Differences

This shows you the differences between two versions of the page.

--- tcp_udp [2017/05/02 21:03] – [TCP Client & Server] dwallace
+++ tcp_udp [2017/05/02 22:45] (current) – dwallace
@@ Line 10: / Line 10: @@
 {{ dylanw:tcp_vs_udp.jpg?600 }}
 \\
-The photo above depicts the differences between TCP & UDP connections which allow you to create different forms of communications. The big picture problem is high-latency or low-bandwidth telerobotics. Solving this partially or completely is important because it will allow remote operation of space robotics, disaster relief, etc. This tutorial shows you how to create a TCP client & server, and a UDP talker & listener, and takes approximately 2 hours to complete.
+The photo above depicts the differences between TCP & UDP connections which allow you to create different forms of communications. The big picture problem is high-latency or low-bandwidth telerobotics. Solving this partially or completely is important because it will allow remote operation of space robotics, disaster relief, etc. This tutorial shows you how to create a TCP client & server, and a UDP talker & listener, and takes approximately 3 hours to complete.
 \\
 ===== Motivation and Audience =====
@@ Line 93: / Line 93: @@
 ==== Server ====
-Let's break down the server code. The first block is where we include all the necessary libraries and define essential constants.
+Let's break down the server code. The server is a program that binds to a certain port, and send a predefined message to any client that connects to it. The first block is where we include all the necessary libraries and define essential constants.
 <code c>
@@ Line 254: / Line 254: @@
 Note, don't forget that all of the code after the first two blocks belongs in main( ), and that we must return 0 at the end of main( ).
+Here is what it looks like when we launch the server:
+{{ dylanw:tcp_server_launch.jpg?500 }}
+\\
+And here is what it looks like when something connects to the server:
+{{ dylanw:tcp_server_connect.jpg?500 }}
 ==== Client ====
-Now, let's go over the client. This program is much simpler, as it only need to handle one connection during its run. We start out with the includes and the constant declarations, just as with the server.
+Now, let's go over the client. The client is a program that connects to server port at a given IP address, and receives the message that the server sends. This program is much simpler, as it only need to handle one connection during its run. We start out with the includes and the constant declarations, just as with the server.
 <code c>
@@ Line 373: / Line 381: @@
 Just as with the server, we must ensure that everything after the first two blocks is in the main( ) function, and that we return 0 at the end.
+Here is what it looks like when the client is launched, and receives a message:
+{{ dylanw:tcp_client_launch.jpg?700 }}
 ===== UDP Talker & Listener =====
+Now that we have established the TCP server & client, we can move onto the UDP talker & listener. The code for these is fairly similar, with only some minor changes.
+The code for the UDP Talker & Listener can be found below:
+[[https://github.com/D-Wazzle/Simple-C-Communications/tree/master/UDP|UDP Talker & listener]]\\
+==== Listener ====
+This listener has very similar style to the TCP server we created. The listener is a program that binds to a predefined port, and listens to any messages received on that port, like a "reverse server". As usual it starts with the includes and constant declarations.
+<code c>
+#include <stdio.h>      // Standard Input/Output library
+#include <stdlib.h>     // Standard library
+#include <unistd.h>     // Unix API
+#include <errno.h>      // Error library
+#include <string.h>     // String library
+#include <sys/types.h>  // System types library
+#include <sys/socket.h> // Sockets library
+#include <netinet/in.h> // Main internet library
+#include <arpa/inet.h>  // The Internet library
+#include <netdb.h>      // Internet database
+#define MYPORT "4812"   // The port users will be connecting to
+#define MAXBUFLEN 100   // The maximum receiving buffer
+</code>
+And again, the get_in_addr function.
+<code c>
+// Function to get sockaddr, IPv4 or IPv6
+void *get_in_addr(struct sockaddr *sa)
+{
+    if (sa->sa_family == AF_INET) { // If IPv4
+        return &(((struct sockaddr_in*)sa)->sin_addr); // Return IPv4
+    }
+    return &(((struct sockaddr_in6*)sa)->sin6_addr); // Return IPv6
+}
+</code>
+Now, we move onto the main( ) function. We start out with the usual data declarations, fill out IP address structures, and getaddrinfo for our IP address.
+<code c>
+    int sockfd; // Socket descriptor
+    struct addrinfo hints, *servinfo, *p; // Used to store IP address info
+    int rv; // Used for error checking
+    int numbytes; // Number of bytes received
+    struct sockaddr_storage their_addr; // Store the talker IP address
+    char buf[MAXBUFLEN]; // Receiving buffer
+    socklen_t addr_len; // IP address length
+    char s[INET6_ADDRSTRLEN]; // String to store IP address
+    memset(&hints, 0, sizeof hints); // Fill structure with empty info
+    hints.ai_family = AF_UNSPEC; // Don't care if IPv4 or IPv6 (set to AF_INET to force IPv4)
+    hints.ai_socktype = SOCK_DGRAM; // UDP type
+    hints.ai_flags = AI_PASSIVE; // Use my IP to listen to the socket
+    if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) { // If getaddrinfo fails
+        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv)); // Give getaddrinfo error
+        return 1;
+    }
+</code>
+Then we use a for loop to search through the servinfo linked-list, until we find a socket that is created and bound successfully.
+<code c>
+    // Loop through all the results and bind to the first we can
+    for(p = servinfo; p != NULL; p = p->ai_next) {
+        if ((sockfd = socket(p->ai_family, p->ai_socktype,
+                p->ai_protocol)) == -1) { // If socket creation error
+            perror("listener: socket"); // Give socket error
+            continue;
+        }
+        if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) { // If bind fails
+            close(sockfd); // Close the socket
+            perror("listener: bind"); // Give bind error
+            continue;
+        }
+        break;
+    }
+</code>
+Now, we check to see if anything was actually created and bound, and if so, free up the structure and print the waiting message.
+<code c>
+    if (p == NULL) { // If nothing ever bound to the socket
+        fprintf(stderr, "listener: failed to bind socket\n"); // Give failure to bind message
+        return 2;
+    }
+    freeaddrinfo(servinfo); // Free the structure
+    printf("listener: waiting to recvfrom...\n"); // Give waiting to receive message
+</code>
+Finally, we attempt to receive a message, waiting until one is received, and then print where the message was received from, how many bytes were sent, and what the message was. Finally, we close the socket.
+<code c>
+    addr_len = sizeof their_addr;
+    if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0,
+        (struct sockaddr *)&their_addr, &addr_len)) == -1) { // If receiving error
+        perror("recvfrom"); // Give receive error
+        exit(1);
+    }
+    printf("listener: got packet from %s\n",
+        inet_ntop(their_addr.ss_family,
+            get_in_addr((struct sockaddr *)&their_addr),
+            s, sizeof s)); // Print received message and converted IP address
+    printf("listener: packet is %d bytes long\n", numbytes); // Give number of bytes in the packet
+    buf[numbytes] = '\0'; // Add null-terminator
+    printf("listener: packet contains \"%s\"\n", buf); // Print packet contents
+    close(sockfd); // Close the socket
+</code>
+As with the others, make sure to include all of this in main( ), and return 0 at the end.
+Here is what it looks like when the listener is launched:
+{{ dylanw:udp_listener_launch.jpg?500 }}
+\\
+And here is what it looks like when the listener receives a message:
+{{ dylanw:udp_listener_receive.jpg?500 }}
+==== Talker ====
+The talker is probably the simplest program of all of these, with just 65 lines of code. The talker is a program that send  given message to a predefined port on the given IP address. It doesn't assume anything is listening, although it is useless without anything listening. As usual, we start with includes and constant declarations.
+<code c>
+#include <stdio.h>      // Standard Input/Output library
+#include <stdlib.h>     // Standard library
+#include <unistd.h>     // Unix API
+#include <errno.h>      // Error library
+#include <string.h>     // String library
+#include <sys/types.h>  // System types library
+#include <sys/socket.h> // Sockets library
+#include <netinet/in.h> // Main internet library
+#include <arpa/inet.h>  // The Internet library
+#include <netdb.h>      // Internet database
+#define SERVERPORT "4812" // The port users will be connecting to
+</code>
+Unlike all of the other programs, no need to define a get_in_addr function, because we are simply sending a UDP packet to the given IP address at the defined port.
+So we move onto the main( ) function, with command line arguments. We start off with the usual: declaring structures, checking argument count, filling out the IP address structure, and calling getaddrinfo for the given IP address.
+<code c>
+    int sockfd; // Socket descriptor
+    struct addrinfo hints, *servinfo, *p; // Structs to store IP address info
+    int rv; // Error checking
+    int numbytes; // Number of bytes to send
+    if (argc != 3) { // Check command line arguments
+        fprintf(stderr,"usage: talker hostname message\n"); // Print usage if needed
+        exit(1);
+    }
+    memset(&hints, 0, sizeof hints); // Fill out empty structure
+    hints.ai_family = AF_UNSPEC; // We don't care if IPv4 or IPv6 (set to AF_INET to force IPv4)
+    hints.ai_socktype = SOCK_DGRAM; // UDP type
+    if ((rv = getaddrinfo(argv[1], SERVERPORT, &hints, &servinfo)) != 0) { // If getaddrinfo fails
+        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv)); // Give getaddrinfo error
+        return 1;
+    }
+</code>
+Next, we loop through the servinfo linked-list trying to create a socket, and then check if we actually created one.
+<code c>
+    // Loop through all the results and make a socket
+    for(p = servinfo; p != NULL; p = p->ai_next) {
+        if ((sockfd = socket(p->ai_family, p->ai_socktype,
+                p->ai_protocol)) == -1) { // If socket create fails
+            perror("talker: socket"); // Give socket error
+            continue;
+        }
+        break;
+    }
+    if (p == NULL) { // Check if anything actually ever was created
+        fprintf(stderr, "talker: failed to create socket\n"); // Give failure to create message
+        return 2;
+    }
+</code>
+Finally, we attempt to send the message packet to the IP address given and the port defined, free the IP address structure, print the message detailing the number of bytes sent, and finally close the socket.
+<code c>
+    if ((numbytes = sendto(sockfd, argv[2], strlen(argv[2]), 0,
+             p->ai_addr, p->ai_addrlen)) == -1) { // If sending error
+        perror("talker: sendto"); // Give sending error
+        exit(1);
+    }
+    freeaddrinfo(servinfo); // Free the IP address structure
+    printf("talker: sent %d bytes to %s\n", numbytes, argv[1]); // Print number of bytes sent to the IP address
+    close(sockfd); // Close the socket
+</code>
+As usual, make sure that the code besides the first block is within main( ), with Cl arguments, and you return 0 at the end.
+Here is what it looks like when the talker is launched, and sends a message:
+{{ dylanw:udp_talker_launch.jpg?700 }}
+This is basically it. Once you understand the basic network calls, and the looping structure, then it should be very easy to implement this yourself. Try out the program given yourself, and then try writing your own, and see if you can make it more efficient, or unique in your own way. Again, this is just the basics, and more difficult concepts will be covered in the future.
 ===== Final Words =====