lib/libsys/shm_open.2

34 .Nd "shared memory object operations"
62 shared memory object named
85 then a new shared memory object named
89 the shared memory object is created with mode
96 flags are specified and a shared memory object named
105 If an existing shared memory object is opened with
110 then the shared memory object will be truncated to a size of zero.
130 In this case, an anonymous, unnamed shared memory object is created.
137 the shared memory object will be garbage collected when the last reference to
138 the shared memory object is removed.
139 The shared memory object may be shared with other processes by sharing the
144 Attempting to open an anonymous shared memory object with
158 object is always backed by aligned, physically contiguous chunks of memory.
159 This ensures that the object can be mapped using so-called
170 Largepage shared memory objects behave slightly differently from non-largepage
172 .Bl -bullet -offset indent
174 Memory for a largepage object is allocated when the object is
177 system call, whereas memory for regular shared memory objects is allocated
189 must be large page size-aligned, and the length of the operation must be a
215 to allocate memory for the object fails.
217 .Bl -tag -offset indent -width SHM_
219 If the (non-blocking) memory allocation fails because there is insufficient free
220 contiguous memory, the kernel will attempt to defragment physical memory and
230 If the memory allocation fails,
239 However, it is possible for physical memory to be fragmented such that the
248 commands can be used with a largepage shared memory object to get and set
251 .Bd -literal
269 works by creating a regular shared memory object using
277 system call atomically removes a shared memory object named
284 .Bl -tag -offset indent -width Er
298 system call removes a shared memory object named
303 function creates an anonymous shared memory object, identical to that created
335 .Bl -tag -width MFD_ALLOW_SEALING
353 both return a non-negative integer,
359 All functions return -1 on failure, and set
383 are guaranteed to access the same shared memory object if and only if
404 on a shared memory object, or on the descriptor returned by
415 also supports zero-copy transmission of data from shared memory
419 Neither shared memory objects nor their contents persist across reboots.
421 Writes do not extend shared memory objects, so
429 .Bd -literal -compact
449 .Bl -tag -width Er
476 .Bl -tag -width Er
493 was specified while creating an anonymous shared memory object via
509 is not specified and the named shared memory object does not exist.
514 are specified and the named shared memory object does exist.
520 and attempted to create a named shared memory object.
526 .Bl -tag -width Er
533 .Bl -tag -width Er
543 The shared memory object at
558 .Bl -tag -width Er
566 The named shared memory object does not exist.
570 requires write permission to the shared memory object.
592 .St -p1003.1b-93 .
605 The functions were reimplemented as system calls using shared memory objects