Posted on 2009-12-18 16:54
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C++
1. realloc失敗的時候，返回NULL
2. realloc失敗的時候，原來的內存不改變，也就是不free或不move，(這個地方很容易出錯)
3. 假如原來的內存后面還有足夠多剩余內存的話，realloc的內存=原來的內存+剩余內存,realloc還是返回原來內存的地址; 假如原來的內存后面沒有足夠多剩余內存的話，realloc將申請新的內存，然后把原來的內存數據拷貝到新內存里，原來的內存將被free掉,realloc返回新內存的地址
4. 如果size為0，效果等同于free()
5. 傳遞給realloc的指針必須是先前通過malloc(), calloc(), 或realloc()分配的
6. 傳遞給realloc的指針可以為空，等同于malloc。
/*?realloc.c?-?C?standard?library?routine.
???Copyright?(c)?1989,?1993??Michael?J.?Haertel
???You?may?redistribute?this?library?under?the?terms?of?the
???GNU?Library?General?Public?License?(version?2?or?any?later
???version)?as?published?by?the?Free?Software?Foundation.
???THIS?SOFTWARE?IS?PROVIDED?"AS?IS"?WITHOUT?ANY?EXPRESS?OR?IMPLIED
???WARRANTY.??IN?PARTICULAR,?THE?AUTHOR?MAKES?NO?REPRESENTATION?OR
???WARRANTY?OF?ANY?KIND?CONCERNING?THE?MERCHANTABILITY?OF?THIS
???SOFTWARE?OR?ITS?FITNESS?FOR?ANY?PARTICULAR?PURPOSE.?*/
#include?<limits.h>
#include?<stddef.h>
#include?<stdlib.h>
#include?<string.h>
#include?"malloc.h"
#ifdef?__ELF__
#pragma?weak?realloc?=?__libc_realloc
#endif
#define?MIN(A,?B)?((A)?<?(B)???(A)?:?(B))
/*?Resize?the?given?region?to?the?new?size,?returning?a?pointer
???to?the?(possibly?moved)?region.??This?is?optimized?for?speed;
???some?benchmarks?seem?to?indicate?that?greater?compactness?is
???achieved?by?unconditionally?allocating?and?copying?to?a
???new?region.?*/
void?*
__libc_realloc?(void?*ptr,?size_t?size)
{
????void?*result,?*previous;
????int?block,?blocks,?type;
????int?oldlimit;
????if?(!ptr)
????????return?__libc_malloc(size);
????if?(!size)?{
????????__libc_free(ptr);
????????return?__libc_malloc(0);
????}
????block?=?BLOCK(ptr);
????switch?(type?=?_heapinfo[block].busy.type)?{
????case?0:
????????/*?Maybe?reallocate?a?large?block?to?a?small?fragment.?*/
????????if?(size?<=?BLOCKSIZE?/?2)?{
????????????if?((result?=?__libc_malloc(size))?!=?NULL)?{
????????????????????memcpy(result,?ptr,?size);
#if?1
????????????????????__libc_free(ptr);
#else
????????????????????_free_internal(ptr);
#endif
????????????}
????????????return?result;
????????}
????????/*?The?new?size?is?a?large?allocation?as?well;?see?if
???????????we?can?hold?it?in?place.?*/
????????blocks?=?BLOCKIFY(size);
????????if?(blocks?<?_heapinfo[block].busy.info.size)?{
????????????/*?The?new?size?is?smaller;?return?excess?memory
???????????????to?the?free?list.?*/
????????????_heapinfo[block?+?blocks].busy.type?=?0;
????????????_heapinfo[block?+?blocks].busy.info.size
????????????????=?_heapinfo[block].busy.info.size?-?blocks;
????????????_heapinfo[block].busy.info.size?=?blocks;
#if?1
????????????__libc_free(ADDRESS(block?+?blocks));
#else
????????????_free_internal(ADDRESS(block?+?blocks));
#endif
????????????return?ptr;
????????}?else?if?(blocks?==?_heapinfo[block].busy.info.size)
????????????/*?No?size?change?necessary.?*/
????????????return?ptr;
????????else?{
????????????/*?Won't?fit,?so?allocate?a?new?region?that?will.??Free
???????????????the?old?region?first?in?case?there?is?sufficient?adjacent
???????????????free?space?to?grow?without?moving.?*/
????????????blocks?=?_heapinfo[block].busy.info.size;
????????????/*?Prevent?free?from?actually?returning?memory?to?the?system.?*/
????????????oldlimit?=?_heaplimit;
????????????_heaplimit?=?0;
#if?1
????????????__libc_free(ptr);
#else
????????????_free_internal(ptr);
#endif
????????????_heaplimit?=?oldlimit;
????????????result?=?__libc_malloc(size);
????????????if?(!result)?{
????????????????/*?Now?we're?really?in?trouble.??We?have?to?unfree
???????????????????the?thing?we?just?freed.??Unfortunately?it?might
???????????????????have?been?coalesced?with?its?neighbors.?*/
????????????????if?(_heapindex?==?block)
????????????????????__libc_malloc(blocks?*?BLOCKSIZE);
????????????????else?{
????????????????????previous?=?malloc((block?-?_heapindex)?*?BLOCKSIZE);
????????????????????__libc_malloc(blocks?*?BLOCKSIZE);
#if?1
????????????????????__libc_free(previous);
#else
????????????????????_free_internal(previous);
#endif
????????????????}????????????
????????????????return?NULL;
????????????}
????????????if?(ptr?!=?result)
????????????????memmove(result,?ptr,?blocks?*?BLOCKSIZE);
????????????return?result;
????????}
????????break;
????default:
????????/*?Old?size?is?a?fragment;?type?is?logarithm?to?base?two?of
???????????the?fragment?size.?*/
????????if?((size?>?1?<<?(type?-?1))?&&?(size?<=?1?<<?type))
????????????/*?New?size?is?the?same?kind?of?fragment.?*/
????????????return?ptr;
????????else?{
????????????/*?New?size?is?different;?allocate?a?new?space,?and?copy
???????????????the?lesser?of?the?new?size?and?the?old.?*/
????????????result?=?__libc_malloc(size);
????????????if?(!result)
????????????????return?NULL;
????????????memcpy(result,?ptr,?MIN(size,?1?<<?type));
????????????__libc_free(ptr);
????????????return?result;
????????}
????????break;
????}
}
</code>
以上就是?libc??庫中?recalloc()?函數的實現，發現在新的內存申請成功的時候，程序是在做了?p=malloc(size);?
????memcpy(p,?ptr,?size);
????free(ptr);
三步操作的。