LCC编译器的源程序分析(68)内存分配链表
LCC采用大块内存的方法，那它分配内存也是比较特殊的，它的源程序如下：
#001 //大块内存结构。
#002 struct block
#003 {
#004
struct block *next; //后继块指针。
#005
char *limit; //尾位置
#006
char *avail; //可用的开始位置.
#007 };
#008
#009 //共用最大的类型.
#010 union align
#011 {
#012
long l;
#013
char *p;
#014
double d;
#015
int (*f)(void);
#016 };
#017
#018 //
#019 union header
#020 {
#021
struct block b;
#022
union align a;
#023 };
#024
#025 #ifdef PURIFY
#026 union header *arena[3];
#027
#028 void *allocate(unsigned long n, unsigned a) {
#029
union header *new = malloc(sizeof *new + n);
#030
#031
assert(a < NELEMS(arena));
#032
if (new == NULL) {
#033
error("insufficient memory/n");
#034
exit(1);
#035
}
#036
new->b.next = (void *)arena[a];
#037
arena[a] = new;
#038
return new + 1;
#039 }
#040
#041 void deallocate(unsigned a) {
#042
union header *p, *q;
#043
#044
assert(a < NELEMS(arena));
#045
for (p = arena[a]; p; p = q) {
#046
q = (void *)p->b.next;
#047
free(p);
#048
}
#049
arena[a] = NULL;
#050 }
#051
#052 void *newarray(unsigned long m, unsigned long n, unsigned a) {
#053
return allocate(m*n, a);
#054 }
#055 #else
#056
#057 //三大块内存开始头.
#058 static struct block first[] = {
#059
{ NULL }, { NULL }, { NULL }
#060 };
#061
#062 //三大块内存的尾指针.
#063 static struct block *arena[] = { &first[0], &first[1], &first[2] };
#064
#065 //空闲块的内存头指针.
#066 static struct block *freeblocks;
#067
#068 //分配n个字节在a区域里.
#069 void *allocate(unsigned long n, unsigned a)
#070 {
#071
struct block *ap;
#072
#073
assert(a < NELEMS(arena));
#074
assert(n > 0);
#075
#076
//获取尾块指针。
#077
ap = arena[a];
#078
#079
//分配需要使用的内存和内存头。
#080
n = roundup(n, sizeof (union align));
#081
#082
//空闲内存是否大于需要分配的。
#083
while (n > (unsigned long)(ap->limit - ap->avail))
#084
{
#085
//如果有空闲块在列表里。
#086
if ((ap->next = freeblocks) != NULL)
#087
{
#088
//取得已经分配过的大块空闲内存。
#089
freeblocks = freeblocks->next;
#090
ap = ap->next;
#091
}
#092
else
#093
{
#094
//没有大块内存，开始分配大块内存。
#095
unsigned m = sizeof (union header) + n + roundup(10*1024, sizeof (union align));
#096
#097
//内存真实开始地址.
#098
ap->next = (block*)malloc(m);
#099
#100
//指向尾指针.
#101
ap = ap->next;
#102
#103
//分配内存是否出错.
#104
if (ap == NULL)
#105
{
#106
error("insufficient memory/n");
#107
exit(1);
#108
}
#109
#110
//内存块尾地址.
#111
ap->limit = (char *)ap + m;
#112
}
#113
#114
//实际上可以使用内存开始位置.
#115
ap->avail = (char *)((union header *)ap + 1);
#116
#117
//下一块内存为空.
#118
ap->next = NULL;
#119
arena[a] = ap;
#120
}
#121
#122
//移动大块内存的空闲开始位置,n是需要分配的内存.
#123
ap->avail += n;
#124
#125
//返回分配的内存地址开始位置.
#126
return ap->avail - n;
#127 }