摘自：https://blog.csdn.net/Yeshangzhu/article/details/81099440
若用户在使用GNU 的ReadLine库，则可以在开发的CLI中使用Tab键对命令自动搜索匹配项并补全，其后台运行机制分析如下：
1.首先看函数 rl_completion_func_t * rl_attempted_completion_function
gnu帮助文档对其描述如下：
Variable: rl_completion_func_t * rl_attempted_completion_function
A pointer to an alternative function to create matches. The function is called with text, start, and end. start and end are indices in rl_line_buffer defining the boundaries of text, which is a character string. If this function exists and returns NULL, or if this variable is set to NULL, then rl_complete() will call the value of rl_completion_entry_function to generate matches, otherwise the array of strings returned will be used. If this function sets the rl_attempted_completion_over variable to a non-zero value, Readline will not perform its default completion even if this function returns no matches.
首先，这是一个可选的创建匹配字符串的函数，如果该函数指针未赋值（即为空）或者它存在但是返回NULL，那么创建匹配项系统库函数 rl_complete()会调用函数 rl_completion_entry_function() 来完成创建匹配项。否则，该函数会生效并替换完成系统库函数 rl_complete()的功能。
其次，该函数包含三个参数：char *text, int start, int end。text表示用户输入有待匹配的不完整字符串，start 和 end 表示rl_line_buffer 的界限。
2.再看函数 char ** rl_completion_matches (const char *text, rl_compentry_func_t *entry_func)
gnu帮助文档对其描述如下：
Function: char ** rl_completion_matches (const char *text, rl_compentry_func_t *entry_func)
Returns an array of strings which is a list of completions for text. If there are no completions, returns NULL. The first entry in the returned array is the substitution for text. The remaining entries are the possible completions. The array is terminated with a NULL pointer.
entry_func is a function of two args, and returns a char *. The first argument is text. The second is a state argument; it is zero on the first call, and non-zero on subsequent calls. entry_func returns a NULL pointer to the caller when there are no more matches.
该函数会完成命令匹配项的搜索和创建，它包含两个参数，text表示带匹配的字符串，函数指针参数entry_func则会完成最终的匹配项创建，返回值为匹配的字符串数组（且第一个字符串为text本身）。
entry_func的函数原型为： char *rl_compentry_func_t (const char *, int);
第一个参数为待匹配字符串，第二个整型参数表示多次调用时的状态值：第一次调用时为0，后续调用时为正整数。需要指出的是该函数的返回字符串必须通过malloc的方式分配产生，ReadLine库函数匹配完成后会通过free释放掉，若为静态变量或全局变量则会导致释放失败，官方给出的描述如下：
The generator function is called repeatedly from rl_completion_matches(), returning a string each time. The arguments to the generator function are text and state. text is the partial word to be completed. state is zero the first time the function is called, allowing the generator to perform any necessary initialization, and a positive non-zero integer for each subsequent call. The generator function returns (char *)NULL to inform rl_completion_matches() that there are no more possibilities left. Usually the generator function computes the list of possible completions when state is zero, and returns them one at a time on subsequent calls. Each string the generator function returns as a match must be allocated with malloc(); Readline frees the strings when it has finished with them. Such a generator function is referred to as an application-specific completion function.
至此，Tab匹配过程完成，总结如下：
参考gnu帮助链接：
http://tiswww.case.edu/php/chet/readline/readline.html#SEC47
GNU ReadLine help链接