CODEWITHSUNDEEP
cshared-memorysystem-calls
Locke
Locke

Reputation: 544

How to share a piece of memory across multiple processes using shmget

So I am making a program that will have multiple processes that all need to access a struct, 'node', that I have defined. So I initialize the memory as follows:

    sharedMemory = (node*)malloc(sizeof(node));//sharedMemory is a global node
    sharedMemory->syskey = sys_key;//just a variable from a function
    segment_id = shmget(sharedMemory->syskey, size, S_IRUSR | S_IWUSR | IPC_CREAT);
    sharedMemory = (node*)shmat(segment_id, NULL, 0);

And this seems to go fine. However I then need to access this piece of memory later. So what I tried to do was this:

    segment_id = shmget(sharedMemory->syskey, size, S_IRUSR | S_IWUSR | IPC_CREAT);
    sharedMemory = (node*)shmat(segment_id, NULL, 0);

And then use sharedMemory. However when I did this everything in sharedMemory seems to have been reset to 'null' (arrays and such). I am sure that I am just doing something stupid but the documentation is a bit confusing for me. Any help would be appreciated.

Upvotes: 1

Views: 115

Answers (1)

Locke
Locke

Reputation: 544

I figured out that I was doing it correctly except for the fact that sharedMemory was a global variable. I needed to just re-declare this whenever I called shmat and attach it. Seems to be working now.

Upvotes: 1

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