shmopen

shm_open is a POSIX API function used to open or create a named shared memory object. It allows multiple processes to a priori share memory by agreeing on a common name and size, and then mapping that object into each process’s address space with mmap. The function returns a file descriptor that can be used for subsequent operations on the object, and the memory contents are shared among all processes that map the object.

The function prototype is int shm_open(const char *name, int oflag, mode_t mode). The name must begin with a single leading slash and contain no other slashes (for example, "/myshm"). The oflag argument can include O_RDONLY or O_RDWR and may include O_CREAT to create the object and O_EXCL to fail if it already exists. When O_CREAT is used, mode specifies the access permissions (as for creating files). If O_CREAT is not used, mode is ignored. If O_CREAT and O_EXCL are used and the object already exists, shm_open will fail with EEXIST.

After obtaining a descriptor, the size of the shared memory object must be set with ftruncate(fd, size). The object persists until it is unmapped and all descriptors are closed; it can be explicitly removed from the namespace with shm_unlink(name), which makes the name unavailable for new openings while existing mappings and descriptors keep using the object.

Mapping the shared memory into a process uses mmap with MAP_SHARED and appropriate protections (e.g., PROT_READ | PROT_WRITE). Synchronization between processes is the responsibility of the application, commonly using semaphores or mutexes.

shm_open is a core component of POSIX shared memory, contrasting with System V shared memory primitives. It is widely available on POSIX systems, with occasional platform-specific linking requirements and behavior variations.