C.Interface.And.Implementations—memory(arena版)的实现

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1、This chapter describes a memory-management interface and an imple-mentation that uses arena-based algorithms, which allocate memory from an arena and deallocate entire arenas at once.

2、With the arena-based allocator, there’s no obligation to call  free for every call to  malloc ; there’s only a single call that deallocates all the memory allocated in an arena since the last deallocation. 

arena的基本思想就是“以栈的形式不断申请新的空间，然后一次性释放内存”。我们可以通过如下图可以看出内存的分布图：

         ================================arena.h===============================

#ifndef ARENA_INCLUDED
#define ARENA_INCLUDED
#include "except.h"#define T Arena_T
typedef struct T *T;extern const Except_T Arena_NewFailed;
extern const Except_T Arena_Failed;//exported functions
extern T     Arena_new    (void);
extern void  Arena_dispose(T *ap);
extern void *Arena_alloc  (T arena, long nbytes,const char *file, int line);
extern void *Arena_calloc (T arena, long count,long nbytes, const char *file, int line);
extern void  Arena_free   (T arena);#undef T
#endif

==============================arena.c==============================

#include <stdlib.h>
#include <string.h>
#include "assert.h"
#include "except.h"
#include "arena.h"
#define T Arena_Tconst Except_T Arena_NewFailed = { "Arena Creation Failed" };
const Except_T Arena_Failed = { "Arena Allocation Failed" };//macros
#define THRESHOLD 10//types
struct T{T prev;char *avail;char *limit;
};union align{int i;long l;long *lp;void *p;void (*fp)(void);float f;double d;long double ld;
};union header{struct T b;union align a;
};//data
static T freechunks;
static int nfree;//functions
T Arena_new(void){T arena = malloc(sizeof (*arena));if(arena == NULL)RAISE(Arena_NewFailed);arena->prev = NULL;arena->limit = arena->avail = NULL;return arena;
}void Arena_dispose(T *ap){assert(ap && *ap);Arena_free(*ap);free(*ap);*ap = NULL;
}void *Arena_alloc(T arena, long nbytes,const char *file, int line){assert(arena);assert(nbytes > 0);//round nbytes up to an alignment boundarynbytes = ((nbytes + sizeof(union align) - 1) /(sizeof (union align))) * (sizeof (union align));while(nbytes > arena->limit - arena->avail){//get a new chunkT ptr;char *limit;//ptr<-a new chunkif((ptr = freechunks) != NULL){freechunks = freechunks->prev;nfree--;limit = ptr->limit;}else{long m = sizeof (union header) + nbytes + 10*1024;ptr = malloc(m);if(ptr == NULL){if(file == NULL)RAISE(Arena_Failed);elseExcept_raise(&Arena_Failed, file, line);}limit = (char *)ptr + m;}*ptr = *arena;arena->avail = (char *)((union header *)ptr + 1);arena->limit = limit;arena->prev = ptr;}arena->avail += nbytes;return arena->avail - nbytes;
}void *Arena_calloc(T arena, long count, long nbytes,const char *file, int line){void *ptr;assert(count > 0);ptr = Arena_alloc(arena, count*nbytes, file, line);memset(ptr, '\0', count*nbytes);return ptr;
}void Arena_free(T arena){assert(arena);while(arena->prev){struct T tmp = *arena->prev;//free the chunk described by arenaif(nfree < THRESHOLD){arena->prev->prev = freechunks;freechunks = arena->prev;nfree++;freechunks->limit = arena->limit;}else{free(arena->prev);}*arena = tmp;}assert(arena->limit == NULL);assert(arena->avail == NULL);
}

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