TCP 连接似乎接收到不完整的数据

我已经设置了一个简单的 TCP 文件传输。一切似乎都工作正常，除了接收到的文件大小偶尔小于发送的文件大小。接收到的文件的大小似乎没有任何规律。

（在下面的代码中，请注意典型的客户端/服务器滚动是相反的） 我的客户端代码如下：

#define kMaxBacklog (5)
// fill out the sockadd_in for the server
struct sockaddr_in servAdddress;
//memcpy() to fill in the sockaddr

//setup the socket
int sockd, returnStatus;    
sockd = socket(AF_INET, SOCK_STREAM, 0);
if (sockd == -1)
    NSLog(@"could not create client socket");
else
    NSLog(@"created client socket");

returnStatus = connect(sockd, (struct sockaddr*)&servAdddress, sizeof(servAdddress));
if (returnStatus == -1)
    NSLog(@"could not connect to server - errno:%i", errno);
else
    NSLog(@"connected to server"); 

NSData *dataWithHeader = [self getDataToSend];
returnStatus = send(sockd, [dataWithHeader bytes], [dataWithHeader length], 0);
if (returnStatus == -1)
    NSLog(@"could not send file to server");
else if( returnStatus < [dataWithHeader length])
    NSLog(@"ONLY PARTIAL FILE SENT");
else
    NSLog(@"file sent of size: %i", returnStatus);

shutdown(sockd, SHUT_WR);
close(sockd);

客户端方法始终报告它发送了整个文件。

对于服务器：

#define MAXBUF (10000)
int _socket;
_socket = socket(AF_INET, SOCK_STREAM, 0); // set up the socket 

struct sockaddr_in addr; 
bzero(&addr, sizeof(addr)); 
addr.sin_len = sizeof(addr); 
addr.sin_family = AF_INET; 
addr.sin_addr.s_addr = INADDR_ANY;
addr.sin_port = htons(0); 

int retval = bind(_socket, (struct sockaddr *)&addr, sizeof(addr)); 
    if (retval == -1)
        NSLog(@"server could not bind to socket");
    else 
        NSLog(@"server socket bound");

socklen_t len = sizeof(addr); 
retval = getsockname(_socket, (struct sockaddr *)&addr, &len);
    if (retval == -1)
        NSLog(@"server could not get sock name");
    else 
        NSLog(@"server socket name got");

    int socket1, socket2, clientAddrLen, returnStatus;
    struct sockaddr_in servAdddress, clientAddress;
    clientAddrLen = sizeof(servAdddress);

    socket1 = _socket;

    returnStatus = listen(socket1, kMaxBacklog);
    if (returnStatus == -1)
        NSLog(@"server could not listen on socket");
    else
        NSLog(@"server socket listening");

while(1){
    FILE *fd;
    int i, readCounter;
    char file[MAXBUF];

    NSLog(@"server blocking on accept()");
    socket2 = accept(socket1, (struct sockaddr*)&clientAddress, (socklen_t*)&clientAddrLen);
    if (socket2 == -1)
        NSLog(@"server could not accpet the connection");
    else
        NSLog(@"server connection accepted");

    i = 0;
    readCounter = recv(socket2, file, MAXBUF, 0);
    if(!readCounter)
        NSLog(@"server connection cancelled, readCount = 0");

        else if (readCounter == -1){
        NSLog(@"server could not read filename from socket");
        close(socket2);
        continue;
    }
    else
        NSLog(@"server reading file of size: %i", readCounter);


    fd = fopen([myfilePathObject cStringUsingEncoding:NSASCIIStringEncoding], "wb");

    if(!fd){
        NSLog(@"server could not open the file for creating");
        close(socket2);
        continue;
    }
    else
        NSLog(@"server file open for creating");

    returnStatus = fwrite([myData bytes], 1, [myData length], fd);
    if (returnStatus == -1)
        NSLog(@"Error writing data to server side file: %i", errno);
    else
        NSLog(@"file written to disk);

    readCounter = 0;
    //close (fd);
    returnStatus = fclose(fd);
    if(returnStatus)
        NSLog(@"server error closing file");

因此，偶尔，readCounter 变量将不会包含与发送的文件相同的大小，但有时会包含相同的大小。

如果重要的话，文件传输是在 iPhone 和 iPhone 模拟器之间进行的，两者都是通过 WIFI 进行的。无论手机是服务器还是模拟器是服务器，都会发生这种情况。

如果有人能帮助我理解为什么会发生这种情况，我将不胜感激。我认为TCP的全部目的就是为了避免这种问题。

（为了给予应有的认可，我的服务器和客户端代码大量借鉴了 Apress 的 Davis、Turner 和 Yocom 所著的《Linux 网络编程权威指南》一书）

The recv函数只能接收 1 个字节，您可能需要多次调用它才能获取整个有效负载。因此，您需要知道您期望有多少数据。虽然你can通过关闭连接来发出完成信号，这并不是一个好主意。

Update:

我还应该提到的是send函数具有相同的约定recv：你必须循环调用它，因为你不能假设它会发送你的所有数据。虽然它可能在您的开发环境中始终有效，但这种假设会在以后困扰您。