1樓:
to store any member of the basicexecution character set. If a member of the basic execution character set is stored in a char object, its value is guaranteed to be positive. If any other character is stored in a char object, the resulting value is implementation-dened but shall be within the range of values that can be represented in that type.
所以,sizeof的結果其實告訴我們的是:多少個char,而不是多少個byte。但是,如果結合sizeof後半段關於array和struct都用了bytes,所以,唯一合理的解釋就是:
乙個char就是乙個byte。(我個人估計標準委員會那幫傢伙寫暈頭了?)
然後,sizeof('a')一定是sizeof(int)。
6.4.4.4 Character constants
An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in 'x'.
Aninteger characterconstant has typeint.
接著,再解釋一下sizeof(*s+0)。
這其實是發生了算數提公升:
6.3.1 Arithmetic operands
The following may be used in an expression wherever an int or unsigned int may be used:
— An object or expression with aninteger typewhose integer conversion rank isless
than the rank of int and unsigned int.也就是,小於int的整形(char也是),在運算中會提公升為int運算,那結果自然也是int了。
最後,再解釋一下byte和bit的關係:
3.61 byte
addressable unit of data storage large enough to hold any member of the basic character set of the execution environment
2 NOTE 1 It is possible to express the address of each individual byte of an object uniquely.
3 NOTE 2 A byte is composed of a contiguous sequence of bits,the number of which is implementation-dened. The least signicant bit is called the low-order bit; the most signicant bit is called the high-order bit.但是這個實現相關也不是天馬行空的,至少規定了下限:
large enough to hold any member of the basic character set
而在5.2.1的定義,basic character set有95個:
26*2的大小寫字母,10個數字、29個符號、4個特殊字元,那也就是起碼要7個bit。再結合6.2.
5中char的定義,必須要求是正數,那就要再加乙個符號位:
If a member of the basic execution character set is stored in a char object, its value isguaranteed to be positive那也就是說,1個byte起碼要8個bit。也就是說,理論上可能會出現1byte=9bit的系統,但是不可能出現1byte=7bit的系統。
2樓:
char*s
="hello"
;printf
("sizeof char is %lu, sizeof char* is %lu, sizeof 'a' is %lu\n"
,sizeof
(char
),sizeof
(char*),
sizeof
('a'
));//char 字元大小 1byte char* 指標 64位的機子為 8byte 『a』為int 4byte
printf
("sizeof s is %lu\n"
,sizeof(*
s+0));
//*s為陣列第乙個,'h' +0 轉化為 int 4byteprintf
("sizeof s is %lu\n"
,sizeof(*
s));
//*s 陣列第乙個元素 』h『 1byte
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