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$ man -k direct | grep read
$ man 3 opendir
$ man 3 readdir
$ man 3 closedir

实现思路：
打开目录，读取每一条记录(对应一个文件或子目录)

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <dirent.h>

void do_ls(char []);

int main(int ac, char * av[]) {
    if (ac == 1) {
        do_ls(".");
    } else {
        while (--ac) {
            do_ls(* ++av);
        }   
    }   
    return 0;
}

void do_ls(char dirname[]) {
    DIR *dirp;
    struct dirent *dir;
    // 判断dirname是否存在
    if ((dirp = opendir(dirname)) == NULL) {
        printf("can't open %s\n", dirname);
        return;
    }   
    while((dir = readdir(dirp)) != NULL) {
        printf("%s\n", dir->d_name);
    }   
    closedir(dirp);
}