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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