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kmem

kmem is a libk module that contains various functions for memory allocation and deallocation. it uses the short naming convention with the glyph m.

description

kmem allocators can work in several different ways. they can allocate memory directly from the heap (like kmheapa() and kmlina()), use a header that has already been allocated by another function, or allocate memory only from a pre-allocated pool. linear allocation with pool allocation is particularly useful, as it permits the very rapid allocation and deallocation of lots of objects with only a few adjustments to the heap, and no possibility of fragmentation or need for expensive algorithms like malloc() or kmheapa(). functions that can return a value and an error condition always return a value of type kmres, a struct that contains the field kmcond with an error code, and one of raw (for functions that return raw pointers) or ptr (for functions that return kmptr objects).

module functions

kmem supplies two module-level functions, used to interact with the kmptr container type.

  • kmfree(kmptr) → kmcond - free, downref, or ignore the passed object as appropriate
  • kmshred(kmptr) → void - free, downref, or zero the passed object as appropriate. if downref'ing, mark underlying object to be shredded. otherwise, zero its contents, then deallocate if appropriate.
  • kmstat(void*) → kmptr - convenience function to wrap a pointer to a non-managed object in a kmptr struct, so it can be passed to functions that accept arbitrary objects. kmptr p = kmstat(raw) is equivalent to kmptr p = { kmkind_none, raw, NULL }.
  • kmtaint(&kmptr) → void - "taints" a kmptr object by setting it to be shredded when freed. this may be desirable if the object pointed to contains privileged information.
  • kmzero(void*,sz) → void - zeroes a region of memory
  • kmozero(kmptr) → void - zeroes an object in memory
  • kmcopy(void* dest, void* src, sz) → void - copies one region of memory to another
  • kmdup(kmptr) → kmptr - duplicates an object in memory, allocating it as sibling of the original

types

kmem defines the following types:

  • enum kmkind - enumerates allocation strategies
  • struct kmptr - abstract pointer object
  • struct kmcell - abstract memory cell
  • struct kmref - a reference-counted cell
  • struct kmnode - a node in an allocation tree
  • struct kmpool - a memory pool

kmptr and kmcell are both very similar. the difference is that a kmptr points to a region in memory and can be passed around freely. a kmcell is the actual in-memory representation of an allocation cell. a kmcell cannot be usefully instantiated; rather, it is downcast from an actual cell type (e.g. kmnode n; kmcell* s = (kmcell*)(&n))

kmkind

kmkind is an enum that specifies an allocation function.

  • kmkind_none - no allocation
  • kmkind_lin - linear heap allocation
  • kmkind_heap - random heap allocation
  • kmkind_pool - pool allocation
  • kmkind_ref - reference-counting allocation
  • kmkind_tree - tree allocation

kmptr

kmem functions can operate on both raw pointers and the kmptr struct type. kmptr is a generic struct that can contain any kind of pointer. this is useful if you wish to allocate different objects in different manners, but pass them on into a single interface.

memory pointed at by kmptr pointers can be freed either with the usual specialized function, or by passing the kmptr structure itself to the generic function kmfree, which will handle it appropriately, even if it's a pointer to a garbage-collected object or to a static region of memory.

a kmptr has the following layout:

  • kmkind kind - codes the type of pointer; kmkind_none indicates a non-allocated pointer to a static (global or on-stack) object.
  • kmshred shred - an enum. if kmshred_yes, the value will be zeroed or otherwise made unreadable on free. if no, kmfree will consult src for shred policy if it is not NULL.
  • void* ref - the raw pointer enclosed by cell
  • kmcell* cell - a pointer to an object enclosure, typically either a memory pool or a referencing-counting object. NULL if not needed.

the convenience function kmstat(void*) → kmptr wraps a pointer to a static object in a kmptr struct.

struct kmcell

kmcell is a stub struct used to disambiguate between source types. a "source" is an object that can hold an allocated object, such as the heap, a memory pool, a fixed-length array on stack, or a fixed-length global array. all values produced by a kmem allocation function can be cast to kmcell*, and have an intial field id that contains a kmcell.

  • kmkind kind - kind of cell
  • size_t size - size of cell (data plus all fields)
  • kmshred shred - shredding flag

struct kmref

kmref is a struct that constitutes the in-memory representation of a reference-counted cell.

  • kmcell id = { .kind = kmkind_ref, … } - kind of cell
  • size_t refs - number of active references
  • kmcell* src - source, if any
  • char data[] - content of cell

struct kmnode

kmnode is the header struct for tree nodes. all tree nodes pointers can yield a kmnode structure by subtracting sizeof (kmnode) from the pointer. a utility function and macro are made available to automate this safely.

  • kmcell id = { .kind = kmkind_tree, … } - kind of cell
  • kmnode* parent - parent node
  • kmnode* child - first child node
  • kmnode* lastchild - last child node
  • kmnode* prev - previous sibling, NULL if first
  • kmnode* next - next sibling, NULL if last

struct kmpool

  • kmcell id = { .kind = kmkind_pool, … } - kind of cell
  • size_t cellsz - size of individual pool cells
  • kmpoolcell* top - pointer to most recently allocated pool cell
  • kmpoolcell* bottom - pointer to most recently freed pool cell
  • kmpoolcell data[] - content of cell

struct kmpoolcell

  • kmpoolcell* last - pointer to last element allocated before this one
  • char data[] - pool data

enum kmshred

kmshred is an enum used to indicate whether an object should be "shredded" (written over) in memory when it's deleted. this is a useful means to ensure that privileged information is not accidentally left in memory after use. if the shredding mechanism is not useful, compile libk with the flag KFmem_noshred to exclude its functions and fields.

  • kmshred_no = 0 - marks an object not to shred on free
  • kmshred_yes = 1 - marks an object to shred on free

naming convention

kmem function names are based on the method of allocation and the action being performed. methods are listed in the section below. kmem defines a number of standardized actions, though not every method uses every action. the character listed in brackets is suffixed to the name of the method to produce a function name: for instance, kmheapa will allocate memory on the heap, while kmrefd will decrement the reference count of its argument.

  • initialize [i] - initializes a memory store on the heap
  • initialize fixed [if] - initialize a memory store on the stack or in a fixed-size global
  • allocate [a] - allocate a new region of memory of the given size, ready to write, and return a struct kmres containing an error code and, if the call succeeded, a pointer in the field raw. always check the cond field of its return value to ensure allocation succeeded. contents of the region specified by the raw field are undefined. takes parameter size_t sz.
  • allocate pointer object [o] - like allocate, but yields a kmptr in the ptr field of its return value instead of a raw void*. takes parameters size_t sz.
  • zero [z] - allocate a new region of memory and zero it before returning it for writing.
  • zero pointer object [zo] - like zero, but returns a kmptr instead of a raw void*.
  • free [f] - free a section of memory, either decrementing a reference count or returning it to whatever pool it came from.
  • shred [s] - destroy whatever was in the segment of memory, then return it to the pool it came from.
  • destroy [x] - tears down a memory store
  • upref [u] - increments a reference counter

methods

kmem currently supports the following methods of memory management, along with which methods are defined for it. (note that a implies z and f implies s). a method may be excluded from a libk binary by defining the flag KFmem_no[name], e.g. KFmem_noheap.

the fastest allocator is the linear allocator, which should be sufficient for most simple programs. it allocates and deallocates memory simply by resizing the stack; there is no fragmentation, but objects must be freed in the order they are allocated. however, entire groups of objects can be freed at once at very little cost.

caveat scriptor: the linear allocation method is unfortunately complicated by the fact that the methods it uses make it fundamentally incompatible with use in the program of the "malloc" allocator used by libc. this should not be a problem for programs that only link libk, but if they link in any external libraries that depend on libc and which use malloc internally, the linear allocator cannot be used.

  • lin [iax] - linear heap allocator
    • kmlini - return a pointer to the current top of the heap
    • kmlina - allocate space on the heap and increase its size appropriately
    • kmlinz - allocate zero-filled space on the heap and increase its size appropriately
    • kmlinx - returns the top of the heap to the location specified, freeing all memory allocated since the call to kmlini() or kmlina() that produced it
  • heap [af] - random heap allocation
    • kmheapa - allocate
    • kmheapz - zero-allocate
    • kmheapo - allocate pointer object
    • kmheapzo - zero-allocate pointer object
    • kmheapf - free
    • kmheaps - shred
  • ref [afu] - reference-counted heap object
    • kmrefa - allocate
    • kmrefz - zero-allocate
    • kmrefo - allocate pointer object
    • kmrefzo - zero-allocate pointer object
    • kmreff - downref; free if last ref
    • kmrefs - downref and mark for shred on last ref
  • pool [ixaf] - memory pool
    • kmpooli(kmcell*, size_t sz, size_t n) → kmpool* - initialize a fixed memory pool (a pool of n cells of length sz)
    • kmpoolx - tear down a memory pool
    • kmpoola - allocate from pool
    • kmpoolz - zero-allocate from pool
    • kmpoolo - allocate pointer object
    • kmpoolzo - zero-allocate pointer object
    • kmpoolf - downref; free if last ref
    • kmpools - downref and mark for shred on last ref
  • tree [af] - uses a node-child strategy. when a node is freed, all of its children are automatically freed as well.
    • kmtreea - create a tree node. if parent is NULL, the node will the top of a new tree. if src is null, allocate on-heap.
    • kmtreez - like kmtreea but zeroed
    • kmtreeao - like kmtreea but returns a kmptr
    • kmtreezo - like kmtreez but returns a kmptr
    • kmtreef - frees a node and all its children

macros

kmem defines the following macros.

  • Kmsz(array) - a convenience macro to return the number of elements in a static array. inserts the text ( sizeof (array) / sizeof (array) [0] )
  • Kmszs(type, struct) - a convenience macro to return the size of a struct. requires compound literals.
  • Kmszsa(type, array) - calculates the number of elements in an array of a given struct type. inserts the text ( sizeof ( (type) array ) / sizeof (type) )
  • Kmpsa(type, struct) - a convenience macro to insert a "pascal struct array" - that is, a struct array prefixed with a size value. this is equivalent to Kmszsa(type, array), array