C++ ::  Send Binary Data Over TCP/IP

I am writing a client-server program in C++ using C libraries. The server listens for a connection and when one is found, it is supposed to send a binary file(an image in my case) to the client. The server is sending the binary file perfectly after opening it and reading its contents into a buffer that is dynamically allocated in the SERVER file.

The problem begins when the client file is supposed to recv() the file, I do not know how I can allocate a buffer big enough for the file to be received. I know how to use malloc() and new, I prefer malloc() for executable size customization. Assuming the file being sent is 11000 bytes(note that this is an assumption and the client can not know the file size because the file size is dynamically generated by server).

How can the client generate a dynamically allocated buffer big enough to hold 11000 bytes?
OR
How can the client store the data in a buffer using recv() without knowing the file size.

Below is part of the code that does the send()'ing and recv()'ing.

SERVER

FILE *img = fopen("img.jpg", "rb");
fseek(img, 0, SEEK_END);
unsigned long filesize = ftell(img);
char *buffer = (char*)malloc(sizeof(char)*filesize);
rewind(img);
// store read data into buffer

[Code] ....

This outputs all the binary code perfectly but it is a bug because it can't be written to a file. Is there a way to dynamically increase the buffer until all filesize is received? Certainly, buffer needs to be dynamically allocated or program crashes ...

Also one more thing, when I telnet the server from command line, telnet displays all the output perfectly as well

Maybe telnet is storing data into a buffer behind the scenes, if so, how?