extern shared memory

In C++, the extern keyword is used to declare a variable or function that is defined in another source file or module. This keyword is typically used in conjunction with shared memory, which is a mechanism that allows multiple processes to access and manipulate the same block of memory.

To use extern with shared memory, we follow these steps:

  1. First, we define a shared memory segment using a system call or library function, such as shmget() or CreateFileMapping(). This creates a shared memory object that can be accessed by multiple processes.

  2. Next, we attach the shared memory segment to the current process using another system call or library function, such as shmat() or MapViewOfFile(). This operation maps the shared memory object to the virtual address space of the process, allowing it to read from and write to the shared memory.

  3. Once the shared memory is attached, we can access it by declaring a pointer of the desired type and assigning it the address of the shared memory. This is where the extern keyword comes into play. We use extern to declare the shared memory variable, indicating that its definition is located in another source file or module.

  4. Now, we can use the shared memory variable like any other variable in our program. We can read from or write to it using the pointer we declared earlier. Any changes made to the shared memory by one process will be visible to all other processes that have attached to the same shared memory segment.

  5. Finally, when we no longer need to access the shared memory, we detach it from the process using the appropriate system call or library function, such as shmdt() or UnmapViewOfFile(). This operation frees up system resources and ensures that the shared memory is no longer accessible to the process.

This is a high-level overview of using extern with shared memory in C++. Remember that the specific system calls and library functions used may vary depending on the operating system or platform you are working with. Additionally, proper synchronization mechanisms, such as semaphores or mutexes, should be used to coordinate access to the shared memory to avoid race conditions and other synchronization issues.