POSIX Shared Memory¶

POSIX shared memory provides a named memory object that can be mapped into multiple processes.

Unlike message-based IPC, shared memory allows processes to access the same memory region directly. This can be very efficient, but it requires explicit synchronization.

What Problem It Solves¶

Use POSIX shared memory when multiple processes need to exchange larger or frequently updated data with low overhead.

Typical use cases:

shared ring buffers
producer/consumer pipelines
high-throughput local IPC
shared state between cooperating processes

Core Concepts¶

Concept	Description
`shm_open()`	Creates or opens a POSIX shared memory object
`ftruncate()`	Sets the size of the shared memory object
`mmap()`	Maps the object into the process address space
`munmap()`	Unmaps the memory region
`shm_unlink()`	Removes the named shared memory object

Basic Flow¶

shm_open()
    ↓
ftruncate()
    ↓
mmap()
    ↓
read/write shared memory
    ↓
munmap() / close() / shm_unlink()

Synchronization Requirement¶

Shared memory only shares bytes. It does not provide locking, notification, or ordering by itself.

Use one of the following with shared memory:

process-shared pthread mutex
POSIX semaphore
eventfd
futex-based synchronization

Common Pitfalls¶

Warning

Do not assume shared memory access is automatically safe. Concurrent readers and writers still need synchronization.

Note

shm_unlink() removes the name, but existing mappings remain valid until unmapped and closed.