IPC (System V)

Purpose

ipc/ implements the System V IPC primitives: message queues (msgget/msgsnd/msgrcv), semaphores (semget/semop), and shared memory (shmget/shmat). It also implements POSIX message queues (mq_open, mq_send, mq_receive).

This is a small directory by code volume but historically important. Modern code uses other primitives (futex, eventfd, pipe, file-backed mappings, shm_open).

Directory layout

ipc/
├── Makefile
├── util.{c,h}         # Common idr / shared init
├── ipc_sysctl.c       # /proc/sys/kernel/* knobs (msgmax, semmsl, shmmax, …)
├── msg.c              # SysV msgqueues
├── msgutil.c          # Message helper
├── sem.c              # SysV semaphores
├── shm.c              # SysV shared memory (uses shmem.c under mm)
├── mqueue.c           # POSIX message queues
├── mq_sysctl.c        # POSIX mq sysctls
├── compat.c           # 32-bit compat shims
├── namespace.c        # IPC namespace lifecycle
└── syscall.c          # The IPC multiplex syscall on archs that have it

Key abstractions

Symbol	File	Purpose
struct ipc_namespace	include/linux/ipc_namespace.h	Per-namespace tables (msg, sem, shm).
struct kern_ipc_perm	include/linux/ipc.h	Common header (uid/gid/mode/key) for IPC objects.
struct shmid_kernel	ipc/shm.c	A SysV shared memory segment.
struct msg_queue	ipc/msg.c	A SysV message queue.
struct sem_array	ipc/sem.c	A SysV semaphore set.

How it works

graph LR
    APP[User app] -->|msgget/semget/shmget| IPC[ipc/*.c]
    IPC -->|namespace| NS[ipc_namespace]
    IPC -->|allocate id| IDR[idr]
    APP -->|shmat/shmdt| SHM[ipc/shm.c]
    SHM --> SHMEM[mm/shmem.c]
    APP -->|mq_send/mq_receive| MQ[ipc/mqueue.c]
    MQ --> VFS[fs/]

• SysV objects are addressed by integer IDs, allocated in an idr per namespace. Their kern_ipc_perm carries a key (IPC_PRIVATE or user-supplied).
• SysV shared memory is implemented on top of tmpfs/shmem (mm/shmem.c). shmat maps the underlying file into the caller's address space.
• POSIX message queues (mq_*) are exposed through a separate mqueue filesystem mounted at /dev/mqueue.
• Sysctls in /proc/sys/kernel/ (msgmax, msgmnb, msgmni, semmsl, semopm, sem*, shmmax, shmall, shmmni) are owned here.

Integration points

• kernel/: namespaces, sysctl.
• mm/: SysV shm uses shmem.
• fs/: POSIX mqueue is its own filesystem.
• security/: LSM hooks for ipc_perm checks.

Key source files

File	Purpose
ipc/util.c	Shared id allocation, lookup, init.
ipc/shm.c	SysV shared memory.
ipc/sem.c	SysV semaphores.
ipc/msg.c	SysV message queues.
ipc/mqueue.c	POSIX mqueues.
ipc/namespace.c	IPC namespace lifecycle.
ipc/ipc_sysctl.c	sysctl knobs.

Entry points for modification

• Most modification here is bug fixes, namespace fixes, or adding new sysctls. New SysV primitives are not added; the modern primitives (futex, pipe, memfd, io_uring) are preferred.

Related pages

• Memory management — shmem backs SysV shm.
• Filesystems — POSIX mqueue is a filesystem.
• Kernel core — namespaces.