@@ -1,24 +1,26 @@ # 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()` -## module functions +# module functions kmem supplies two module-level functions, used to interact with the `kmptr` container type. - * `kmfree(kmptr) → void` - free, downref, or ignore the pasted object as appropriate - * `kmshred(kmptr) → void` - free, downref, or ignore the pasted object as appropriate. if deallocating, zero its contents + * `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 +# types kmem defines the following types: * `enum kmkind` - enumerates allocation strategies @@ -30,9 +32,9 @@ `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 `kmkind` is an enum that specifies an allocation function. * `kmkind_none` - no allocation @@ -41,9 +43,9 @@ * `kmkind_pool` - pool allocation * `kmkind_ref` - reference-counting allocation * `kmkind_tree` - tree allocation -### kmptr +## 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. @@ -56,17 +58,17 @@ * `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 +## 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 +## 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 @@ -73,9 +75,9 @@ * `size_t refs` - number of active references * `kmcell* src` - source, if any * `char data[]` - content of cell -### struct kmnode +## 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 @@ -84,36 +86,36 @@ * `kmnode* lastchild` - last child node * `kmnode* prev` - previous sibling, NULL if first * `kmnode* next` - next sibling, NULL if last -### struct kmpool +## 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 +### struct kmpoolcell * `kmpoolcell* last` - pointer to last element allocated before this one * `char data[]` - pool data -### enum kmshred +## 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 +# 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] - return a raw pointer to a new region of memory of the given size, ready to write, or NULL if not possible. contents of that region undefined. takes parameter (size_t sz). - * allocate pointer object [ao] - like *allocate*, but returns a `kmptr` instead of a raw `void*`. + * allocate [a] -a llocate a new region of memory of the given size, ready to write, and write a pointer to it into argument `where`. returns a value of `kmcond`; always check this to ensure allocation succeeded. contents of that region undefined. takes parameters `void** where, size_t sz`. + * allocate pointer object [o] - like *allocate*, but fills in a `kmptr` instead of a raw `void*`. takes parameters `kmptr* where, 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.