Number of bytes on ReadBytes

Is there a way to know how much bytes I should wait for (in advance) in a TCP stream, using :

ReadBytes(BUFFER_tcp,HANDLE_tcp,0,num_bytes)

I mean I don't know in advance how many bytes the server will send. Thank you.

you can get the number of bytes received by function: ReadAvail(), but if you want to know how long is the "fail" you are going to receive then you'll need to send the length of it.