How does one keep an int and an array in shared memory in C?

You are going to need to increase the size of your shared memory. How big an array do you need? Whatever value it is, you're going to need to select it before creating the shared memory segment - dynamic memory isn't going to work too well here When you attach to shared memory, you get a pointer to the start address.It will be sufficiently well aligned to be used for any purpose.

So, you can create pointers to your two variables and array along these lines (cribbing some of the skeleton from your code example) - note the use of pointers to access the shared memory: enum { ARRAY_SIZE = 1024 * 1024 }; int segmentId; int *sharedInt1; int *sharedInt2; int *sharedArry; const int shareSize = sizeof(int) * (2 + ARRAY_SIZE); /* Allocate shared memory segment */ segmentId = shmget(IPC_PRIVATE, shareSize, S_IRUSR | S_IWUSR); /* attach the shared memory segment */ sharedInt1 = (int *) shmat(segmentId, NULL, 0); sharedInt2 = sharedInt1 + 1; sharedArry = sharedInt1 + 2; /* Rest of code will go here */ ...fork your child processes... ...the children can use the three pointers to shared memory... ...worry about synchronization... ...you may need to use semaphores too - but they *are* complex... ...Note that pthreads and mutexes are no help with independent processes... /* detach shared memory segment */ shmdt(sharedInt1); /* remove shared memory segment */ shmctl(segmentId, IPC_RMID, NULL).

You are going to need to increase the size of your shared memory. How big an array do you need? Whatever value it is, you're going to need to select it before creating the shared memory segment - dynamic memory isn't going to work too well here.

When you attach to shared memory, you get a pointer to the start address. It will be sufficiently well aligned to be used for any purpose. So, you can create pointers to your two variables and array along these lines (cribbing some of the skeleton from your code example) - note the use of pointers to access the shared memory: enum { ARRAY_SIZE = 1024 * 1024 }; int segmentId; int *sharedInt1; int *sharedInt2; int *sharedArry; const int shareSize = sizeof(int) * (2 + ARRAY_SIZE); /* Allocate shared memory segment */ segmentId = shmget(IPC_PRIVATE, shareSize, S_IRUSR | S_IWUSR); /* attach the shared memory segment */ sharedInt1 = (int *) shmat(segmentId, NULL, 0); sharedInt2 = sharedInt1 + 1; sharedArry = sharedInt1 + 2; /* Rest of code will go here */ ...fork your child processes... ...the children can use the three pointers to shared memory... ...worry about synchronization... ...you may need to use semaphores too - but they *are* complex... ...Note that pthreads and mutexes are no help with independent processes... /* detach shared memory segment */ shmdt(sharedInt1); /* remove shared memory segment */ shmctl(segmentId, IPC_RMID, NULL).

This looks pretty great. Thanks so much! – Josh Nov 4 '09 at 3:23.

This guide looks useful: cs.cf.ac.uk/Dave/C/node27.html. It includes some example programs. There are also Linux man pages online.

Thanks but unfortunately that's one of those pages that thoroughly confused me. – Josh Nov 4 '09 at 2:37 Have you tried copy and pasting the example code there and compiling it? If you can get that to compile you are probably half way there to understanding it.

– user181548 Nov 4 '09 at 2:40 My problem with that example is that it seems to be different programs talking in a client-server manner. I hadn't clarified what I need at the time in my original question but in my code I will be creating multiple processes from the same program and need them to communicate. – Josh Nov 4 '09 at 3:07 I think you need to break your problem down in to steps and then try to solve them one at a time.

– user181548 Nov 4 '09 at 4:13.

Shared memory is just a segment of memory allocated by one process, with a unique id, and the other process will also make the allocation, with the same id, and the size of the memory is the sizeof the structure that you are using, so you would have a structure with 2 integers and an integer array. Now they both have a pointer to the same memory, so the writes of one will overwrite whatever else was there, and the other has immediate access to it. So what part is getting you confused?

Do you have some code of what you are trying to do?

Josh no, comment formatting sucks moldy rocks -- but then you're supposed to edit your question to clarify it, instead of commenting, so you can format your code prettily in an edit of your answer, and delete these comments! – Alex Martelli Nov 4 '09 at 2:47 I'd recommend you add clarification to your original question. – Carl Norum Nov 4 '09 at 2:47 @Josh - you may want to look at this tutorial: ecst.csuchico.Edu/~beej/guide/ipc/shmem.

Html, as he also shows how to use ftok to create a key for the call. – James Black Nov 4 '09 at 3:15 That guide looks pretty good. Thanks for all the help!

– Josh Nov 4 '09 at 3:24.

. Try something like: #include #include ... int segid = shmget((key_t)0x0BADDOOD, shareSize, IPC_CREAT); if (segid */ } int *p = (int*) shmat(segid, 0, 0); if (!p) { /* insert error processing here! */ }.

That looks interesting. I'm not the greatest when it comes to C so will p be the int array in this case? I'm also curious about that hex value you have in the shmget.

– Josh Nov 4 '09 at 3:05 IPC_PRIVATE works if the parent creates the shared memory and then forks the children who are going to share it. – Jonathan Leffler Nov 4 '09 at 3:06 @Josh: you need a 32-bit value as the key for the shared memory. You can either use ftok() to create one, or choose a fixed name.No Bad Dood is as good a name as any for a piece of shared memory as any other.

0xDEADBEEF is another well known value. But IPC_PRIVATE is good too - as long as the parent process creates and attaches the shared memory before forking. – Jonathan Leffler Nov 4 '09 at 3:08 Oh!

I didn't even get that reference. Pretty sweet haha – Josh Nov 4 '09 at 3:12 @Jonathan, yep -- if you're forking (and not exec'ing: per linux.die.Net/man/2/shmget , exec detaches all shm segments) IPC_PRIVATE is fine. But fork-w/o-exec is a pretty rare case in my experience.

– Alex Martelli Nov 4 '09 at 4:22.

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