You can retrieve the size of an object by using posix_typed_mem_get_info().
The posix_typed_mem_get_info() call fills in a posix_typed_mem_info structure, which includes the posix_tmi_length field, which contains the size of the typed memory object.
As specified by POSIX, the length field is dynamic and contains the current allocatable size for that object (in effect, the free size of the object for POSIX_TYPED_MEM_ALLOCATE and POSIX_TYPED_MEM_ALLOCATE_CONTIG). If you opened the object with a tflag of 0 or POSIX_TYPED_MEM_MAP_ALLOCATABLE, the length field is set to zero.
When you map in a typed memory object, you usually pass an offset to mmap(). The offset is the physical address of the location in the object where the mapping should commence. The offset is appropriate only when opening the object with a tflag of 0 or POSIX_TYPED_MEM_MAP_ALLOCATABLE. If you opened the typed memory object with POSIX_TYPED_MEM_ALLOCATE or POSIX_TYPED_MEM_ALLOCATE_CONTIG, a nonzero offset causes the call to mmap() to fail with an error of EINVAL.