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命令源码解析

//一般情况下,源码目录下会有一个Makeile,然后make之后,就能得到一些二进制文件//根据make的过程,可以判断出哪些文件被编译//在文件内,一般先定义一些变量,定义很多宏//在main函数内.//1.做初始化//2.设置设置成权限x,前提是the real user ID 是 root	uid = getuid();//returns the real user ID of the calling process.	if (setuid(uid)) {
   //sets the effective user ID of the calling process		perror("ping: setuid");		exit(-1);	}//3.解析参数//getopt//getopt_longwhile ((ch = getopt(argc, argv, COMMON_OPTSTR "bRT:")) != EOF){
   		switch(ch) {
   		case 'b':			broadcast_pings = 1;			break;		case 'Q':			settos = parsetos(optarg);			if (settos &&			    (setsockopt(icmp_sock, IPPROTO_IP, IP_TOS,					(char *)&settos, sizeof(int)) < 0)) {
   				perror("ping: error setting QOS sockopts");				exit(2);			}			break;		case 'R':			if (options & F_TIMESTAMP) {
   				fprintf(stderr, "Only one of -T or -R may be used\n");				exit(2);			}			options |= F_RROUTE;			break;		case 'T':			if (options & F_RROUTE) {
   				fprintf(stderr, "Only one of -T or -R may be used\n");				exit(2);			}			options |= F_TIMESTAMP;			if (strcmp(optarg, "tsonly") == 0)				ts_type = IPOPT_TS_TSONLY;			else if (strcmp(optarg, "tsandaddr") == 0)				ts_type = IPOPT_TS_TSANDADDR;			else if (strcmp(optarg, "tsprespec") == 0)				ts_type = IPOPT_TS_PRESPEC;			else {
   				fprintf(stderr, "Invalid timestamp type\n");				exit(2);			}			break;		case 'I':		{
   #if 0			char dummy;			int i1, i2, i3, i4;			if (sscanf(optarg, "%u.%u.%u.%u%c",				   &i1, &i2, &i3, &i4, &dummy) == 4) {
   				__u8 *ptr;				ptr = (__u8*)&source.sin_addr;				ptr[0] = i1;				ptr[1] = i2;				ptr[2] = i3;				ptr[3] = i4;				options |= F_STRICTSOURCE;			} else {
   				device = optarg;			}#else			if (inet_pton(AF_INET, optarg, &source.sin_addr) > 0)				options |= F_STRICTSOURCE;			else				device = optarg;#endif			break;		}		case 'M':			if (strcmp(optarg, "do") == 0)				pmtudisc = IP_PMTUDISC_DO;			else if (strcmp(optarg, "dont") == 0)				pmtudisc = IP_PMTUDISC_DONT;			else if (strcmp(optarg, "want") == 0)				pmtudisc = IP_PMTUDISC_WANT;			else {
   				fprintf(stderr, "ping: wrong value for -M: do, dont, want are valid ones.\n");				exit(2);			}			break;		case 'V':			printf("ping utility, iputils-ss%s\n", SNAPSHOT);			exit(0);		COMMON_OPTIONS			common_options(ch);			break;		default:			usage();		}	}	argc -= optind;//optind是解析到的参数的个数,argc是参数的总数,减去之后argc是错误的参数的个数	argv += optind;//当getopt返回后,此时argv已已经重新排序.argv+optind之后,指向了首个没有解析出来的参数//接下来对改变的各种参数进行判断并做各种动作getopt会做下面几件事1/对argv进行分析,每次返回一个argv[i]2/解析成功,optind加13/解析不成功,将argv[i]放入argv的后面4/接着分析-a 后面的参数,以optarg代入,且一个 -a 后面只能跟一个参数  //main函数里面一般做的是初始化,设置权限,然后解析参数,设置状态位.然后接着,调用实际功能函数

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