shmget

shmget is a system call in the System V Inter-Process Communication (IPC) API that obtains a shared memory segment identifier or creates a new segment. It returns a non-negative shmid on success, which can be used in subsequent operations to attach to or manage the segment. If an error occurs, it returns -1 and sets errno.

The function signature is int shmget(key_t key, size_t size, int shmflg). The key argument specifies a key for the segment and can be produced with ftok, or the special value IPC_PRIVATE to request a private, non-persistent segment. The size argument indicates the number of bytes to allocate, but its effect is only relevant when a new segment is being created. If an existing segment is found, the size parameter is ignored. The shmflg argument provides access permissions and creation flags, including the read/write permission bits (the lower nine bits) and options like IPC_CREAT to create the segment if it does not exist, and IPC_EXCL to fail if a segment with the given key already exists when used with IPC_CREAT.

Key behavior includes: if IPC_CREAT is specified and no segment exists for the key, shmget creates a new one with the requested size; if a segment exists, shmget returns its shmid and does not change its size (unless IPC_CREAT alone without IPC_EXCL is used and the segment was created just now). If IPC_EXCL is used with IPC_CREAT and a segment already exists, shmget fails with EEXIST. Common errors include EACCES, EINVAL, ENOENT, and ENOSPC.

After obtaining a shmid, processes typically attach with shmat, detach with shmdt, and mark the segment for removal with shmctl(..., IPC_RMID). The header files required are sys/ipc.h and sys/shm.h. This interface is distinct from POSIX shared memory APIs such as shm_open and shm_unlink.