目前我正在尝试通过TCP连接模拟时间协议RFC 868。 我用Java完成了服务器，我在C中做客户端。 以下是服务器的主要代码行。
private static final long SECONDS_FROM_1900_TO_1970 = 2208988800L; //Long I need in order to calculate the
//time past since 1900. It is because time protocol sets the epoch at 1900 and the java Date class at
//1970.
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
try{
int serverPort = 37000;
ServerSocket receivingConnections = new ServerSocket (serverPort);
for(;;) { //Infinity loop equivalent to while(true)
clientConnection = receivingConnections.accept();
System.out.println("Connection established with client IP: " +clientConnection.getInetAddress());
try{
in = new DataInputStream(clientConnection.getInputStream());
out = new DataOutputStream(clientConnection.getOutputStream());
Date actualTime = new Date();//Date gives us the second for the epoch until 1970. Had to import utils.date
long secondsSince1970 = (actualTime.getTime())/1000;//I divide the time by 1000 because actualTime.getTime()
//returns me the result in ms
long secondsFromTheBeginning = secondsSince1970 + SECONDS_FROM_1900_TO_1970; //Total seconds i do need to calculate
//the time from 1990 in order to respect RFC 868 specification
byte[] actualTimeToBytes = new byte[4]; //Byte structure I need to send to the client. 4 bytes = 32 bits.
//With the next 4 lines of code I will introduce the whole long into 4 bytes. A long takes 4 bytes = 32 bits.
actualTimeToBytes[0] = (byte) ((secondsFromTheBeginning & 0x00000000FF000000L) >> 24);
actualTimeToBytes[1] = (byte) ((secondsFromTheBeginning & 0x0000000000FF0000L) >> 16);
actualTimeToBytes[2] = (byte) ((secondsFromTheBeginning & 0x000000000000FF00L) >> 8);
actualTimeToBytes[3] = (byte) (secondsFromTheBeginning & 0x00000000000000FFL);
String s = new String(actualTimeToBytes);
System.out.println(secondsFromTheBeginning); //The actual time being sent over the socket..
System.out.println(s); // The actual byte being sent over the socket.
out.write(actualTimeToBytes);//Here I send to the receiver the time in Byte form via the socket
out.flush();
System.out.println("The time was indeed sent to: " +clientConnection.getInetAddress());
} catch (IOException e){System.out.println("Error: "+e.getMessage()); }
finally { if (clientConnection != null) {
System.out.println("Connection finished with client IP: " +clientConnection.getInetAddress());
clientConnection.close();
}
}
}
} catch (IOException ex) {
System.err.println(ex);
}
}
服务器工作正常，但在客户端执行时会出现问题。 客户代码：
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <unistd.h>
#include <time.h>
/*****************************************************
* Reproduced from Unix Network Programming
W.Richard Stevens
Prentice Hall Software Series 1990
* Definitions for TCP and UDP client/server programs
*/
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
/*****************************************************/
#define SERVERPORT 37000
#define SERVERIP "192.168.0.18"
#define TIMELENGTHBYTES 4
/*
* Program a new function that reads four bytes from the socket:
* int readDate(...)
*/
int readDate(int *sockfd){
int n;
char buffer[4];
unsigned long lorovrigida;
n = read(*sockfd, buffer, 4);
lorovrigida = (buffer[0]<<24) + (buffer[1]<<16) + (buffer[2]<<8) + (buffer[3]);
lorovrigida-=2208988800;
printf(ctime(&lorovrigida));
if ( n < 0 ) {
printf("TIMECLNT: Error in read()\n");
exit(EXIT_FAILURE);
}
}
int connectToServer(char *ip, int port, int *sockfd) {
int rv = 0;
struct sockaddr_in serv_addr;
char buf[4];
bzero((char *) &serv_addr, sizeof (serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = inet_addr(SERVERIP);
serv_addr.sin_port = htons(SERVERPORT);
if ((*sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
rv = -1;
} else {
if (connect(*sockfd, (struct sockaddr *) &serv_addr, sizeof (serv_addr)) < 0) {
rv = -2;
}
}
return rv;
}
int main(int argc, char** argv) {
int e, sockfd;
if ((e = connectToServer(SERVERIP, SERVERPORT, &sockfd)) != 0) {
printf("Error on call to connectToServer: %d\n", e);
exit(e);
}
printf("Successfully connected to server\n");
if(sockfd==NULL)
printf("El socket esta vacio\n");
fflush(stdout);
/*
* Call the readDate() function and print out the result:
*/
readDate(&sockfd);
return (EXIT_SUCCESS);
}
主要问题是我真的不知道如何阅读4个收到的字节。 当我打印出lorovrigida时，它似乎是一个巨大而随机的数字。
我怎样才能正确阅读？是因为我使用的是无符号长？我只是被卡住了，因为我尝试了很多不同的方法，没有任何作用。
感谢您的帮助。
答案 0 :(得分：1)
我不认为这是端点问题，因为java server处理了这个问题。
在您的客户端char buffer[4];中，您将其声明为signed，当进行位移时，当某个字节的最高位为1时，您没有去获取正确的结果。定义为是unsigned char buffer[4];
unsigned char ua = 0x80;
unsigned int b = ua << 1;
printf("%u\n", b);
char a = 0x80;
unsigned int c = a << 1;
printf("%u\n", c);
结果：
256
4294967040
答案 1 :(得分：0)
32位结果的重建是一个问题，从可能签名的4 char开始。
假设char已签名并且是2＆＃39>补充......
添加buffer[3]时，首先会进行常规整数提升。这意味着像1000_0000（ - 128）这样的位模式变为1111_1111_...1000_0000。前导1位的数量取决于int位宽。
要避免所有此符号扩展，最好使用无符号类型。
// Method 1 - cast buffer
int readDate(int *sockfd) {
char buffer[4];
int n;
unsigned long lorovrigida;
n = read(*sockfd, buffer, 4);
if ( n < 0 ) {
printf("TIMECLNT: Error in read()\n");
exit(EXIT_FAILURE);
}
lorovrigida = ((unsigned char)buffer[0]<<24) + ((unsigned char)buffer[1]<<16) +
((unsigned char)buffer[2]<<8) + ((unsigned char)buffer[3]);
....
}
// Method 2 - use unsigned char type
int readDate_2(int *sockfd) {
unsigned char buffer[4];
...
lorovrigida = (buffer[0]<<24) + (buffer[1]<<16) +
(buffer[2]<<8) + (buffer[3]);
....
}
// Method 3 - exact size types
int readDate_2(int *sockfd) {
uint8_t buffer[4];
...
lorovrigida = (buffer[0]<<24) + (buffer[1]<<16) +
(buffer[2]<<8) + (buffer[3]);
....
}
// Method 4 - * by unsigned
int readDate_2(int *sockfd) {
uint8_t buffer[4];
...
lorovrigida = buffer[0]*0x1000000LU + buffer[1]*0x10000LU +
buffer[2]*0x100u + buffer[3];
....
}
// Method 5 - successive or-ing
int readDate_2(int *sockfd) {
uint8_t buffer[4];
...
lorovrigida = buffer[0];
lorovrigida = lorovrigida << 8 | buffer[1];
lorovrigida = lorovrigida << 8 | buffer[2];
lorovrigida = lorovrigida << 8 | buffer[3];
....
}
如果系统使用16位无符号，则方法1 - 3会受到可移植性的影响。
优化编译器可以为这些方法生成类似的代码。
此外：代码可能会受益于我的编译器额外使用uint32_t而不是unsigned long（64位）等固定宽度。