Send binary file over TCP/IP connection

Question

I will rephrase the whole question here so that it is answerable.

I am able to copy binary file perfectly in the same machine not using sockets but just making a sim

Answer 1

We could avoid the header that contains the image size, but we just read to the end of the sent data. About the buffer size, we could use a fixed number such as 10 * 1024, when we received some data from the server, we just save it into a file according to the actual received data length.

// please open a file ...
FILE * fp;
// ...
const int LENGTH = 10 * 1024;

int len = 0;
char * buffer = (char *)malloc(LENGTH);
while ((len = recv(socket, buffer, LENGTH, 0)) > 0) {
    fwrite(buffer, 1, len, fp);
}
free(buffer);
// close the file

@T.C: I guess we cannot allocate a buffer according to the size sent from the server in case the image is too large to save inside the client's memory. Not mention the server is fake, and intended to make any attack.

Answer 2

You need to loop both the sending and receiving. Neither send() nor recv() are guaranteed to send/read as many bytes as you requested.

You also should send the file size before the file data so the receiver knows how many bytes to expect and when to stop reading.

Try something more like this:

SERVER

bool senddata(SOCKET sock, void *buf, int buflen)
{
    unsigned char *pbuf = (unsigned char *) buf;

    while (buflen > 0)
    {
        int num = send(sock, pbuf, buflen, 0);
        if (num == SOCKET_ERROR)
        {
            if (WSAGetLastError() == WSAEWOULDBLOCK)
            {
                // optional: use select() to check for timeout to fail the send
                continue;
            }
            return false;
        }

        pbuf += num;
        buflen -= num;
    }

    return true;
}

bool sendlong(SOCKET sock, long value)
{
    value = htonl(value);
    return senddata(sock, &value, sizeof(value));
}

bool sendfile(SOCKET sock, FILE *f)
{
    fseek(f, 0, SEEK_END);
    long filesize = ftell(f);
    rewind(f);
    if (filesize == EOF)
        return false;
    if (!sendlong(sock, filesize))
        return false;
    if (filesize > 0)
    {
        char buffer[1024];
        do
        {
            size_t num = min(filesize, sizeof(buffer));
            num = fread(buffer, 1, num, f);
            if (num < 1)
                return false;
            if (!senddata(sock, buffer, num, 0))
                return false;
            filesize -= num;
        }
        while (filesize > 0);
    }
    return true;
}

FILE *filehandle = fopen("imagefile.jpg", "rb");
if (filehandle != NULL)
{
    sendfile(clientsocket, filehandle);
    fclose(filehandle);
}

CLIENT

bool readdata(SOCKET sock, void *buf, int buflen)
{
    unsigned char *pbuf = (unsigned char *) buf;

    while (buflen > 0)
    {
        int num = recv(sock, pbuf, buflen, 0);
        if (num == SOCKET_ERROR)
        {
            if (WSAGetLastError() == WSAEWOULDBLOCK)
            {
                // optional: use select() to check for timeout to fail the read
                continue;
            }
            return false;
        }
        else if (num == 0)
            return false;

        pbuf += num;
        buflen -= num;
    }

    return true;
}

bool readlong(SOCKET sock, long *value)
{
    if (!readdata(sock, value, sizeof(value)))
        return false;
    *value = ntohl(*value);
    return true;
}

bool readfile(SOCKET sock, FILE *f)
{
    long filesize;
    if (!readlong(sock, &filesize))
        return false;
    if (filesize > 0)
    {
        char buffer[1024];
        do
        {
            int num = min(filesize, sizeof(buffer));
            if (!readdata(sock, buffer, num))
                return false;
            int offset = 0;
            do
            {
                size_t written = fwrite(&buffer[offset], 1, num-offset, f);
                if (written < 1)
                    return false;
                offset += written;
            }
            while (offset < num);
            filesize -= num;
        }
        while (filesize > 0);
    }
    return true;
}

FILE *filehandle = fopen("imagefile.jpg", "wb");
if (filehandle != NULL)
{
    bool ok = readfile(clientsocket, filehandle);
    fclose(filehandle);

    if (ok)
    {
        // use file as needed...
    }
    else
        remove("imagefile.jpg");
}