A shared memory system lets two or more execution contexts access the same region of memory. In operating-system interprocess communication (IPC), that usually means separate processes map a common region into their address spaces. The mechanism makes data available to multiple participants; it does not, by itself, coordinate their access or prevent conflicting updates.
What does “shared memory” mean?
The phrase has different meanings in different areas of computing. This article uses “shared memory system” primarily to mean an IPC mechanism for sharing data between processes. The Linux man-pages documentation defines the POSIX form this way: “The POSIX shared memory API allows processes to communicate information by sharing a region of memory.”
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GPU programming uses the same phrase for a different memory space. In CUDA, shared memory is accessible to threads within a thread block or cluster, rather than being the POSIX process-shared memory described below.
How does process-shared memory work?
With POSIX shared memory on Linux, processes use a named shared-memory object and map it into their own virtual address spaces. A typical sequence is:
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shm_open()creates or opens the object and returns a file descriptor.ftruncate()sets the object’s size.mmap()maps the region into the calling process’s address space.- Each participating process opens and maps the same object so it can access the shared region.
munmap()removes a process’s mapping when it no longer needs it;shm_unlink()removes the object’s name.
The Linux man-pages overview also documents related operations such as close(), fstat(), fchmod(), and fchown(). On Linux, POSIX shared-memory objects are created in a tmpfs virtual filesystem normally mounted at /dev/shm; that is a Linux implementation detail, not a universal rule. See the Linux man-pages overview of POSIX shared memory.
Why does shared memory need synchronization?
Mapping a common region gives processes access to the same data, but does not establish which process may read or write at a particular moment. If processes update shared data concurrently, their operations can conflict or expose partially updated state. The application needs a separate synchronization protocol, for example one based on POSIX semaphores, to coordinate access. Allocation, mapping, and synchronization are distinct responsibilities.
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How do POSIX and System V shared memory differ?
POSIX and System V are separate operating-system interface families for process-to-process shared-memory IPC. They serve the same broad purpose but use different object and management models.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problems| Interface | How a process identifies and accesses the region | Lifecycle model |
|---|---|---|
| POSIX | A named object is opened with shm_open() and mapped with mmap(). |
The documented workflow includes sizing with ftruncate(), unmapping with munmap(), and removing the name with shm_unlink(). Linux objects have kernel persistence subject to shutdown and unlinking/unmapping conditions. |
| System V | A segment is identified by an identifier and managed with attach, detach, and control operations. | Uses the System V IPC management model rather than POSIX object naming. See the Linux man-pages overview of System V IPC. |
For POSIX on Linux, the man page says an object persists until system shutdown or until all processes have unmapped it and it has been deleted with shm_unlink(). Other operating systems may document different behavior, so check the target platform’s rules instead of assuming Linux semantics apply everywhere.
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Is GPU shared memory the same thing?
No. CUDA shared memory is a GPU programming memory space shared among threads in a thread block or cluster. NVIDIA’s CUDA Programming Guide states: “The shared memory is accessible by all threads within a thread block or cluster.” It is allocated at thread-block level, and its size and behavior depend on GPU architecture. It is not another name for a POSIX region shared by independent operating-system processes. See NVIDIA’s CUDA programming model documentation.
How does shared memory differ from Unified Memory and page deduplication?
- CUDA Unified Memory: A separate CUDA facility for managing memory across supported CPU and GPU arrangements. NVIDIA’s documented IPC-capable, system-allocated-memory technique does not share memory between different hosts and their devices. It should not be treated as synonymous with POSIX shared memory. See NVIDIA’s Unified Memory documentation.
- Kernel Samepage Merging (KSM): A Linux kernel feature that merges eligible identical memory pages under kernel policy. It is page deduplication, not the application-level POSIX IPC interface. See the Linux kernel KSM documentation.
When is a shared memory system useful?
Process-shared memory is useful when multiple processes need access to common data through a shared region. It can be a fit when the application can define both the region’s layout and a synchronization protocol. The documentation cited here defines the interfaces and behavior but does not establish a general performance advantage over other IPC approaches; performance depends on the application and platform.
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