C - shared memory - dynamic array inside shared struct

Question

i\'m trying to share a struct like this
example:

typedef struct {
    int* a;
    int b;
    int c;
} ex;

between processes, the prob

Answer 1

You need to use shared memory/memory mapped files/whatever your OS gives you. In general, IPC and sharing memory between processes is quite OS dependent, especially in low-level languages like C (higher-level languages usually have libraries for that - for example, even C++ has support for it using boost). If you are on Linux, I usually use shmat for small amount, and mmap (http://en.wikipedia.org/wiki/Mmap) for larger amounts. On Win32, there are many approaches; the one I prefer is usually using page-file backed memory mapped files (http://msdn.microsoft.com/en-us/library/ms810613.aspx)

Also, you need to pay attention to where you are using these mechanism inside your data structures: as mentioned in the comments, without using precautions the pointer you have in your "source" process is invalid in the "target" process, and needs to be replaced/adjusted (IIRC, pointers coming from mmap are already OK(mapped); at least, under windows pointers you get out of MapViewOfFile are OK).

EDIT: from your edited example: What you do here:

e = (ex*)shmat(mid, NULL, 0);

(other process)

int shmid = shmget(key, sizeof(ex), 0);
ex* e = (ex*)shmat(shmid, NULL, 0);

is correcty, but you need to do it for each pointer you have, not only for the "main" pointer to the struct. E.g. you need to do:

e->a = (int*)shmat(shmget(another_key, dim_of_a, IPC_CREAT | 0666), NULL, 0);

instead of creating the array with malloc. Then, on the other process, you also need to do shmget/shmat for the pointer. This is why, in the comments, I said that I usually prefer to pack the structs: so I do not need to go through the hassle to to these operations for every pointer.