shared memory (ipc)in linux

Shared memory means one process will create a mem开发者_开发技巧ory portion oher process can acess.

My question is: how will the other process know the created shared memory shmid (i.e shmid=shmget(key-t ,size , permission);)?

To simplify: The process that creates the shared memory defines the name and permissions (process of the logged in user).

Basically you create a file:

file_descriptor = shm_open("/some.shared.memory", 
                   (O_CREAT | O_RDWR),
                   (S_IREAD | S_IWRITE))  

Or if it exists you can open it:

file_descriptor = shm_open("/some.shared.memory",
                   (O_CREAT | O_EXCL | O_RDWR),
                   (S_IREAD | S_IWRITE))

Thus, the name "/some.shared.memory" is your way to find the shared segment. (Note the flags)

• Have a look here http://www.kernel.org/doc/man-pages/online/pages/man7/shm_overview.7.html
• Do a Google search on "shm_open example" and you'll find lots of examples.
• The process that creates the file will determine the permissions (owner+group) See the chown command and fchmod functions which you can call to set the permissions. Also have a look at umask.

They need to use the same way to derive the key_t; usually you use the ftok function where both sides use the same file name and id value, so they end up with the same key.