shmdt

shmdt is a system call used in the System V IPC shared memory interface to detach a previously attached shared memory segment from the calling process’s address space. It is invoked after a successful shmat, which attaches a segment at a specific address. The detachment is per-process: once detached, the process can no longer access that memory region through the old pointer, though the underlying segment may still exist if other processes are attached or if the segment has not been marked for deletion.

The function signature is int shmdt(const void *shmaddr). On success, it returns 0. On error, it returns -1 and sets errno. Common error indications include EFAULT, if shmaddr lies outside the process’s address space, and EINVAL, if the specified address does not correspond to an attached shared memory segment for the calling process.

Detachment does not remove the shared memory segment from the system. The segment remains in the kernel as long as at least one process is still attached, or until an explicit deletion request (for example, via shmctl with IPC_RMID) marks the segment for removal. If the segment is removed while still attached to some processes, those processes will fail to access it after detachment.

Context and usage: shmdt is part of the traditional System V IPC API, alongside shmget (to create or access a segment), shmat (to attach), and shmctl (to control or delete). Proper handling of permissions and synchronization is important, as multiple processes may attach to the same segment and detach independently. Using shmdt correctly helps prevent resource leaks in applications that rely on shared memory for interprocess communication.