分析C++ Command-line參數

預注：命令行（commandline）被操做系統的命令分析器（/日後簡稱cmdlineparser）分解到命令參數argv[0]…[n]，這裏，commandline是入料，argv是出品.

Microsoft C/C++ 程序引導代碼使用如下規則解析操做系統命令行中給出的參數:

• cmdlineparser用空白字符從commandline中分隔出argv；空白字符能夠是一個空格(0x20)或製表符(0x09)；注意，空白字符不必定就分割了argv，由於空白字符多是argv的一部分
• 相比0x20和0x09，字符^(0x5E) 未被識別爲轉義符或者分隔符；出品argv以前，commandline由cmdlineparser徹底處理
• commandline中，雙引號括起來的字符串"string"被解釋爲單個參數，即便其中包含空格0x20，譬如"a string"，解析爲a string； 帶引號的字符串能夠嵌入在參數內，譬如d"e f"g，將被cmdlineparser解析爲de fg
• commandline中，前面有反斜槓(0x5C)的雙引號 (\") 被解釋爲argv中的雙引號字符 (")
• 承4.，反斜槓在argv中按其原義解釋，除非它們緊位於雙引號以前
• commandline中，若是偶數個反斜槓後跟一個雙引號，每對反斜槓將被cmdlineparser解析爲argv中的一個反斜槓；而緊跟後面的那個雙引號將被cmdlineparser看成分隔符，等價於commandline中的空白字符
• commandline中，若是奇數個反斜槓後跟一個雙引號，每對反斜槓將被cmdlineparser解析爲argv中的一個反斜槓；剩下的反斜槓+雙引號按4.被轉義解釋爲雙引號

以上這段文字翻譯自http://msdn.microsoft.com/en-us/library/17w5ykft.aspx ,主要仍是本人理解的語義。原文以下

Microsoft C/C++ startup code uses the following rules when interpreting arguments given on the operating system command line:

• Arguments are delimited by white space, which is either a space or a tab.
• The caret character (^) is not recognized as an escape character or delimiter. The character is handled completely by the command-line parser in the operating system before being passed to the argv array in the program.
• A string surrounded by double quotation marks ("string") is interpreted as a single argument, regardless of white space contained within. A quoted string can be embedded in an argument.
• A double quotation mark preceded by a backslash (\") is interpreted as a literal double quotation mark character (").
• Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
• If an even number of backslashes is followed by a double quotation mark, one backslash is placed in the argv array for every pair of backslashes, and the double quotation mark is interpreted as a string delimiter.
• If an odd number of backslashes is followed by a double quotation mark, one backslash is placed in the argv array for every pair of backslashes, and the double quotation mark is "escaped" by the remaining backslash, causing a literal double quotation mark (") to be placed in argv.

示例

下面的過程演示如何經過命令行參數:

  
 
 
    
 
   // 
 
    command_line_arguments.cpp  
 
   // 
 
    compile with: /EHsc 
 
     
 
   #include  
 
   < 
 
   iostream 
 
   > 
 
     
 
   using 
 
     
 
   namespace 
 
    std;  
 
   int 
 
    main(  
 
   int 
 
    argc,  
 
   // 
 
    Number of strings in array argv 
 
     
 
     
 
   char 
 
     
 
   * 
 
   argv[],  
 
   // 
 
    Array of command-line argument strings 
 
     
 
     
 
   char 
 
     
 
   * 
 
   envp[] )  
 
   // 
 
    Array of environment variable strings 
 
     
 
   {  
 
   int 
 
    count;  
 
   // 
 
    Display each command-line argument. 
 
     
 
    cout  
 
   << 
 
     
 
   " 
 
   \nCommand-line arguments:\n 
 
   " 
 
   ;  
 
   for 
 
   ( count  
 
   = 
 
     
 
   0 
 
   ; count  
 
   < 
 
    argc; count 
 
   ++ 
 
    ) cout  
 
   << 
 
     
 
   " 
 
    argv[ 
 
   " 
 
     
 
   << 
 
    count  
 
   << 
 
     
 
   " 
 
   ]  
 
   " 
 
     
 
   << 
 
    argv[count]  
 
   << 
 
     
 
   " 
 
   \n 
 
   " 
 
   ; }  

  

下表顯示示例輸入，並預期的輸出，演示上面的規則列表

命令行輸入       |   argv [1]  |   argv [2]   |   argv [3]
-----------------|-------------|--------------|---------------
"abc" d e        |   abc       |   d          |   e
a\\b d"e f"g h   |   a\\b      |   de fg      |   h
a\\\"b c d       |   a\"b      |   c          |   d
a\\\\"b c" d e   |   a\\b c    |   d          |   e

/////////////////////////////////////////////////

又：

有關連在一塊兒的多個雙引號的解析，很是狗血，請參考討論

• http://www.daviddeley.com/autohotkey/parameters/parameters.htm (/爲便於閱讀，但請把你瀏覽器的字符集設置爲ISO-8859-1，而後ZoomIn視圖)
• http://blogs.msdn.com/b/oldnewthing/archive/2010/09/17/10063629.aspx
• http://www.cplusplus.com/articles/DEN36Up4/
• http://www.boost.org/doc/libs/1_36_0/doc/html/program_options/howto.html#id3453269
• http://www.cplusplus.com/forum/articles/13355/

尤爲是 http://www.daviddeley.com/autohotkey/parameters/parameters.htm 中的這個補充說明：

• And here's the missing undocumented rule:
  If a closing " is followed immediately by another ", the 2nd " is accepted literally and added to the parameter.

及其算法：

5.10  The Microsoft C/C++ Command Line Parameter Parsing Algorithm
The following algorithm was reverse engineered by disassembling a small C program compiled using Microsoft Visual C++ and examining the disassembled code:

1. Parse off parameter 0 (the program filename)
    * The entire parameter may be enclosed in double quotes (it handles double quoted parts)
      (Double quotes are necessary if there are any spaces or tabs in the parameter)
    * There is no special processing of backslashes (\)

2. Parse off next parameter:
    a. Skip over multiple spaces/tabs between parameters
      LOOP
    b. Count the backslashes (\). Let m = number of backslashes. (m may be zero.)
    c. IF next character following m backslashes is a double quote:
           If m is even (or zero)
                if currently in a double quoted part
                   IF next character is also a "
                        move to next character (the 2nd ". This character will be added to the parameter.)
                   ELSE
                        set flag to not add this " character to the parameter
                   ENDIF
                    toggle double quoted part flag
               else
                    set flag to not add this " character to the parameter
               endif
           Endif
            m = m/2 (floor divide e.g. 0/2=0, 1/2=0, 2/2=1, 3/2=1, 4/2=2, 5/2=2, etc.)
       ENDIF
    d. add m backslashes
    e. add this character to our parameter
      ENDLOOP