records are present in the database and therefore I can't know the size of the shared array before the allocation. For this reason I have to reallocate big array while producer is reading row by row from database. After the memory is shared and filled, another applications reads data from shared array. Sometimes size of this big shared array could be changed and be replenished with new data.

Answer 1

Is it possible to share already existing dynamic array within POSIX model without copying?

No, that is not how shared memory works. Read shm_overview(7) & mmap(2) .

Copying two billion doubles might take a few seconds.

Perhaps you could use mremap(2) .

BTW, for POSIX shared memory, most computers limit the size of the of the segment shared with shm_open(3) to a few megabytes (not gigabytes). Heuristically the maximal shared size (on the whole computer) should be much less than half of the RAM available.

My feeling is that your design is inadequate and you should not use shared memory in your case. You did not explain what problem you are trying to solve and how is the data modified (Did you consider using some RDBMS ?). What are the synchronization issues?

Your question smells a lot like some XY problem , so you really should explain more, motivate it much more, and also give a broader, higher-level picture.