c - 非终止 while 循环，同时使用 recv

Question

我正在尝试从客户端程序接收一条大消息，两者都使用 SOCK_STREAM 套接字，因此，我必须在 recv 调用中使用 while 循环。但是，它卡住了，并且似乎没有在循环内打印任何内容。

这是服务器的代码

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h> 
#include <stdlib.h>
#include <arpa/inet.h>

#define BUFSIZE 1024
extern int errno;
void error(char *msg)
{
    perror(msg);
    exit(0);
}

int main(int argc, char *argv[])
{
     int sock, newsock, len, fromlen, n;
     unsigned short port;
     struct sockaddr_in server, from;
     char buffer[BUFSIZE];
     char *msg = "I Got your message";

     if (argc < 2) {
         fprintf(stderr,"usage %s portnumber\n",argv[0]);
         exit(0);
     }
     port = (unsigned short) atoi(argv[1]);
     sock=socket(AF_INET, SOCK_STREAM, 0);
     if (sock < 0) error("Opening socket");
     server.sin_family=AF_INET;
     server.sin_addr.s_addr=INADDR_ANY;
     server.sin_port=htons(port);  
     len=sizeof(server);
     if (bind(sock, (struct sockaddr *)&server, len) < 0) 
          error("binding socket");
     fromlen=sizeof(from);
     if (listen(sock,5) < 0) 
          error("listening");

     // accept connections loop
     while (1) {
         newsock=accept(sock, (struct sockaddr *)&from, &fromlen);
         if (newsock < 0) error("Accepting");
         printf("A connection has been accepted from %s\n",
                 inet_ntoa((struct in_addr)from.sin_addr));

         // receive long messages loop
         while ((n = recv(newsock,buffer,BUFSIZE-1,0)) > 0){
            buffer[n] = '\0';
            printf("\n message: \n%s\n", buffer);
         }

        len = strlen(msg);
        n = send(newsock,msg,len,0);
        if (n < len) error("Error writing");

       if (close(newsock) < 0) error("closing");           
     }
     return 0; // we never get here 
}

这是客户端的代码

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h> /* for atoi */

char *msg = "Very long message";
void error(char *msg)
{
perror(msg);
exit(0);
}

int main(int argc, char *argv[])
{
int sock, n;
unsigned short port;
struct sockaddr_in server;
struct hostent *hp;
char buffer[1024];

if (argc != 3) { 
     printf("Usage: %s server port\n", argv[0]);
     exit(1);
}
sock= socket(AF_INET, SOCK_STREAM, 0);
if (sock < 0) error("Opening socket");

server.sin_family = AF_INET;
hp = gethostbyname(argv[1]);
if (hp==NULL) error("Unknown host");
memcpy((char *)&server.sin_addr,(char *)hp->h_addr,         
      hp->h_length);
port = (unsigned short)atoi(argv[2]);
server.sin_port = htons(port);
if (connect(sock, (struct sockaddr *)&server, sizeof server) < 0)
         error("Connecting");
n = send(sock, msg, strlen(msg),0);
printf("sent %d bytes\n", n);
if (n < strlen(msg))
         error("Writing to socket");

n = recv(sock, buffer, 1023,0);
if (n < 1) error("reading from socket");
buffer[n]='\0';
printf("The message from the server is %s\n",buffer);
if (close(sock) < 0) error("closing");
printf("Client terminating\n");

return 0;
}

最终目标是在服务器端接收完整的消息，然后向客户端发送确认消息已收到。

Answer 1

这个条件:

 (n = recv(newsock,buffer,BUFSIZE-1,0)) > 0

将始终为真，除非远程关闭套接字或系统调用返回错误。否则，如果客户端没有发送数据，它将阻塞。

在 STREAM 套接字上，您必须在不做任何假设的情况下期待一个字节流，它将如何在对 recv() 的多次调用中分块。一种常见的解决方案是首先传输(和接收)长度信息，然后调用 recv 直到总共接收到恰好 length 个字节。

您的特殊情况的另一个选项(“快速修复”)可能是在发送大消息后关闭客户端套接字的发送方向:

shutdown(fd, SHUT_WR);

之后，recv 系统调用确实返回 0，服务器可以发送确认。但是，客户端在调用 shutdown() 后将无法发送数据。