shmat

shmat is a function in the System V interprocess communication (IPC) API used to attach a shared memory segment to the address space of the calling process. A shared memory segment must first be created or obtained with shmget, and its identifier is provided to shmat via the shmid parameter.

The function prototype is void *shmat(int shmid, const void *shmaddr, int shmflg). The shmid identifies the desired shared memory segment. The shmaddr argument suggests a preferred attach address; if it is NULL, the system selects the address. The shmflg argument can include SHM_RDONLY to attach the segment in read-only mode; otherwise the default is read–write. The return value is a pointer to the attached memory region on success, or (void *) -1 on failure, in which case errno is set to indicate the error.

Common errors include EACCES (permission denied to access the segment), EINVAL (invalid shmid or incompatible shmflg, or an address that cannot be used), and EFAULT (shmaddr points outside the process’s address space). ENOMEM or other memory-related errors may occur if there is insufficient memory for the mapping.

Lifecycle and related calls: shmat is part of the System V IPC trio that also includes shmget (create or locate a segment) and shmdt (detach the segment). shmctl with IPC_RMID marks the segment for destruction; the segment is removed when no processes remain attached. System V IPC is traditional across many Unix-like systems, but modern code often uses POSIX shared memory (shm_open and mmap) for improved portability and naming.

In summary, shmat enables interprocess sharing by mapping a shared memory segment into a process’s address space, with lifecycle managed through accompanying IPC controls.