FTP client -- CR characters in gif is removed after get - c++

Question

I'm creating an FTP client.
I'm getting a gif from the server, but after that the gif is corrupted.
When I change the file extension to look at the diff, I see that the
CR/LF characters are gone.
How could this be? I made sure to use image mode.
Here's my read code in TCP socket.
string TCPSocket::long_read()
{
pollfd ufds;
ufds.fd = sd;
ufds.events = POLLIN;
ufds.revents = 0;
ssize_t bytesRead = 0;
string result;
char* buf = new char[LONGBUFLEN];
do {
bzero(buf, LONGBUFLEN);
bytesRead = ::read(sd, buf, LONGBUFLEN);
if (bytesRead == 0) {
break;
}
if (bytesRead > 0) {
result = result + string(buf, bytesRead);
}
} while (poll(&ufds, 1, 1000) > 0);
return result;
}
Here my get code in main.cpp
else if (command == command::GET) {
string filename;
cin >> filename;
string dataHost;
int dataPort;
if (enterPassiveMode(dataHost, dataPort)) {
dataSocket = new TCPSocket(dataHost.c_str(), dataPort);
if (fork() == 0) {
string result = dataSocket->long_read();
size_t length = result.size();
char* resultArr = new char[length];
memcpy(resultArr, result.data(), length);
// mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
FILE* file = fopen(filename.c_str(), "w+b");
if (file) {
fwrite(resultArr, length, 1, file);
fclose(file);
}
else {
cout << "open failed";
}
break;
}
else {
writeAndImmediateRead(rfc959::TYPE_I);
controlSocket->write(rfc959::RETRIVE(filename));
string result = controlSocket->read();
cout << result;
int reply = Parser::firstDigit(result);
// I'll remove incomplete local file if request fails
if (reply != rfc959::POSITIVE_PRELIMINARY_REPLY) {
remove(filename.c_str());
continue;
}
wait(NULL);
cout << controlSocket->long_read();
}
}
}
EDIT
I did make sure to use Binary mode. And when I transferred a text file(though of a smaller size), it doesn't have this problem. Here's the output:
EDIT 2
Output from Wireshark showing Request: TYPE I and Response: Opening BINARY mode

Answer 1

By default, FTP servers and clients perform data transfers as "ASCII mode", which means that any CRLF sequence is translated on-the-fly to the host's ASCII line ending (e.g. just bare LF on Unix mmachines). This behavior is mandated by RFC 959; see Section 3.1.1.1.
To transfer your data as binary, and avoid the ASCII mode translation, your FTP client will want to send the TYPE command first, e.g.:
TYPE I
Your .gif file should then be transferred as is, with no replacements/transformations on any CRLF sequences.
Hope this helps!