C++ Sending PNG File via HTTP - c++

Question

I'm trying to create a webserver to learn how HTTP functions. I am trying to send a png file to the browser, however the image successfully makes it.
Here is my png sending code:
std::ifstream in("P:/server"+location, std::ios::binary);
if(!in.is_open()) {
std::cout << "failed to open file" << std::endl;
in.close();
}
in.seekg(0, std::ios::end);
int length = in.tellg();
in.seekg(0, std::ios::beg);
char *data = new char[length];
in.read(data, length);
in.close();
std::string headers = "HTTP/1.1 200 OK\n\rContent-Length: " + std::to_string(length) + "\n\rConnection: keep-alive\n\rContent-Type: image/png\n\r\n\r";
int totalLength = headers.length() + length;
char *allData = new char[totalLength];
std::strcpy(allData, headers.data());
std::strcat(allData, data);
int bytes = send(socket, data, totalLength, NULL);
Once the server should have sent the image, it shows up as the missing image icon.
I have checked to make sure that all the bytes are being sent, and that the image is being loaded.
Any help would be very much appreciated!

Answer 1

There are quite a few mistakes in your code.
You are still trying to process the file if is_open() returns false.
You are using \n\r when you need to use \r\n instead.
You are using strcat() to append data to allData. When appending binary data, like a PNG, strcat() will truncate on the first 0x00 byte encountered. You need to use memcpy() (or equivalent) instead.
Your call to send() is sending data when it should be sending allData instead. So you are you not sending the HTTP headers at all, and you are sending data using the wrong length.
You are assuming send() will send all of the data you give it in a single operation. That is almost never the case, especially fo large amounts of data. send() returns the number of bytes it actually accepted for sending, so you need to call it in a loop until all of the data has been accepted.
You are not sending an error message to the client if something goes wrong while preparing the file.
That being said, allData is actually unnecessary, and is a waste of memory. TCP is a byte stream, so you can send headers and data individually one after the other. How many times you call send() doesn't affect how the other party receives the data.
You might also consider minimizing memory usage further by changing data to be a fixed-sized buffer so you can send() the content of in while reading from it in smaller chunks.
Try something more like this instead:
int sendData(int sckt, const void *data, int datalen)
{
const char *ptr = static_cast<const char*>(data);
while (datalen > 0) {
int bytes = send(sckt, ptr, datalen, 0);
if (bytes <=0) return -1;
ptr += bytes;
datalen -= bytes;
}
return 0;
}
int sendStr(int sckt, const std::string &s)
{
return sendData(sckt, s.c_str(), s.size());
}
...
std::string filename = "P:/server"+location;
if (!fileExists(filename)) // <- you need to implement this
{
if (sendStr(socket, "HTTP/1.1 404 Not Found\r\nContent-Length: 0\r\nConnection: keep-alive\r\n\r\n") == -1) {
close(socket);
}
}
else
{
std::ifstream in(filename, std::ios::binary);
if (!in.is_open())
{
std::cout << "failed to open file" << std::endl;
if (sendStr(socket, "HTTP/1.1 500 Error\r\nContent-Length: 0\r\nConnection: keep-alive\r\n\r\n") == -1) {
close(socket);
}
}
else
{
in.seekg(0, std::ios::end);
std::size_t length = in.tellg();
in.seekg(0, std::ios::beg);
if (in.fail())
{
std::cout << "failed to get size of file" << std::endl;
if (sendStr(socket, "HTTP/1.1 500 Error\r\nContent-Length: 0\r\nConnection: keep-alive\r\n\r\n") == -1) {
close(socket);
}
}
else if (sendStr(socket, "HTTP/1.1 200 OK\r\nContent-Length: " + std::to_string(length) + "\r\nConnection: keep-alive\r\nContent-Type: image/png\r\n\r\n") == -1)
{
close(socket);
}
else if (length > 0)
{
char data[1024];
do
{
if (!in.read(data, std::min(length, sizeof(data))))
{
close (socket);
break;
}
int bytes = in.gcount();
if (sendData(socket, data, bytes) == -1)
{
close(socket);
break;
}
length -= bytes;
}
while (length > 0);
}
}
}