Memory management demo from TCPL

  1  /*
  2     simple memory manage demo from TCPL
  3     should be compiled in unix
  4     debug to see how it works
  5    */
  6 
  7 #include <stdio.h>
  8 #include <stdlib.h>
  9 #include <unistd.h>
 10 
 11 typedef long Align;
 12 
 13 union header
 14 {
 15     struct
 16     {
 17         union header* ptr;
 18         unsigned size;
 19     } s;
 20     Align x;
 21 };
 22 
 23 typedef union header Header;
 24 
 25 
 26 static Header base;
 27 static Header* freep = NULL;
 28 static Header* morecore(unsigned);
 29 
 30 void* malloc(unsigned nbytes)
 31 {
 32     Header* p;
 33     Header* prevp;
 34     unsigned nunits;
 35     nunits = (nbytes + sizeof(Header) - 1) / sizeof(Header) + 1;
 36     if ((prevp = freep) == NULL)
 37     {
 38         base.s.ptr = freep = prevp = &base;
 39         base.s.size = 0;
 40     }
 41 
 42     for (p = prevp->s.ptr;; prevp = p, p = p->s.ptr)
 43     {
 44         if (p->s.size >= nunits)
 45         {
 46             if (p->s.size == nunits)
 47                 prevp->s.ptr = p->s.ptr;
 48             else
 49             {
 50                 p->s.size -= nunits;
 51                 p += p->s.size;
 52                 p->s.size = nunits;
 53             }
 54             freep = prevp;
 55 
 56             return (void*)(p + 1);
 57         }
 58         if (p == freep)
 59             if ((p == morecore(nunits)) == NULL)
 60                 return NULL;
 61     }
 62 }
 63 
 64 
 65 #define NALLOC 1024
 66 
 67 static Header* morecore(unsigned nu)
 68 {
 69     char* cp;
 70     Header* up;
 71 
 72     if (nu < NALLOC)
 73         nu = NALLOC;
 74     cp =(char*) sbrk(nu * sizeof(Header));
 75     if (cp == (char*) - 1)
 76         return NULL;
 77     up = (Header*)cp;
 78     up->s.size = nu;
 79     free((void*)(up + 1));
 80 
 81     return freep;
 82 }
 83 
 84 void free(void* ap)
 85 {
 86     Header* bp;
 87     Header* p;
 88 
 89     bp = (Header*)ap - 1;
 90     for (p = freep; !(bp > p && bp < p->s.ptr); p = p->s.ptr)
 91         if (p >= p->s.ptr && (bp > p || bp < p->s.ptr))
 92             break;
 93     if (bp + bp->s.size == p->s.ptr)
 94     {
 95         bp->s.size += p->s.ptr->s.size;
 96         bp->s.ptr = p->s.ptr->s.ptr;
 97     }
 98     else
 99         bp->s.ptr = p->s.ptr;
100 
101     if (p + p->s.size == bp)
102     {
103         p->s.size += bp->s.size;
104         p->s.ptr = bp->s.ptr;
105     }
106     else
107         p->s.ptr = bp;
108 
109     freep = p;
110 }
111 
112 int main(int argc, char* argv[])
113 {
114     char* str;
115     str=malloc(100*sizeof(char));
116 
117     char* str2=malloc(10*sizeof(char));
118 
119     free(str);
120     free(str2);
121 
122     return 0;
123 }

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原文地址：https://www.cnblogs.com/invisible/p/2744705.html