The following classes accessing memory segments are available:
The class MemoryAccess(3bobcat) defines an object interfacing to a shared memory segment (cf. shmget(2)). If offers functionality to set and retrieve its id, and write and read bytes from its shared memory segment. MemoryAccess objects are used by MemoryBridge(3bobcat) objects keeping track of which segments actually contain the data bytes.
MemoryAccess objects are used by MemoryBridge and are normally not directly constructed by user programs.
The class MemoryBridge (cf. figure images/sharedsegment1.jpg) interfaces to shared memory using MemoryAccess objects. Its main member function is the static member create, returning a pointer to a newly allocated MemoryMemory object.
The member create receives as its input parameters the number of blocks, the size of the segments allocated by MemoryBridge, and the segment's access mode (as used by, e.g., chmod(1)). It returns a pointer to the allocated MemoryBridge, and its ID (set using a pointer to an int, passed as its first parameter).
Although the MemoryBridge class defines an array of a single MemoryAccess(3bobcat) object, in fact it will contain d_nAccesss MemoryAccess objects. The member create first allocates a block of raw memory of the required total size. Next it initializes the first part of this raw, shared memory, using the MemoryBridge constructor and placement new. Next, the remaining amount of raw shared memory is initialized with d_nAccesss - 1 MemoryAccess objects, again using placement new (cf. figure images/sharedsegment2.jpg in the source distribution).
Following read/write operations the offset is updated accordingly. There's only one offset, which is used by [IO]?MemoryStream objects when requesting or updating offsets. a MemoryMemory object offers safeguards against inappropriate use. E.g., if no memory has been allocated yet it may thow exceptions. Also, offsets can never exceed the MemoryMemory's maximum possible offset.
The class MemoryBuf implements a std::streambuf specialization interfacing to a MemoryMemory object. In addition to a default constructor which is an empty stub the class offers constructors which immediately interface to a MemoryBridge object.
The MemoryStream class offers stream-facilities operating on shared memory maintained by a MemoryBuf object.
An overview of currently defined shared memory segments is shown by the command ipcs -ma. To erase a defined shared memory segment the command ipcrm -m <id> can be used, where <id> is the shared memory segment's ID.
Images referred to in this man-page are located in the source distribution in bobcat/documents/images, or they can be found in the standard location of documentation files in your distribution (e.g., they are located in /usr/share/doc/libbobcat6/images).
Debian Bobcat project files: