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 }

 

 

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