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1、After the call to free, p holds a dangling pointer— a pointer that refers to memory that logically does not exist. Subse-quently dereferencing p is an error, although if the block hasn’t been reallocated for another purpose, the error might go undetected.
2、another error: deallocating free memory.
3、Another error is deallocating memory that wasn’t allocated by malloc , calloc , or realloc.
针对以上错误，此版本memory实现较为复杂，使用了哈希表数据结构。
这个是内存的结构。整体而言用哈希表+链表进行存储。同时，释放的空间用freelist循环链表进行连接。有阴影的部分表示目前已经申请正在使用的空间。C函数中几个函数都是在这个数据结构上进行操作的。
==============================mem.h=====================================
#ifndef MEM_INCLUDED
#define MEM_INCLUDED
#include "except.h"//exported exceptions
extern const Except_T Mem_Failed;//exported functions
extern void *Mem_alloc(long nbytes,const char *file, int line);
extern void *Mem_calloc(long count, long nbytes,const char *file, int line);
extern void Mem_free(void *ptr,const char *file, int line);
extern void *Mem_resize(void *ptr, long nbytes,const char *file, int line);
//exported macros
#define ALLOC(nbytes) \Mem_alloc((nbytes), __FILE__, __LINE__)
#define CALLOC(count, nbytes) \Mem_calloc((count), (nbytes), __FILE__, __LINE__)
#define NEW(p) ((p) = ALLOC((long)sizeof *(p)))
#define NEW0(p) ((p) = CALLOC(1, (long)sizeof *(p)))
#define FREE(ptr) ((void)(Mem_free((ptr),\__FILE__, __LINE__), (ptr) = 0))
#define RESIZE(ptr, nbytes)((ptr) = Mem_resize((ptr),\(nbytes), __FILE__, __LINE__))
#endif
=============================memchk.c==================================
#include <stdlib.h>
#include <string.h>
#include "assert.h"
#include "except.h"
#include "mem.h"//checking types
union align{int i;long l;long *lp;void *p;void (*fp)(void);float f;double d;long double ld;
};//checking macros
#define hash(p, t) (((unsigned long)(p)>>3) & \(sizeof (t)/sizeof((t)[0])-1))
#define NDESCRIPTORS 512
#define NALLOC((4096 + sizeof(union align)-1)/ \(sizeof(union align)))*(sizeof (union align))//data
const Except_T Mem_Failed = { "Allocation Failed" };//checking data
static struct descriptor{struct descriptor *free;struct descriptor *link;const void *ptr;long size;const char *file;int line;
} *htab[2048];
static struct descriptor freelist = { &freelist };//checking functions
static struct descriptor *find(const void *ptr){struct descriptor *bp = htab[hash(ptr, htab)];while(bp && bp->ptr != ptr)bp = bp->link;return bp;
}void Mem_free(void *ptr, const char *file, int line){if(ptr){struct descriptor *bp;if(((unsigned long)ptr)%(sizeof (union align)) != 0|| (bp = find(ptr)) == NULL || bp->free)Except_raise(&Assert_Failed, file, line);bp->free = freelist.free;freelist.free = bp;}
}void *Mem_resize(void *ptr, long nbytes,const char *file, int line){struct descriptor *bp;void *newptr;assert(ptr);assert(nbytes > 0);if(((unsigned long)ptr)%(sizeof (union align)) != 0|| (bp = find(ptr)) == NULL || bp->free)Except_raise(&Assert_Failed, file, line);newptr = Mem_alloc(nbytes, file, line);memcpy(newptr, ptr, nbytes < bp->size ? nbytes : bp->size);Mem_free(ptr, file, line);return newptr;
}void *Mem_calloc(long count, long nbytes,const char *file, int line){void *ptr;assert(count > 0);assert(nbytes > 0);ptr = Mem_alloc(count*nbytes, file, line);memset(ptr, '\0', count*nbytes);return ptr;
}static struct descriptor *dalloc(void *ptr, long size, const char *file, int line){static struct descriptor *avail;static int nleft;if(nleft <= 0){avail = malloc(NDESCRIPTORS * sizeof (*avail));if(avail == NULL)return NULL;nleft = NDESCRIPTORS;}avail->ptr = ptr;avail->size = size;avail->file = file;avail->line = line;avail->free = avail->link = NULL;nleft--;return avail++;
}void *Mem_alloc(long nbytes, const char *file, int line){struct descriptor *bp;void *ptr;assert(nbytes > 0);//round nbytes up to an alignment boundary>nbytes = ((nbytes + sizeof(union align) - 1)/(sizeof(union align)))*(sizeof(union align));for(bp = freelist.free; bp; bp = bp->free){if(bp->size > nbytes){//use the end of the block at bp->ptrbp->size -= nbytes;ptr = (char*)bp->ptr + bp->size;if((bp = dalloc(ptr, nbytes, file, line)) != NULL){unsigned h = hash(ptr, htab);bp->link = htab[h];htab[h] = bp;return ptr;}else{if(file == NULL)RAISE(Mem_Failed);elseExcept_raise(&Mem_Failed, file, line);}}if(bp == &freelist){struct descriptor *newptr;//<newptr <- a block of size NALLOC + nbytes >if((ptr = malloc(nbytes + NALLOC)) == NULL|| (newptr = dalloc(ptr, nbytes+NALLOC,__FILE__, __LINE__)) == NULL){if(file == NULL)RAISE(Mem_Failed);elseExcept_raise(&Mem_Failed, file, line);}newptr->free = freelist.free;freelist.free = newptr;}}assert(0);return NULL;
}
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