The really cool, game-changing thing about the Internet is that it allows us to leverage the true N^2 value of large networks. Specifically, whenever a computer has a persistent Internet connection, it can exchange information with any other Internet-connected device at any time, quickly and easily.<\/p>\n\n\n\n
When writing Internet-aware applications, such communications are often done with TCP or UDP sockets. These virtual connections represent a specific connection from one computer to another, through which data can be transferred.<\/p>\n\n\n\n
What is needed for TCP\/IP (UDP is similar but without verification steps) is a server program (to listen for incoming connections) and a client program (to contact a server with a request). As an easily-modified example, here is a server program in C that accepts a single byte, increments it (allowing rollover), and sends it back.<\/p>\n\n\n\n
\/* server.c : minimal TCP server that receives one uint8,\n applies myFunction() (here: ++ with wrap-around),\n and sends it back. *\/\n\n#include <stdio.h>\n#include <stdlib.h>\n#include <stdint.h>\n#include <unistd.h>\n#include <string.h>\n#include <arpa\/inet.h>\n#include <sys\/socket.h>\n#include <netinet\/in.h>\n\n#define PORT 10000 \/* listen port; change if you like *\/\n\n\/* Your pluggable transform ---------------------------------- *\/\nstatic uint8_t myFunction(uint8_t x) {\n return x + 1; \/* uint8_t overflow wraps naturally *\/\n}\n\/* ----------------------------------------------------------- *\/\n\nint main(void){\n\n int srv_fd = socket(AF_INET, SOCK_STREAM, 0);\n if (srv_fd < 0) { perror(\"socket\"); exit(EXIT_FAILURE); }\n\n \/* allow quick restart (otherwise TIME_WAIT delays) *\/\n int opt = 1;\n setsockopt(srv_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof opt);\n\n struct sockaddr_in addr = {0};\n addr.sin_family = AF_INET;\n addr.sin_addr.s_addr = htonl(INADDR_ANY); \/* 0.0.0.0 *\/\n addr.sin_port = htons(PORT);\n\n if (bind(srv_fd, (struct sockaddr *)&addr, sizeof addr) < 0) {\n perror(\"bind\"); exit(EXIT_FAILURE);\n }\n if (listen(srv_fd, 1) < 0) { perror(\"listen\"); exit(EXIT_FAILURE); }\n\n printf(\"Server listening on port %d \u2026\\n\", PORT);\n\n for (;;) {\n struct sockaddr_in cli_addr;\n socklen_t cli_len = sizeof cli_addr;\n int cfd = accept(srv_fd, (struct sockaddr *)&cli_addr, &cli_len);\n if (cfd < 0) { perror(\"accept\"); continue; }\n\n uint8_t in_byte;\n ssize_t n = recv(cfd, &in_byte, 1, 0);\n if (n == 1) {\n uint8_t out_byte = myFunction(in_byte);\n send(cfd, &out_byte, 1, 0);\n printf(\" %u -> %u from %s\\n\",\n in_byte, out_byte, inet_ntoa(cli_addr.sin_addr));\n }\n close(cfd);\n }\n}\n<\/code><\/pre>\n\n\n\nHere is a client program, which calls the server, sends a byte, and reports what it gets back.
(Change the IP address if the programs are not running on the same machine.)<\/p>\n\n\n\n
\/* client.c : minimal TCP client that sends a uint8\n and prints the transformed result. *\/\n\n#include <stdio.h>\n#include <stdlib.h>\n#include <stdint.h>\n#include <unistd.h>\n#include <arpa\/inet.h>\n#include <sys\/socket.h>\n#include <netinet\/in.h>\n\n#define PORT 10000 \/* must match server *\/\n#define SERVER_IP \"127.0.0.1\" \/* change to LAN IP if needed *\/\n\nint main(int argc, char **argv){\n\n uint8_t value = 42; \/* default test value *\/\n if (argc == 2) value = (uint8_t)atoi(argv[1]);\n\n int fd = socket(AF_INET, SOCK_STREAM, 0);\n if (fd < 0) { perror(\"socket\"); exit(EXIT_FAILURE); }\n\n struct sockaddr_in srv = {0};\n srv.sin_family = AF_INET;\n srv.sin_port = htons(PORT);\n if (inet_pton(AF_INET, SERVER_IP, &srv.sin_addr) != 1) {\n fprintf(stderr, \"Bad server IP\\n\"); exit(EXIT_FAILURE);\n }\n\n if (connect(fd, (struct sockaddr *)&srv, sizeof srv) < 0) {\n perror(\"connect\"); exit(EXIT_FAILURE);\n }\n\n if (send(fd, &value, 1, 0) != 1) { perror(\"send\"); exit(EXIT_FAILURE); }\n\n uint8_t reply;\n if (recv(fd, &reply, 1, 0) != 1) { perror(\"recv\"); exit(EXIT_FAILURE); }\n\n printf(\"Sent %u, got %u\\n\", value, reply);\n\n close(fd);\n return 0;\n}\n<\/code><\/pre>\n\n\n\n(Code provided by GPT-o3.)<\/p>\n","protected":false},"excerpt":{"rendered":"
TCP and UDP sockets can enable Internet-aware programs to communicate directly with each other… Continue reading