Page-level protection needs to be set on the mapping. Private mapping is required. The segments can be from 1 byte to 256M bytes in size. The process can attach these segments into the address space for the size of the segment. Another segment can be attached at the end of the first one in the same 256M byte region.
Some restrictions exist on the use of the extended shmat feature.
The mmap services are specified by various standards and commonly used as the file-mapping interface of choice in other operating system implementations. However, the system's implementation of the mmap subroutine may differ from other implementations. Mapping into the process private area is not supported. Mappings are not implicitly unmapped. Mapping of character devices or use of an mmap region as a buffer for a read-write operation to a character device is not supported.