For CLI(command line interface) tools, command line options are popular. libapr provides APIs to handle command line options easily. Here is an excerpted code from getopt-sample.c.

在CLI（command line interface）工具中，命令行选项是比较流行的。APR提供了一些API来简化命令行选项的处理，这是getopt-sample.c示例文件中的一些代码。

/* excerpted from getopt-sample.c */
static const apr_getopt_option_t opt_option[] = {
    /* long-option, short-option, has-arg flag, description */
        { "in", 'i', TRUE, "input file" },      /* -i name or --in name */
        { "out", 'o', TRUE, "output file" },    /* -o name or --out name */
        { "help", 'h', FALSE, "show help" },    /* -h or --help */
        { NULL, 0, 0, NULL }, /* end (a.k.a. sentinel) */
};

At first, we should supply an array of apr_getopt_option_t elements. We call it option-list. Each element has four variables, i.e. long option, short option, flag of the trailing argument, and description. A long option is specified as '--help'. A short option is specified as '-h'. Short options are mandatory and long options are optional. We can set a long option to NULL. The third variable is flag of the existence of trailing argument. If a command line option works as '--in filename', i.e. the trailing argument is required, we have to set the flag to TRUE. If you run the program without the required argument, e.g. './a.out -in', it causes an error. The option-list must have a sentinel element as described above.

首先，我们应该提供一个apr_getopt_option_t类型的数组，我们叫他选项列表，每一个元素有4个变量组成，他们是完整的选项名称（long option），缩写的选项名称（short option），是否为带参数选项的标志位（trailing argument）和选项描述（description）。完整的选项名称类似于“--help”，缩写的选项名称如“-h”，缩写的选项名称是强制必须存在的，而完整的选项名称是可选的，你可以设置完整的选型名称为NULL.第三个变量是一个是否为带参数选项的标志位，如果命令行选项需要这样的格式“--in filename”，则这个标志位必须设置为TRUE，如果你运行这个程序没有填写这个参数，如“./a.out -in”，这将会导致发生错误。选项列表必须有一个哨兵元素（sentinel element）作为结束。

To parse actual command line options based on the option-list, we have to call apr_getopt_init() at first. That initializes apr_getopt_t object. Next, we keep calling apr_getopt_long() while it returns APR_SUCCESS. Here is an excerpted code from getopt-sample.c.

解析一个实际的命令行必须在一个选项列表的基础上，我们首先需要调用apr_getopt_init()来初始化apr_getopt_init()对象，接下来我们要保证在调用apr_getopt_long()时他的返回值是APR_SUCCESS，这里有一些示例代码出自getopt-sample.c文件。

/* excerpted from getopt-sample.c */

/* initialize apr_getopt_t */
apr_getopt_t *opt;
apr_getopt_init(&opt, mp, argc, argv);

/* parse the all options based on opt_option[] */
while ((rv = apr_getopt_long(opt, opt_option, &optch, &optarg)) == APR_SUCCESS) {
    switch (optch) {
        case 'i':
        ...OMIT

During the loop, apr_getopt_long() processes the actual command line option one by one. If the option found is in the option-list, apr_getopt_long() returns APR_SUCCESS and set optch's value. When the option has the trailing argument, apr_getopt_long() parses it and set optarg's value.

在这个循环当中，apr_getopt_long()函数一个接一个的处理实际的命令行选项。如果一个选项在选项列表中被找到，apr_getopt_long()将返回APR_SUCCESS并且设置optch的值，当这个选项为一个带参数的选项时，apr_getopt_long()解析后出该参数时将该值设置到optarg中。

I show you an example. Let's think about the case that you run the program as './getopt-sample -h -i foo.txt'. At the first loop, apr_getopt_long() finds 'h' in the option-list. Then, apr_getopt_long() returns APR_SUCCESS and set optch to 'h'. At the next loop, apr_getopt_long() finds 'i' in the option-list and 'i' requiring the trailing argument, so it parses the trailing argument, 'foo.txt'. Thus, apr_getopt_long() returns APR_SUCCESS, and set optch to 'i' and optarg to "foo.txt". At the next loop, apr_getopt_long() finds no more options, so returns APR_EOF.

通过这个示例让我们考虑一下这个程序（'./getopt-sample -h -i foo.txt'）运行时发生的情况，在第一次循环的时，apr_getopt_long()在选项列表中找到了’h’选项，这个时候apr_getopt_long()返回APR_SUCCESS，并且将optch设置为’h’。在下一次循环中apr_getopt_long()发现了在选项列表中发现了’i’，’i’是一个带参数的选项，所以解析出参数’foo.txt’，这是apr_getopt_long()返回APR_SUCCESS，并且将optch设置成‘i’将optarg设置成"foo.txt"，在下一次循环中apr_getopt_long()发现没有选项了，所以返回APR_EOF。