Shared memory is an inter-process communication (IPC) method that permits separate programs to exchange data by directly reading from and writing to a common area of memory. Its primary advantage is speed, as direct access bypasses slower methods that require copying data between processes. This high-throughput, low-latency channel is useful when large volumes of information must be shared or when programs need to collaborate closely. The operating system manages the shared memory segment, but once established, processes interact with it without constant kernel intervention.
The Core Mechanism of Shared Memory
Shared memory’s foundation is in how modern operating systems manage memory. Each process operates in a private virtual address space, a conceptual map of memory locations. The operating system, with hardware assistance, translates these virtual addresses into physical addresses in RAM. This separation prevents one process from interfering with another’s memory.
Shared memory works when the operating system kernel allocates a segment of physical RAM. The kernel then maps this same physical segment into the virtual address space of each participating process. While the virtual addresses for this segment may differ between processes, they all point to the identical block of physical memory.
This is like a physical whiteboard in a room. Multiple people, representing processes, can view the board through their own unique window. Each person’s perspective (their virtual address) is private, but the whiteboard they all view and write on is the same physical object. This direct, shared access avoids the need for a courier to copy information between private notes.
The Creation Process
Creating a shared memory object begins with a request to the operating system to create a new region or open an existing one. Under the POSIX standard, this is done with the `shm_open()` system call. This function requires a unique name, like a file path such as `/my_shared_mem`, which allows unrelated processes to find the same object.
`shm_open()` returns a file descriptor, an integer that acts as a handle to the object’s managing kernel structure. At this point, the shared memory object exists but has a size of zero bytes. No physical memory has been allocated for its contents, so no data can be stored yet.
Since the object is created without a size, space must be allocated using the `ftruncate()` system call. The process provides the file descriptor from `shm_open()` and specifies the desired size in bytes. For instance, calling `ftruncate()` with a size of 4096 allocates a 4-kilobyte segment, giving the object a defined capacity.
Mapping and Accessing the Shared Memory
After a shared memory object is created and sized, it is not yet part of any process’s address space. A process must map the object into its virtual memory using the `mmap()` system call. This step connects the kernel object to a memory range the process can directly read from and write to.
The `mmap()` function requires the file descriptor, the mapping size, and behavior flags. The `MAP_SHARED` flag ensures modifications by one process are visible to all others sharing the object. A successful `mmap()` call returns a pointer to the starting address of the new memory block in the process’s virtual address space.
Once mapped, the process uses this pointer to interact with the shared memory just as it would with other dynamically allocated memory. It can write data to this location or read data placed there by another process. The operating system handles the underlying work of ensuring all changes are propagated correctly to the shared physical memory.
Synchronization and Lifecycle Management
When multiple processes can concurrently read and write to the same memory, there is a risk of data corruption. For example, one process might read a multi-byte value while another is still writing it, resulting in partial data. This is a race condition, and to prevent it, access to the shared memory must be coordinated.
Coordination is achieved with synchronization primitives like mutexes or semaphores. A mutex acts like a key, allowing only the process holding it to access the shared resource. A process locks the mutex before writing and unlocks it afterward, ensuring the operation is atomic. These synchronization objects are often placed within the shared memory segment for easy access by all processes.
Proper lifecycle management of the shared memory object is also necessary. When a process finishes with the shared segment, it detaches it from its address space using the `munmap()` system call. This call invalidates the pointer from `mmap()`, so the process can no longer access the memory. Detaching the memory only severs the connection for one process and does not destroy the object.
To remove the shared memory object from the system, the `shm_unlink()` function is used. This call removes the object’s name, preventing new processes from opening it. The system waits until all processes have unmapped the object with `munmap()` and closed their file descriptors. Once the last reference is gone, the operating system deallocates the physical memory, completing the lifecycle.