added to tutorial

psk/tutorial-tcp-psk.md

@@ -3,3 +3,60 @@ TCP/PSK Tutorial
 
 < TODO >
 
+# **Concurrent Server**
+
+
+The main thread accepts clients and for each client accepted a new thread is spawned that then handles the typical server processes.
+
+
+1. To use multiple threads include the pthread header file.
+        **#include <pthread.h>**
+
+
+2. When creating multiple threads the state of variables can become an issue. Since in the example, CYASSL_CTX* is not changed after being initially set we can make it a global variable and allow all threads read access while they are processing without having to lock the memory.
+
+
+3. After the main thread accepts a client, call the pthread_create function.
+```c
+pthread_create(pthread_t* thread, int attribute, void* function, void* arg)
+```
+4. In the example the function passed to pthread_create accepts one void * argument which is the socket the client is on. The function then performs the process of creating a new SSL object, reading and writing to the client, freeing the SSL object, and then terminating the thread.
+```c
+ /*
+  *Process handled by a thread.
+  */
+  void* cyassl_thread(void* fd)
+  {
+          CYASSL* ssl;
+          int connfd = (int)fd;
+          int  n;                  /* length of string read */
+          char buf[MAXLINE];   /* string read from client */
+          char response[22] = "I hear ya for shizzle";
+ 
+          /* create CYASSL object and respond */
+          if ((ssl = CyaSSL_new(ctx)) == NULL)
+              err_sys("CyaSSL_new error");
+          CyaSSL_set_fd(ssl, connfd);            
+                                             
+          /* respond to client */                            
+          n = CyaSSL_read(ssl, buf, MAXLINE);                
+          if (n > 0) {                                       
+              printf("%s\n", buf);                           
+              if (CyaSSL_write(ssl, response, 22) > 22) {
+                  err_sys("respond: write error");
+              }
+          }
+          if (n < 0) {
+              err_sys("respond: read error");
+          }
+                                                       
+          /* closes the connections after responding */
+          CyaSSL_shutdown(ssl);
+         CyaSSL_free(ssl);           
+         if (close(connfd) == -1)   
+             err_sys("close error");
+         pthread_exit( NULL);
+ }
+```
+5. Void* arg is the argument that gets passed into cyassal_thread when pthread_create is called. In this example that argument is used to pass the socket value that the client for the current thread is on.
+