C: Winsock's 'send()' not blocking

XProject · #1 22-Mar-2005, 15:40

I am currently trying to write a programin C on Windows 2000 that will send a file between two computers. The code I have right now seems to work, but there is a substanial problem with it. When sending the binary data from the server to the client, sometimes packets will mesh together. This will cause data to be lost because I have predetermined a size for each chunk of the file. Once the data arrives, I have the client only get the normal packet size. To help explain this, here is an example of the server:

CPP / C++ / C Code:

#define CHUNK 256 //define the size of each chunk
char buff[CHUNK];
FILE *fp = fopen("example.exe", "rb");
while(!feof(fp))
{
     fread(&buff, sizeof(char), CHUNK, fp);
     send(sock, buff, CHUNK, 0);
}

And here's how the client looks:

CPP / C++ / C Code:

#define CHUNK 256
char buff[CHUNK];
FILE *fp = fopen("example.exe", "wb");
unsigned long total_bytes = //get file size from server
unsigned long current_byte = 0;
while(1)
{
     recv(sock, buff, CHUNK, 0);
     fwrite(&buff, sizeof(char), CHUNK, fp);
     current_byte += CHUNK;
     if(current_byte >= total_bytes) break;
}

Using Ethreal, here is an example of what happens, supposing CHUNK was set to 10:
First Packet: "this is an" 10 bytes
Second Packet: " example o" 10 bytes
Third Packet: "f the problem. blah." 20 bytes
Normally, this will occur right around the 8 KB mark for some reason. If anyone could expain to me how to fix this problem, it would be greatly appreciated.

nkhambal · #2 22-Mar-2005, 16:53

Assuming you are using TCP to transfer files between client and server, I would suggest following. This is not tested.

Server side

CPP / C++ / C Code:

#define CHUNK 256 //define the size of each chunk
char buff[CHUNK];
int bytes_read;
FILE *fp = fopen("example.exe", "rb");
while(!feof(fp))
{
     if ( (bytes_read = fread(&buff, sizeof(char), CHUNK, fp)) > 0 )
     {
        send(sock, buff, bytes_read, 0);
     }
}

Client Side:

CPP / C++ / C Code:

#define CHUNK 256
char buff[CHUNK];
FILE *fp = fopen("example.exe", "wb");
unsigned long total_bytes = //get file size from server
unsigned long current_byte = 0;
int bytes_received;

while ((bytes_received=recv(sock, buff, CHUNK, 0)) != -1)
{
    if (bytes_received == 0 ) break;  //TCP Connection closed.
    fwrite(&buff, sizeof(char), bytes_received, fp);
}

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