C语言 UDP sendto()和recvfrom()函数没有按预期工作

Question

我想实现一个简单的UDP服务器和客户端，所以我用C语言写了一半的代码。该代码的目的是从客户端向服务器发送消息，如果消息被正确发送，服务器发送一个"确认"。消息发送给客户端。但是，sendto()和recvfrom()的返回值为-1，因此不发送消息。在uppserver .c中，我知道"被策略牢固地覆盖。在udpserver.c中，我可以发送消息，在udpclient.c中，我可以接收消息，但我不知道如何从这里编写代码。你能给我一些具体的代码吗?另外，我希望服务器说Hello，客户端说ACK。

#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#define MYPORT "4567"                      // the port that client  will be connecting to
#define MAXBUFLEN 100
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(void)
{
int sockfd;
struct addrinfo hints, *servinfo, *p;
int rv;
int numbytes;
struct sockaddr_storage their_addr;
char buf[MAXBUFLEN];
int32_t receivedNumber;
socklen_t addr_len;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;             // set to AF_INET to force IPv4
hints.ai_socktype = SOCK_DGRAM;          // UDP 
hints.ai_flags = AI_PASSIVE;             // use my IP

if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %sn", gai_strerror(rv));
return 1;
}
// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) {
perror("server: socket");
continue;
}
if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror("server: bind");
continue;
}
break;
}
if (p == NULL) {
fprintf(stderr, "listener: failed to bind socketn");
exit(1);
}
printf("server: waiting for client...n");
addr_len = sizeof their_addr;
//Receive from client
if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0, (struct sockaddr *)&their_addr, &addr_len)) == -1) {
perror("recvfrom");
exit(1);
}
printf("Received from client: %sn", buf);
strcpy(buf,"ACK");

//Send client an ACK message
if(sendto(sockfd,buf,strlen(buf),0,p->ai_addr,p->ai_addrlen)==-1){
printf("Errorn");
}
/*
unknown code
*/
freeaddrinfo(servinfo);
close(sockfd);
return 0;
} 

#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#define SERVERPORT "4567" // the port that client  will be connecting to
#define MAXBUFLEN 100
int main(int argc, char *argv[])
{
int sockfd;
struct addrinfo hints, *servinfo, *p;
struct sockaddr_storage their_addr;
socklen_t addr_len;
int rv;
int numbytes;
char buf[MAXBUFLEN];
if (argc != 3) {
fprintf(stderr,"usage: talker hostname messagen");
exit(1);
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;
if ((rv = getaddrinfo(argv[1], SERVERPORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %sn", gai_strerror(rv));
return 1;
}
// loop through all the results and make a socket
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) {
perror("talker: socket");
continue;
}
break;
}
if (p == NULL) {
fprintf(stderr, "talker: failed to create socketn");
return 2;
}
// Send to server
if ((numbytes = sendto(sockfd, argv[2], strlen(argv[2]), 0, p->ai_addr, p->ai_addrlen)) == -1) {
perror("client: sendto");
exit(1);
}
//Receive from server
if(recvfrom(sockfd,buf,MAXBUFLEN-1,0,(struct sockaddr *)&their_addr,&p->ai_addrlen)==-1){
printf("Errorn");
}
/* 
unknown code
*/

printf("Received from server: %sn", buf);
freeaddrinfo(servinfo);
close(sockfd);
return 0;
}

gcc udpserver.c
./a.out
server: waiting for client...
Received from client: Hello~
gcc udpclient.c 
./a.out 127.0.0.1 Hello
Received from server: �*��z     ```

Answer 1

如果从通道读入缓冲区并且结果是正的，那么您已经成功地读取了一些字节，但不是以空结尾的字符串.

。

numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1, ...);

numbytes是读取的字节数，现在是时候制作字符串了:

buf[numbytes] = '';

客户端和服务器端都不将空字节添加到buf末尾。