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Yes, two processes can both attach to a shared memory segment. A shared memory segment wouldn’t be much use if that were not true, as that is the basic idea behind a shared memory segment – that’s why it’s one of several forms of IPC (inter-Process communication).
When two processes map the same file in memory, the memory that one process writes is seen by another process, so memory mapped files can be used as an interprocess communication mechanism.
How shared memory can be used in terms of system call and data sharing?
So, shared memory provides a way by letting two or more processes share a memory segment. With Shared Memory the data is only copied twice – from input file into shared memory and from shared memory to the output file. SYSTEM CALLS USED ARE: ftok(): is use to generate a unique key.
To share memory between services and user applications the name of the shared memory must start with the global namespace prefix “Global\\\\”. This global namespace enables processes on multiple client sessions to communicate with a service application. The server component can create the shared memory in the global namespace.
In this module, shared memory refers to “System V style” shared memory blocks (though is not necessarily implemented explicitly as such) and does not refer to “distributed shared memory”. This style of shared memory permits distinct processes to potentially read and write to a common (or shared) region of volatile memory.
How is shared memory used to avoid two copies?
Using shared memory, we can avoid two data copies, but we have to synchronize the access to the shared memory segment. To use shared memory, we have to perform 2 basic steps: Request to the operating system a memory segment that can be shared between processes.
Let’s see a simple example of shared memory use. A server process creates a shared memory object, maps it and initializes all the bytes to a value. After that, a client process opens the shared memory, maps it, and checks that the data is correctly initialized: