a simple GC

先看这篇文章

http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/

一个很小的GC,如何实现的呢?看一下源代码

#include <stdio.h>#include <stdlib.h>#define STACK_MAX 256typedef enum {  OBJ_INT,  OBJ_PAIR} ObjectType;typedef struct sObject {  ObjectType type;  unsigned char marked;  /* The next object in the linked list of heap allocated objects. */  struct sObject* next;  union {    /* OBJ_INT */    int value;    /* OBJ_PAIR */    struct {      struct sObject* head;      struct sObject* tail;    };  };} Object;typedef struct {  Object* stack[STACK_MAX];  int stackSize;  /* The first object in the linked list of all objects on the heap. */  Object* firstObject;  /* The total number of currently allocated objects. */  int numObjects;  /* The number of objects required to trigger a GC. */  int maxObjects;} VM;void assert(int condition, const char* message) {  if (!condition) {    printf("%s\n", message);    exit(1);  }}VM* newVM() {  VM* vm = malloc(sizeof(VM));  vm->stackSize = 0;  vm->firstObject = NULL;  vm->numObjects = 0;  vm->maxObjects = 8;  return vm;}void push(VM* vm, Object* value) {  assert(vm->stackSize < STACK_MAX, "Stack overflow!");  vm->stack[vm->stackSize++] = value;}Object* pop(VM* vm) {  assert(vm->stackSize > 0, "Stack underflow!");  return vm->stack[--vm->stackSize];}void mark(Object* object) {  /* If already marked, we're done. Check this first to avoid recursing     on cycles in the object graph. */  if (object->marked) return;  object->marked = 1;  if (object->type == OBJ_PAIR) {    mark(object->head);    mark(object->tail);  }}void markAll(VM* vm){  for (int i = 0; i < vm->stackSize; i++) {    mark(vm->stack[i]);  }}void sweep(VM* vm){  Object** object = &vm->firstObject;  while (*object) {    if (!(*object)->marked) {      /* This object wasn't reached, so remove it from the list and free it. */      Object* unreached = *object;      *object = unreached->next;      free(unreached);      vm->numObjects--;    } else {      /* This object was reached, so unmark it (for the next GC) and move on to       the next. */      (*object)->marked = 0;      object = &(*object)->next;    }  }}void gc(VM* vm) {  int numObjects = vm->numObjects;  markAll(vm);  sweep(vm);  vm->maxObjects = vm->numObjects * 2;  printf("Collected %d objects, %d remaining.\n", numObjects - vm->numObjects,         vm->numObjects);}Object* newObject(VM* vm, ObjectType type) {  if (vm->numObjects == vm->maxObjects) gc(vm);  Object* object = malloc(sizeof(Object));  object->type = type;  object->next = vm->firstObject;  vm->firstObject = object;  object->marked = 0;  vm->numObjects++;  return object;}void pushInt(VM* vm, int intValue) {  Object* object = newObject(vm, OBJ_INT);  object->value = intValue;  push(vm, object);}Object* pushPair(VM* vm) {  Object* object = newObject(vm, OBJ_PAIR);  object->tail = pop(vm);  object->head = pop(vm);  push(vm, object);  return object;}void objectPrint(Object* object) {  switch (object->type) {    case OBJ_INT:      printf("%d", object->value);      break;    case OBJ_PAIR:      printf("(");      objectPrint(object->head);      printf(", ");      objectPrint(object->tail);      printf(")");      break;  }}void freeVM(VM *vm) {  vm->stackSize = 0;  gc(vm);  free(vm);}void test1() {  printf("Test 1: Objects on stack are preserved.\n");  VM* vm = newVM();  pushInt(vm, 1);  pushInt(vm, 2);  gc(vm);  assert(vm->numObjects == 2, "Should have preserved objects.");  freeVM(vm);}void test2() {  printf("Test 2: Unreached objects are collected.\n");  VM* vm = newVM();  pushInt(vm, 1);  pushInt(vm, 2);  pop(vm);  pop(vm);  gc(vm);  assert(vm->numObjects == 0, "Should have collected objects.");  freeVM(vm);}void test3() {  printf("Test 3: Reach nested objects.\n");  VM* vm = newVM();  pushInt(vm, 1);  pushInt(vm, 2);  pushPair(vm);  pushInt(vm, 3);  pushInt(vm, 4);  pushPair(vm);  pushPair(vm);  gc(vm);  assert(vm->numObjects == 7, "Should have reached objects.");  freeVM(vm);}void test4() {  printf("Test 4: Handle cycles.\n");  VM* vm = newVM();  pushInt(vm, 1);  pushInt(vm, 2);  Object* a = pushPair(vm);  pushInt(vm, 3);  pushInt(vm, 4);  Object* b = pushPair(vm);  /* Set up a cycle, and also make 2 and 4 unreachable and collectible. */  a->tail = b;  b->tail = a;  gc(vm);  assert(vm->numObjects == 4, "Should have collected objects.");  freeVM(vm);}void perfTest() {  printf("Performance Test.\n");  VM* vm = newVM();  for (int i = 0; i < 1000; i++) {    for (int j = 0; j < 20; j++) {      pushInt(vm, i);    }    for (int k = 0; k < 20; k++) {      pop(vm);    }  }  freeVM(vm);}int main(int argc, const char * argv[]) { // test1();  test2(); // test3(); // test4(); // perfTest();  return 0;}

VM有两种类型,int和pair, 我们分配在heap的内存在栈上有引用,而如果然后退出,栈的内容被清除,而heap没有delete,就是所谓的内存泄漏

VM里面有heap的引用链表,对stack上的引用逐个mark,没有被stack到达的heap内存,被VM给delete

原理挺简单的,可是作者很好的抽象并仿真出来,确实是非常好的思路