- How does an HTTP client indicate that it has finished sending the HTTP request?
The HTTP request should end with a blank line.
- How does an HTTP client know what type of content the HTTP server is sending?
The HTTP server should identify the content with a Content-Type header.
- How can an HTTP server identify a file's media type?
A common method of identifying a file's media type is just to look at the file extension. The server is free to use other methods though. When sending dynamic pages or data from a database, there will be no file and therefore no file extension. In this case, the server must know the media type from its context.
- How can you tell whether a file exists on the filesystem and is readable by your program? Is fopen(filename, "r") != 0 a good test?
This is not a trivial problem. A robust program will need to consider system specific APIs carefully. Windows uses special filenames that will trip up a program that relies only on fopen() to check for a file's existence.