[英]UDP server/client in C - sendto error: Address family not supported by protocol
我正在编写一个简单的说明性UDP服务器客户端。 服务器应该根据客户端的输入来计算客户端的网络字节顺序,并将其发送回客户端。 我一直试图整天修复此错误。 任何帮助将不胜感激。 这是服务器的启动和输出:
./udpserver.out 4545
from.sin_family: 12079
Little Endian
ERROR UDP_SRV_004 - can not send message to client: Address family not supported by protocol
客户开始:
./udpclient.out localhost 4545
服务器代码:
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <netinet/in.h>
#define BUFFER_SIZE 80
static void error(char *msg){
perror(msg);
exit(1);
}
int main(int argc, char ** argv){
int socketfd, portno, fromlen, n, length;
struct sockaddr_in server, from;
char buffer[BUFFER_SIZE];
char response[BUFFER_SIZE];
const char *le = "Little Endian\n";
const char *be = "Big Endian\n";
const char *unk = "Unknown \n";
int cli_family;
if(argc!=2){
fprintf(stderr,"Error starting the client, please start server as: %s port\n", argv[0]);
exit(0);
}
if((socketfd=socket(AF_INET,SOCK_DGRAM,0))<0)
error("ERROR UDP_SRV_001 - can not create socket");
portno=atoi(argv[1]);
bzero(&server,sizeof(server));
server.sin_family=AF_INET;
server.sin_addr.s_addr=INADDR_ANY;
server.sin_port=htons(portno);
if(bind(socketfd,(struct sockaddr *)&server,sizeof(server))<0)
error("ERROR UDP_SRV_002 - can not bind server address to a socket file descriptor");
bzero(buffer, BUFFER_SIZE);
bzero(response,BUFFER_SIZE);
n=recvfrom(socketfd,buffer,BUFFER_SIZE-1,0,(struct sockaddr *)&from,&fromlen);
if (n<0)
error("ERROR UDP_SRV_003 - can not receive client's message");
cli_family=from.sin_family;
printf("from.sin_family: %d\n", cli_family);
if (buffer[0] == 1 && buffer[1] == 2)
strcpy(response, be);
else if (buffer[0] == 2 && buffer[1] == 1)
strcpy(response, le);
else
strcpy(response, unk);
length=strlen(response);
printf("%s\n",response);
n=sendto(socketfd,response,length,0,(struct sockaddr *)&from,fromlen);
if(n<0)
error("ERROR UDP_SRV_004 - can not send message to client");
return 0;
}
客户代码:
#include <stdio.h>
#include <sys/types.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <unistd.h>
#define BUFFER_SIZE 80
static void error(char *msg){
perror(msg);
exit(1);
}
typedef union UNIT{
short s;
char c[sizeof(short)];
}unit;
int main(int argc, char ** argv){
int socketfd, portno, n, length;
struct sockaddr_in server, from;
struct hostent *host;
char buffer[BUFFER_SIZE];
unit un;
un.s=0x0102;
if(argc!=3){
fprintf(stderr,"Error starting the client, please start client as: %s hostname port\n", argv[0]);
exit(0);
}
if((host=gethostbyname(argv[1]))==NULL)
error("ERROR UDP_CLI_001 - no such host defined, please check your /etc/hosts file");
portno=atoi(argv[2]);
if((socketfd=socket(AF_INET,SOCK_DGRAM,0))<0)
error("ERROR UDP_CLI_002 - can not create socket");
bzero((char *)&server,sizeof(server));
server.sin_family=AF_INET;
server.sin_port=htons(portno);
bcopy((char *)host->h_addr,(char *)&server.sin_addr,host->h_length);
length=sizeof(server);
if(sizeof(short)!=2){
exit(0);
}
n=sendto(socketfd,un.c,strlen(un.c),0,(struct sockaddr *)&server,length);
if(n<0)
error("ERROR UDP_CLI_003 - can not send to server");
bzero(buffer,BUFFER_SIZE);
n=recvfrom(socketfd,buffer,BUFFER_SIZE,0,(struct sockaddr *)&from, &length);
if(n<0)
error("ERROR UDP_CLI_004 - can receive from server");
printf("%s",buffer);
return 0;
}
在服务器中,您未能在调用sendto
之前初始化fromlen
。 结果, from
没有被填充。
从手册页 :
如果src_addr不为NULL,并且底层协议提供了源地址,则该源地址被填充。当src_addr为NULL时,不填充任何内容; 在这种情况下,不使用addrlen,并且也应为NULL。 参数addrlen是一个值结果参数,调用方应在调用与src_addr相关联的缓冲区大小之前对其进行初始化,并在返回时进行修改以指示源地址的实际大小。 如果提供的缓冲区太小,返回的地址将被截断。 在这种情况下,addrlen将返回一个大于提供给调用的值。
recvfrom
函数尝试读取fromlen
的值,但是由于未初始化它,因此您将调用未定义的行为 。
初始化fromlen
,它将解决该问题。
fromlen = sizeof from;
n=recvfrom(socketfd,buffer,BUFFER_SIZE-1,0,(struct sockaddr *)&from,&fromlen);
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