recv() data loss on localhost

Question

I'm building a P2P file sharing program and I can transfer files between computes connected to my Wifi Router. However, when I run it on my computer with localhost address, the sender part of my program successfully sends all the data to the receiver program but my receiver does not get all the data. I'm running these two on seperate ports on my machine. The problem does not occur when I run sender and receiver on seperate machines.

Here's my receive code snippet

    while ((len = recv(welcomeSocket, buffer, BUFSIZ, 0)) > 0 && remain_data > 0)
    {
            fwrite(buffer, sizeof(char), len, received_file);
            remain_data -= len;
            fprintf(stdout, "Receive %zd bytes and we hope :- %d bytes\n", len, remain_data);

    }

And here's my send code snippet

     while (((sent_bytes = sendfile(fds[i].fd, fd, offset, BUFSIZ)) > 0) && (remain_data > 0))
    {
            fprintf(stdout, "Server sent %u bytes from file's data, offset is now : %jd \n", sent_bytes, (intmax_t)offset);
            remain_data -= sent_bytes;
            fprintf(stdout, "remaining data = %d\n", remain_data);
    }

The output at the sender part is

Server sent 8192 bytes from file's data, offset is now : 0 
remaining data = 30292
Server sent 8192 bytes from file's data, offset is now : 0 
remaining data = 22100
Server sent 8192 bytes from file's data, offset is now : 0 
remaining data = 13908
Server sent 8192 bytes from file's data, offset is now : 0 
remaining data = 5716
Server sent 5716 bytes from file's data, offset is now : 0 
remaining data = 0

And the output at the receiver is

Receive 256 bytes and we hope :- 38228 bytes
Receive 8192 bytes and we hope :- 30036 bytes
Receive 8192 bytes and we hope :- 21844 bytes
Receive 8192 bytes and we hope :- 13652 bytes
Receive 5716 bytes and we hope :- 7936 bytes

Once again, this does not happen when running these programs on different machines and recv() gets all the data. Is the communication path at localhost so fast that the recv() is not able to handle the data and hence misses some of it?

Thank you

Answer 1

Assuming that remain_data is the actual file size, then both of your loops are wrong. They should be more like this:

while (remain_data > 0)
{
    len = recv(welcomeSocket, buffer, min(remain_data, BUFSIZ), 0);
    if (len <= 0) {
        // error handling...
        break;
    }
    fwrite(buffer, sizeof(char), len, received_file);
    remain_data -= len;
    fprintf(stdout, "Receive %zd bytes and we hope :- %d bytes\n", len, remain_data);
}

off_t offset = 0;
while (remain_data > 0)
{
    sent_bytes = sendfile(fds[i].fd, fd, &offset, min(remain_data, BUFSIZ));
    if (sent_bytes <= 0) {
        // error handling...
        break;
    }
    fprintf(stdout, "Server sent %u bytes from file's data, offset is now : %jd \n", sent_bytes, (intmax_t)offset);
    remain_data -= sent_bytes;
    fprintf(stdout, "remaining data = %d\n", remain_data);
}