blocks|key|1338397|text|不，它不能保证是8位的。sizeof(char)保证为1，但这不一定意味着一个8位字节。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1338398|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

No, it is not guaranteed to be 8-bits. sizeof(char) is guaranteed to be 1, but that does not necessarily mean one 8-bit byte.

blocks|key|1554113|text|否，字符数据类型必须至少包含8位(参见ANSI+C规范)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1554114|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

no, char data type must contain at least 8 bits (see ANSI C specification)

blocks|key|1554165|text|C99标准草案规定一个字节必须至少8位宽，因为<limits.h>包含一个宏CHAR_BIT，它产生每个字节的位数，并保证至少为8位(§5.2.4.2.1)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1554166|C%2B%2B标准草案在名称<climits>+(§18.2.2)下包含C的<limits.h>。|1554167|entityMap^0|N|A|12|8|0|A|9|Y|A|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

The C99 standard draft says that a byte must be at least 8-bit wide, because <code>&lt;limits.h&gt;</code> contains a macro <code>CHAR_BIT</code> which yields the number of bits per byte, and is guaranteed to be at least 8 (§5.2.4.2.1).

The C++ standard draft includes C's <code>&lt;limits.h&gt;</code> under the name <code>&lt;climits&gt;</code> (§18.2.2).

blocks|key|1338610|text|我要说的第一件事是，如果你需要一个精确的位数的类型，那么使用一个特定于大小的类型。这取决于你的平台，从Linux上有符号8位类型的__s8到Windows上VC%2B%2B中的__int8。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1338611|现在，根据Robert+Love在"Linux内核开发“中关于可移植性的章节，他指出C标准”将标准类型的大小留给实现，尽管它确实规定了最小大小“。|1338612|然后，在页面底部的脚注中，他说：“除了char，它总是8位的。”|1338613|现在我不确定他是基于什么，但可能是the+ANSI+C+spec中的这一节|1338614|2.2.4.2数值限制|1338615|一致性实现应记录本节中指定的所有限制，这些限制应在标头limits.h和flat.h中指定|1338616|“整数类型的大小限制。h”|1338617|下面给出的值应替换为适合在#if预处理指令中使用的常量表达式。其实现定义的数值应等于或大于所显示的数值(绝对值)，并具有相同的符号。|1338618|非位字段的最小对象的最大位数(字节)|1338619|CHAR_BIT+8|1338620|带符号字符类型的对象的最小值|1338621|SCHAR_MIN+-127|1338622|带符号字符类型的对象的最大值|1338623|SCHAR_MAX+%2B127|1338624|unsigned+char类型的对象的最大值|1338625|UCHAR_MAX+255|1338626|char类型的对象的最小值|1338627|CHAR_MIN见下文|1338628|char类型的对象的最大值|1338629|CHAR_MAX见下文|1338630|对于任何受支持的区域设置，多字节字符的最大字节数|1338631|MB_LEN_MAX+1|1338632|类型为short+int的对象的最小值|1338633|SHRT_MIN+-32767|1338634|类型为short+int的对象的最大值|1338635|SHRT_MAX+%2B32767|1338636|类型为unsigned+short+int的对象的最大值|1338637|USHRT_MAX+65535|1338638|int类型的对象的最小值|1338639|INT_MIN+-32767|1338640|int类型的对象的最大值|1338641|INT_MAX+%2B32767|1338642|unsigned+int类型的对象的最大值|1338643|UINT_MAX+65535|1338644|类型为long+int的对象的最小值|1338645|LONG_MIN+-2147483647|1338646|类型为long+int的对象的最大值|1338647|LONG_MAX+%2B2147483647|1338648|类型为unsigned+long+int的对象的最大值|1338649|ULONG_MAX+4294967295|1338650|如果在表达式中使用char+sign-+expression类型的对象的值，则CHAR_MIN的值应与SCHAR_MIN的值相同，CHAR_MAX的值应与SCHAR_MAX的值相同。如果在表达式中使用char类型的对象时，其值不带符号扩展，则CHAR_MIN的值应为0，CHAR_MAX的值应与UCHAR_MAX的值相同。/7/|1338651|entityMap|0|LINK|mutability|MUTABLE|url|http://flash-gordon.me.uk/ansi.c.txt^0|1T|4|2C|6|0|0|J|4|0|H|F|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|2V|8|@$9|2W|A|2X|B|C]|$9|2Y|A|2Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|30|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|31|8|@$9|32|A|33|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|34|8|@]|D|@$9|35|A|36|1|37]]|E|$]]|$1|L|3|M|5|6|7|38|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|39|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|3A|8|@]|D|@]|E|$]]|$1|R|3|S|5|6|7|3B|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|3C|8|@]|D|@]|E|$]]|$1|V|3|W|5|6|7|3D|8|@]|D|@]|E|$]]|$1|X|3|Y|5|6|7|3E|8|@]|D|@]|E|$]]|$1|Z|3|10|5|6|7|3F|8|@]|D|@]|E|$]]|$1|11|3|12|5|6|7|3G|8|@]|D|@]|E|$]]|$1|13|3|14|5|6|7|3H|8|@]|D|@]|E|$]]|$1|15|3|16|5|6|7|3I|8|@]|D|@]|E|$]]|$1|17|3|18|5|6|7|3J|8|@]|D|@]|E|$]]|$1|19|3|1A|5|6|7|3K|8|@]|D|@]|E|$]]|$1|1B|3|1C|5|6|7|3L|8|@]|D|@]|E|$]]|$1|1D|3|1E|5|6|7|3M|8|@]|D|@]|E|$]]|$1|1F|3|1G|5|6|7|3N|8|@]|D|@]|E|$]]|$1|1H|3|1I|5|6|7|3O|8|@]|D|@]|E|$]]|$1|1J|3|1K|5|6|7|3P|8|@]|D|@]|E|$]]|$1|1L|3|1M|5|6|7|3Q|8|@]|D|@]|E|$]]|$1|1N|3|1O|5|6|7|3R|8|@]|D|@]|E|$]]|$1|1P|3|1Q|5|6|7|3S|8|@]|D|@]|E|$]]|$1|1R|3|1S|5|6|7|3T|8|@]|D|@]|E|$]]|$1|1T|3|1U|5|6|7|3U|8|@]|D|@]|E|$]]|$1|1V|3|1W|5|6|7|3V|8|@]|D|@]|E|$]]|$1|1X|3|1Y|5|6|7|3W|8|@]|D|@]|E|$]]|$1|1Z|3|20|5|6|7|3X|8|@]|D|@]|E|$]]|$1|21|3|22|5|6|7|3Y|8|@]|D|@]|E|$]]|$1|23|3|24|5|6|7|3Z|8|@]|D|@]|E|$]]|$1|25|3|26|5|6|7|40|8|@]|D|@]|E|$]]|$1|27|3|28|5|6|7|41|8|@]|D|@]|E|$]]|$1|29|3|2A|5|6|7|42|8|@]|D|@]|E|$]]|$1|2B|3|2C|5|6|7|43|8|@]|D|@]|E|$]]|$1|2D|3|2E|5|6|7|44|8|@]|D|@]|E|$]]|$1|2F|3|2G|5|6|7|45|8|@]|D|@]|E|$]]|$1|2H|3|2I|5|6|7|46|8|@]|D|@]|E|$]]|$1|2J|3|2K|5|6|7|47|8|@]|D|@]|E|$]]|$1|2L|3|2M|5|6|7|48|8|@]|D|@]|E|$]]|$1|2N|3|-4|5|6|7|49|8|@]|D|@]|E|$]]]|2O|$2P|$5|2Q|2R|2S|E|$2T|2U]]]]

First thing I would say is that if you need a type to be an exact number of bits, then use a size specific type. Depending on your platform that could range from <code>__s8</code> for a signed 8 bit type on Linux to <code>__int8</code> in VC++ on Windows.

Now, according to Robert Love in his chapter on portability in "Linux Kernel Development" he states that the C standard "leaves the size of the standard types up to implementations, although it does dictate a minimum size." 

Then in a footnote at the bottom of the page he says, "With the exception of <code>char</code> which is always 8 bits"

Now I'm not sure what he's basing this on, but maybe it's this section from <a href="http://flash-gordon.me.uk/ansi.c.txt" rel="nofollow noreferrer">the ANSI C spec</a>?

2.2.4.2 Numerical limits

A conforming implementation shall document all the limits specified
in this section, which shall be specified in the headers limits.h
and float.h

"Sizes of integral types limits.h"

The values given below shall be replaced by constant expressions
suitable for use in #if preprocessing directives. Their
implementation-defined values shall be equal or greater in magnitude
(absolute value) to those shown, with the same sign.

maximum number of bits for smallest object that is not a bit-field (byte)

CHAR_BIT 8 

minimum value for an object of type signed char 

SCHAR_MIN -127 

maximum value for an object of type signed char 

SCHAR_MAX +127 

maximum value for an object of type unsigned char 

UCHAR_MAX 255 

minimum value for an object of type char 

CHAR_MIN see below 

maximum value for an object of type char 

CHAR_MAX see below 

maximum number of bytes in a multibyte character, for any supported locale 

MB_LEN_MAX 1 

minimum value for an object of type short int 

SHRT_MIN -32767 

maximum value for an object of type short int 

SHRT_MAX +32767 

maximum value for an object of type unsigned short int 

USHRT_MAX 65535 

minimum value for an object of type int 

INT_MIN -32767 

maximum value for an object of type int 

INT_MAX +32767 

maximum value for an object of type unsigned int 

UINT_MAX 65535 

minimum value for an object of type long int 

LONG_MIN -2147483647 

maximum value for an object of type long int 

LONG_MAX +2147483647 

maximum value for an object of type unsigned long int 

ULONG_MAX 4294967295

If the value of an object of type char sign-extends when used in an
expression, the value of CHAR_MIN shall be the same as that of
SCHAR_MIN and the value of CHAR_MAX shall be the same as that of
SCHAR_MAX . If the value of an object of type char does not
sign-extend when used in an expression, the value of CHAR_MIN shall be
0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX
./7/

问char保证恰好是8位的长度吗？
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问char保证恰好是8位的长度吗？
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