函数解析解决办法

函数解析

C/C++ code

FS_u16  FS__CLIB_str2wstr(short *wstr, const char *str, FS_size_t n){    short *s;    int len = n;    for (s = wstr; (0 < n) && (*str != '\0');--n)    {        *s++ = *str++;    /* copy at most n chars from s2[] */    }        *s++ = L'\0';        return len;}

这段代码最后*s++ = L'\0';是什么意思？

[解决办法]
L'\0' 是宽字符版的 '\0';
这句话就是在字符串末尾加 0;
[解决办法]
类似0X是十六进制常数的前缀，L是w_char类型字符常数的前缀
[解决办法]

探讨

引用:

结尾加一个\0表示字符串结束
\0是字符串结束符，strcpy,等很多跟字符串相关的函数都是识别\0作为结束的


这个当然都知道喽！！关键那个L干嘛用的？

[解决办法]
C++ Character Constants
Character constants are one or more members of the “source character set,” the character set in which a program is written, surrounded by single quotation marks ('). They are used to represent characters in the “execution character set,” the character set on the machine where the program executes.

Microsoft Specific 

For Microsoft C++, the source and execution character sets are both ASCII.

END Microsoft Specific

There are three kinds of character constants: 

Normal character constants


Multicharacter constants


Wide-character constants 
Note Use wide-character constants in place of multicharacter constants to ensure portability.

Character constants are specified as one or more characters enclosed in single quotation marks. For example:

char ch = 'x'; // Specify normal character constant.
int mbch = 'ab'; // Specify system-dependent
// multicharacter constant.
wchar_t wcch = L'ab'; // Specify wide-character constant.

Note that mbch is of type int. If it were declared as type char, the second byte would not be retained. A multicharacter constant has four meaningful characters; specifying more than four generates an error message.

Syntax

character-constant :

'c-char-sequence'
L'c-char-sequence'

c-char-sequence :

c-char
c-char-sequence c-char

c-char :

any member of the source character set except the single quotation mark ('), backslash (\), or newline character
escape-sequence

escape-sequence :

simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence

simple-escape-sequence : one of

\' \" \? \\
\a \b \f \n \r \t \v

octal-escape-sequence :

\octal-digit
\octal-digit octal-digit
\octal-digit octal-digit octal-digit

hexadecimal-escape-sequence :

\xhexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit

Microsoft C++ supports normal, multicharacter, and wide-character constants. Use wide-character constants to specify members of the extended execution character set (for example, to support an international application). Normal character constants have type char, multicharacter constants have type int, and wide-character constants have type wchar_t. (The type wchar_t is defined in the standard include files STDDEF.H, STDLIB.H, and STRING.H. The wide-character functions, however, are prototyped only in STDLIB.H.)

The only difference in specification between normal and wide-character constants is that wide-character constants are preceded by the letter L. For example:

char schar = 'x'; // Normal character constant

wchar_t wchar = L'\x81\x19'; // Wide-character constant

Table 1.2 shows reserved or nongraphic characters that are system dependent or not allowed within character constants. These characters should be represented with escape sequences.

Table 1.2 C++ Reserved or Nongraphic Characters

Character ASCII 
Representation ASCII 
Value Escape Sequence 
Newline NL (LF) 10 or 0x0a \n 
Horizontal tab HT 9 \t 
Vertical tab VT 11 or 0x0b \v 
Backspace BS 8 \b 
Carriage return CR 13 or 0x0d \r 
Formfeed FF 12 or 0x0c \f 
Alert BEL 7 \a 
Backslash \ 92 or 0x5c \\ 
Question mark ? 63 or 0x3f \? 
Single quotation mark ' 39 or 0x27 \' 
Double quotation mark " 34 or 0x22 \" 
Octal number ooo — \ooo 
Hexadecimal number hhh — \xhhh 
Null character NUL 0 \0 


If the character following the backslash does not specify a legal escape sequence, the result is implementation defined. In Microsoft C++, the character following the backslash is taken literally, as though the escape were not present, and a level 1 warning (“unrecognized character escape sequence”) is issued.

Octal escape sequences, specified in the form \ooo, consist of a backslash and one, two, or three octal characters. Hexadecimal escape sequences, specified in the form \xhhh, consist of the characters \x followed by a sequence of hexadecimal digits. Unlike octal escape constants, there is no limit on the number of hexadecimal digits in an escape sequence.

Octal escape sequences are terminated by the first character that is not an octal digit, or when three characters are seen. For example:

wchar_t och = L'\076a'; // Sequence terminates at a
char ch = '\233'; // Sequence terminates after 3 characters

Similarly, hexadecimal escape sequences terminate at the first character that is not a hexadecimal digit. Because hexadecimal digits include the letters a through f (and A through F), make sure the escape sequence terminates at the intended digit.

Because the single quotation mark (') encloses character constants, use the escape sequence \' to represent enclosed single quotation marks. The double quotation mark (") can be represented without an escape sequence. The backslash character (\) is a line-continuation character when placed at the end of a line. If you want a backslash character to appear within a character constant, you must type two backslashes in a row (\\). (SeePhases of Translation in the Preprocessor Reference for more information about line continuation.)

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