Update README.md

README.md

@@ -5,6 +5,22 @@ Created November 1999 by J. David Blackstone.
 
 Modified June 2021 by Fumiama(源文雨)
 
+# Protocol
+
+A necessary subset of `HTTP 1.0` with following options of request header being supported
+### From client
+- Content-Length
+### From server
+- Content-Length
+- Content-Type (only support text/plain image/x-icon text/css text/html)
+- Server
+### Code
+- 200 OK
+- 400 BAD REQUEST
+- 404 NOT FOUND
+- 500 Internal Server Error
+- 501 Method Not Implemented
+
 # Features
 
 1. Serve files
@@ -33,3 +49,21 @@ simple-http-server -d port chdir
 - **-d** - run as daemon
 - **port** - bind server on this port (0 for a random one)
 - **chdir** - change root dir to here
+
+# CGI usage
+
+When you put a executable file into the web path, the server will call `execl` to run it while passing 3 parameters as below
+
+```c
+argv[0] = path;   //Path of the executable file
+argv[1] = method; //request method (GET/POST)
+argv[2] = query_string;   //the query string, like "a=1&b=2&c=3"
+```
+
+The server will read a `4 bytes` unsigned integer from pipe, indicating the `length` of the remaining content. Then it will send `length` bytes of data to the client directly with nothing being decorated, which means that you need to assemble the HTTP header by yourself.
+
+Here is a CGI example [CMoe-Counter](https://github.com/fumiama/CMoe-Counter)
+
+And its realization is here:
+
+<div align=center> <a href="#"> <img src="http://pan.fumiama.top:42412/cmoe?name=shttps&theme=gb" /> </a> </div>