getopt():短选项处理

假设一个选项须要一个关联值。而程序运行时没有提供，返回一个问号（？）,假设将optstring的第一个字符设为冒号（:),这样的情况下，函数会返回冒号而不是问号。

getopt_long():长选项处理

Decode options from the vector argv (whose length is argc). The argument shortopts describes the short options to accept, just as it does in getopt. The argument longopts describes the long options to accept (see above).

When getopt_long encounters a short option, it does the same thing that getopt would do: it returns the character code for the option, and stores the options argument (if it has one) inoptarg.

When getopt_long encounters a long option, it takes actions based on the flag and val fields of the definition of that option.

If flag is a null pointer, then getopt_long returns the contents of val to indicate which option it found. You should arrange distinct values in the val field for options with different meanings, so you can decode these values after getopt_long returns. If the long option is equivalent to a short option, you can use the short option’s character code in val.

If flag is not a null pointer, that means this option should just set a flag in the program. The flag is a variable of type int that you define. Put the address of the flag in the flag field. Put in the val field the value you would like this option to store in the flag. In this case, getopt_long returns 0.

For any long option, getopt_long tells you the index in the array longopts of the options definition, by storing it into *indexptr. You can get the name of the option withlongopts[*indexptr].name. So you can distinguish among long options either by the values in their val fields or by their indices. You can also distinguish in this way among long options that set flags.

When a long option has an argument, getopt_long puts the argument value in the variable optarg before returning. When the option has no argument, the value in optarg is a null pointer. This is how you can tell whether an optional argument was supplied.

struct option {
    char *name;   //长选项的名字
    int has_arg;  // 0/1。标志是否有选项
    int *flag; //上面有具体说明，通常为NULL
    int val;  
};

应用场景分析