whats wrong with the socket recv

Thanks for your response.

But maybe you didn't notice, I used a while(leftsize>0) loop to see whether I get the whole chunck of data.

What I did was fwrite whatever recvbytes I got from one recv call to the file, then loop back to check out whether there are any bytes left until I got one full chunk of data.

The complete condition was if I got the data size equal to zero(which is the 8th unsigned short number of the header, if(leftsize==0)), then the thread will stop receiving.

What I didn't understand was why the server side disconnected the socket before the client side received the stop condition.

What I am doing is like continuously receiving the packet header(which are 8 unsigned short numbers), then the packet data, and write it to a file. Until the newly received header meet the stop condition.

Thanks again.

I tried to recv one byte at a time for the header,
it somehow worked on most of the packets except the last one. the last packet's header has 7 zero fields just as same as the first packet's header, but it only worked on the first packet, not the last packet. Could you help me figure out why?

Below are the modified code:

UINT CDLCtrl::Receive(LPVOID arg)
{
int i=0;
char recvsize[2];
char recvMessage[40000];
char framemarker[500];
unsigned short recvbytes=0;
unsigned short recvnum=0;
unsigned short leftsize=0;
unsigned short year,month,day,hour,min,sec,usec;
int count=0;
FILE * fp;

CDLCtrl *myctrl= (CDLCtrl*)arg;

fp=fopen(myctrl->saveFileName,"wb");

recvMessage[0]='\0';

recvsize[0]='\0';

while(1){
while(count<8){
count++;
if((recvbytes=recv(myctrl->sock, (char*)&recvsize[0], 1, 0)) == SOCKET_ERROR)
{
int res = WSAGetLastError();
fclose(fp);
closesocket(myctrl->sock);
::MessageBox(NULL, "recv() failed","Error", MB_OK);
return 1;

}
if(recvbytes==0){
fclose(fp);
closesocket(myctrl->sock);
::MessageBox(NULL, "Server socket closed", "Message",MB_OK);
return 1;
}
else if(recvbytes==1)
{
if((recvbytes=recv(myctrl->sock, (char *)&recvsize[1], 1, 0)) != SOCKET_ERROR){
if(recvbytes==1)
recvnum=((recvsize[0]&0xff)<<8)|(recvsize[1]&0xff);
else if(recvbytes==0){
fclose(fp);
closesocket(myctrl->sock);
::MessageBox(NULL, "Server socket closed", "Message",MB_OK);
return 1;
}
}
else{
::MessageBox(NULL, "recv() failed","Error", MB_OK);
return 1;
}

}


switch(count){
case 1:
month=recvnum;
break;

case 2:
day=recvnum;
break;

case 3:
year=recvnum;
break;

case 4:
hour=recvnum;
break;
case 5:
min=recvnum;
break;

case 6:
sec=recvnum;
break;

case 7:
usec=recvnum;
break;

case 8:
leftsize=recvnum;

break;

}
}


count=0;

if(leftsize==0){
fclose(fp);
closesocket(myctrl->sock);
::MessageBox(NULL, "Downloading Completed", "Message",MB_OK);
return 0;

}


if(year!=0){
sprintf(framemarker,"--frame;%s;%d/%d/%d;%d:%d:%d:%d;%d\n","Lab-Entry",year,month,day,hour,min,sec,usec,leftsize);

fputs(framemarker,fp);
}


while(leftsize>0){

if((recvbytes=recv(myctrl->sock, recvMessage, leftsize, 0)) == SOCKET_ERROR)
{
fclose(fp);
closesocket(myctrl->sock);
::MessageBox(NULL, "recv() failed","Error", MB_OK);
return 1;
}
else
{
leftsize-=recvbytes;
if(recvbytes==0){
fclose(fp);
closesocket(myctrl->sock);
::MessageBox(NULL, "Server socket closed", "Message",MB_OK);
return 1;

}

else if(year!=0){
fwrite(recvMessage,1,recvbytes,fp);

}
}
}

if(year!=0){
fputs("\n",fp);
}

}

closesocket(myctrl->sock);
fclose(fp);
return 0;
}

Thanks for your advice. I have solved the problem.
For the unsigned short, it was two bytes, at the beginning, I tried to recv two bytes together, now I fixed it with receiving one byte at a time.
Appeciate your help.