Using Share Memory on Hash Table

[Nitish Moleshwari]

Hi,

I am trying to keep my Hash table in a Shared Memory  which can be shared along multiple processes. I am able to open my shared memory and add Hash table  successfully but while closing my shared memory i am getting error of : Invalid argument . 

I am using General macro for adding and finding hash.I hope to hear from you .

Thank You,

Nitish G. Moleshwari

 

[Arthur O'Dwyer]

I don't think this question has anything to do with uthash.

Here's the first Google result for "Invalid argument shared memory"; hope it helps! (If not, some of the other results probably will.)

http://stackoverflow.com/questions/16630261/creating-my-first-unix-server-client-but-getting-a-shmget-invalid-argument-er

–Arthur

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[Nitish Moleshwari]

Hi,

Is there any way, i can share my uthash table with multiple processes? My procedure will be , one process on start will create and  add the  hash table and after this other processes will make changes in that same hash table . 

Please let me know any solution on this.

Thanking You,

Nitish Moleshwari

[Arthur O'Dwyer]

Nitish,

Read these:

http://www.cse.psu.edu/~deh25/cmpsc473/notes/OSC/Processes/shm.html

http://man7.org/linux/man-pages/man7/shm_overview.7.html

However, be warned that:

- Concurrent accesses to shared resources must be guarded by a mutex

http://stackoverflow.com/questions/2389353/do-pthread-mutexes-work-across-threads-if-in-shared-memory

- You'll need to figure out whose responsibility it is to initialize the shared hash; if this requires a shared mutex, then you'll need to figure out whose responsibility it is to initialize the shared mutex, and so on

- All pointers that will be followed by both processes must point to objects located in the shared memory segment

http://www.drdobbs.com/creating-stl-containers-in-shared-memory/184401639

- Not only that, but since uthash uses raw C pointers, both processes must agree on the LOCATION of the shared memory segment, e.g. by hard-coding the first argument to mmap()

http://stackoverflow.com/questions/5384218/c-putting-a-hash-table-into-a-shared-memory-segment

This is all possible, but probably much harder than you ought to be tackling.

I've slapped together a quick(?) example using the POSIX shared-memory API; see the attached zipfile. Notice that it sets up a pthread_mutex_t to control access to the shared memory but then doesn't actually *use* that mutex, because I'm lazy. Also notice that I had to write my own memory allocator, which I did in as lazy a way as possible (e.g. SharedTable_free() is a no-op).

HTH,

Arthur

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