分析C++ Command-line参数

预注：命令行（commandline）被操作系统的命令分析器（/往后简称cmdlineparser）分解到命令参数argv[0]…[n]，这里，commandline是入料，argv是出品.

Microsoft C/C++ 程序引导代码使用以下规则解析操作系统命令行中给出的参数:

• cmdlineparser用空白字符从commandline中分隔出argv；空白字符可以是一个空格(0x20)或制表符(0x09)；注意，空白字符不一定就分割了argv，因为空白字符可能是argv的一部分
• 相比0x20和0x09，字符^(0x5E) 未被识别为转义符或者分隔符；出品argv之前，commandline由cmdlineparser完全处理
• commandline中，双引号括起来的字符串"string"被解释为单个参数，即使其中包含空格0x20，譬如"a string"，解析为a string； 带引号的字符串可以嵌入在参数内，譬如d"e f"g，将被cmdlineparser解析为de fg
• commandline中，前面有反斜杠(0x5C)的双引号 (\") 被解释为argv中的双引号字符 (")
• 承4.，反斜杠在argv中按其原义解释，除非它们紧位于双引号之前
• commandline中，如果偶数个反斜杠后跟一个双引号，每对反斜杠将被cmdlineparser解析为argv中的一个反斜杠；而紧跟后面的那个双引号将被cmdlineparser当作分隔符，等价于commandline中的空白字符
• commandline中，如果奇数个反斜杠后跟一个双引号，每对反斜杠将被cmdlineparser解析为argv中的一个反斜杠；剩下的反斜杠+双引号按4.被转义解释为双引号

以上这段文字翻译自http://msdn.microsoft.com/en-us/library/17w5ykft.aspx ,主要还是本人理解的语义。原文如下

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

下面的过程演示如何通过命令行参数:

   
   
   
   

    
    
    
    
    //
    
    
    
     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
    
    
    
    "
    
    
    
    ; } 
   
   
   
   

下表显示示例输入，并预期的输出，演示上面的规则列表

命令行输入       |   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：

有关连在一起的多个双引号的解析，非常狗血，请参考讨论

• http://www.daviddeley.com/autohotkey/parameters/parameters.htm (/为便于阅读，但请把你浏览器的字符集设置为ISO-8859-1，然后ZoomIn视图)
• 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/

尤其是 http://www.daviddeley.com/autohotkey/parameters/parameters.htm 中的这个补充说明：

• 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