指针一定要初始化，给其分配指向的内存地址！！！！！

ls.c

#include <stdio.h>
#include <dirent.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
	if (argc != 2) {
		printf("Please input directory or file name\n");
		return 1;
	}

	struct stat file_attr;
	struct dirent *file;

	if(lstat(argv[1], &file_attr) < 0) {
		printf("lstat error\n");
		return 1;
	}

	if (S_ISDIR(file_attr.st_mode)) {

		struct dirent *file;
		DIR *dirp = opendir(argv[1]);
		while ((file = readdir(dirp)) != NULL) {
			printf("%s\n", file->d_name);
		}
		closedir(dirp);

	} else {
		printf("%s\n", argv[1]);
	}
	return 0;
}

free错误，透彻理解什么时候需要free！！！！