面試官問：malloc(0)時(shí)程序會(huì)返回什么？

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The order and contiguity of storage allocated by successive calls to the aligned_alloc, calloc, malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated). The lifetime of an allocated object extends from the allocation until the deallocation. Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned to indicate an error, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

/*
??malloc(size_t?n)
??Returns?a?pointer?to?a?newly?allocated?chunk?of?at?least?n?bytes,?or?null
??if?no?space?is?available.?Additionally,?on?failure,?errno?is
??set?to?ENOMEM?on?ANSI?C?systems.

??If?n?is?zero,?malloc?returns?a?minumum-sized?chunk.?(The?minimum
??size?is?16?bytes?on?most?32bit?systems,?and?24?or?32?bytes?on?64bit
??systems.)??On?most?systems,?size_t?is?an?unsigned?type,?so?calls
??with?negative?arguments?are?interpreted?as?requests?for?huge?amounts
??of?space,?which?will?often?fail.?The?maximum?supported?value?of?n
??differs?across?systems,?but?is?in?all?cases?less?than?the?maximum
??representable?value?of?a?size_t.
*/

/*?pad?request?bytes?into?a?usable?size?--?internal?version?*/
#define?request2size(req)?????????????????????????????????????????\
??(((req)?+?SIZE_SZ?+?MALLOC_ALIGN_MASK????MINSIZE?:??????????????????????????????????????????????????????\
???((req)?+?SIZE_SZ?+?MALLOC_ALIGN_MASK)?&?~MALLOC_ALIGN_MASK)

/*?Same,?except?also?perform?an?argument?and?result?check.??First,?we?check
???that?the?padding?done?by?request2size?didn't?result?in?an?integer
???overflow.??Then?we?check?(using?REQUEST_OUT_OF_RANGE)?that?the?resulting
???size?isn't?so?large?that?a?later?alignment?would?lead?to?another?integer
???overflow.??*/

#define?checked_request2size(req,?sz)?\
({????????\
??(sz)?=?request2size?(req);?????\
??if?(((sz)???????||?REQUEST_OUT_OF_RANGE?(sz))?\
????{????????\
??????__set_errno?(ENOMEM);?????\
??????return?0;???????\
????}????????\
})

/*?The?smallest?possible?chunk?*/
#define?MIN_CHUNK_SIZE????????(offsetof(struct?malloc_chunk,?fd_nextsize))
/*?The?smallest?size?we?can?malloc?is?an?aligned?minimal?chunk?*/
#define?MINSIZE??\
??(unsigned?long)(((MIN_CHUNK_SIZE+MALLOC_ALIGN_MASK)?&?~MALLOC_ALIGN_MASK))/*?The?corresponding?bit?mask?value.??*/
#define?MALLOC_ALIGN_MASK?(MALLOC_ALIGNMENT?-?1)
/*?MALLOC_ALIGNMENT?is?the?minimum?alignment?for?malloc'ed?chunks.??It
???must?be?a?power?of?two?at?least?2?*?SIZE_SZ,?even?on?machines?for
???which?smaller?alignments?would?suffice.?It?may?be?defined?as?larger
???than?this?though.?Note?however?that?code?and?data?structures?are
???optimized?for?the?case?of?8-byte?alignment.??*/
#define?MALLOC_ALIGNMENT?(2?*?SIZE_SZ????????__alignof__?(long?double)?:?2?*?SIZE_SZ)

#ifndef?INTERNAL_SIZE_T
#?define?INTERNAL_SIZE_T?size_t
#endif

/*?The?corresponding?word?size.??*/
#define?SIZE_SZ?(sizeof?(INTERNAL_SIZE_T))

/*
??This?struct?declaration?is?misleading?(but?accurate?and?necessary).
??It?declares?a?"view"?into?memory?allowing?access?to?necessary
??fields?at?known?offsets?from?a?given?base.?See?explanation?below.
*/

struct?malloc_chunk?{

??INTERNAL_SIZE_T??????mchunk_prev_size;??/*?Size?of?previous?chunk?(if?free).??*/
??INTERNAL_SIZE_T??????mchunk_size;???????/*?Size?in?bytes,?including?overhead.?*/

??struct?malloc_chunk*?fd;?????????/*?double?links?--?used?only?if?free.?*/
??struct?malloc_chunk*?bk;

??/*?Only?used?for?large?blocks:?pointer?to?next?larger?size.??*/
??struct?malloc_chunk*?fd_nextsize;?/*?double?links?--?used?only?if?free.?*/
??struct?malloc_chunk*?bk_nextsize;
};

//?GCC?提供
/*?Offset?of?member?MEMBER?in?a?struct?of?type?TYPE.?*/
#define?offsetof(TYPE,?MEMBER)?__builtin_offsetof?(TYPE,?MEMBER)

#include?
#include?
int?main(void)?{
????char?*p?=?malloc(0);
????printf("Address:?0x%x.\nLength:?%ld.\n",p,malloc_usable_size(p));
????return?0;
}