Simply storing an integer in a shared memory segment: C

Question

Im just trying to share an integer between two processes, but the memory segment is initialized in program 1 and is used in program 4. Here is the initialization in program 1:

shmid = shmget(key, sizeof(int*), 0666 | IPC_CREAT);
int *data = (int *)shmat(shmid, (void*)0,0);

Here I get a warning of "cast to pointer from integer of different size". Argh.

Simple, I'm assuming, but I'm a big time noob with IPC. And many other things in c....

Then I pass it to another program:

snprintf(shmarg, sizeof(shmarg), "%n", data);
pid_t pid3 = run_cmd4("/home/tropix/hw11-4", shmarg, semarg, pipe_from_p2_2, pipe_to_p5_2);

Not sure how to access it on the other side though. How can I get the int back in Program 4?

osgx · Accepted Answer · 2011-05-10 17:27:16Z

4

Pretty sure you're missing the header that defines shmat() and thus the compiler is guessing that shmat returns an int (not a pointer).

edited May 10, 2011 at 17:27

osgx

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answered May 9, 2011 at 3:51

Mel

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Tony Delroy · Accepted Answer · 2011-05-09 04:29:51Z

Here I get a warning of "cast to pointer from integer of different size". Argh.

Try using NULL instead of (void*)0.

Then I pass it to another program:

snprintf(shmarg, sizeof(shmarg), "%n", data);
pid_t pid3 = run_cmd4("/home/tropix/hw11-4", shmarg,
                      semarg, pipe_from_p2_2, pipe_to_p5_2);

Not sure how to access it on the other side though. How can I get the int back in Program 4?

The pointer itself isn't useful unless the shared memory area happens to be loaded at the exact same virtual memory address in the other program: you probably don't even want to try to arrange that - instead, let the other program (hw11-4) load the shared memory segment and let the OS choose the virtual memory address, then simply look at that address for the int. So, hw11-4 needs to be passed the same shared memory key (e.g. as a command line argument) and itself use shmget to open and get the key for, then shmat to map, the shared memory segment into memory....

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