init

Author: biraj21

Makefile

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+cc=gcc
+flags=-Wall -Werror
+src=src
+bin=bin
+
+all: clean $(bin)/server $(bin)/client
+
+clean:
+	rm -f $(bin)/*
+
+$(bin)/server: $(src)/server.c
+	$(cc) $(flags) -o $@ $<
+
+$(bin)/client: $(src)/client.c
+	$(cc) $(flags) -o $@ $<

README.md

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+# TCP Server in C
+
+This is a simple TCP server written from scratch in C. It is a simple server that listens on a port and echos back whatever it receives.
+
+I've followed the tutorial at [Beej's Guide to Network Programming](https://beej.us/guide/bgnet/html/multi/index.html) to gain an understanding of sockets before writing this server.
+
+Note that currently it only supports one client. Will probably learn and implement an event loop kind of thing to support multiple clients.
+
+## How to run
+
+1. Clone the repository
+
+2. Compile client and server programs (don't worry, there's a Makefile to do this for you)
+
+   ```bash
+   make
+   ```
+
+3. Run the server (it will run on port 3000)
+
+   ```bash
+   ./bin/server
+   ```
+
+4. Run the client in another terminal
+
+   ```bash
+   ./bin/client
+   ```
+
+5. Type something in the client terminal and press enter. You should see the server echoing back what you typed.
+
+6. Ctrl+C to stop the server and client.

src/client.c

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+#include <netdb.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+
+#include "./shared.h"
+
+void cleanup(void);
+void handle_sigint(int sig);
+
+struct addrinfo *server_info;
+
+int main(void) {
+  atexit(cleanup);
+  signal(SIGINT, handle_sigint);
+
+  // to store the return value of various function calls for error checking
+  int rv;
+
+  struct addrinfo hints = {0};     // make sure the struct is empty
+  hints.ai_family = AF_UNSPEC;     // don't care whether it's IPv4 or IPv6
+  hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
+  hints.ai_flags = AI_PASSIVE;     // fill in my IP for me
+
+  // NULL to assign the address of my local host to socket structures
+  // because the server is also running locally
+  rv = getaddrinfo(NULL, PORT, &hints, &server_info);
+  if (rv != 0) {
+    fprintf(stderr, "getaddrinfo(): %s\n", gai_strerror(rv));
+    return EXIT_FAILURE;
+  }
+
+  // create a socket
+  int socket_fd = socket(server_info->ai_family, server_info->ai_socktype,
+                         server_info->ai_protocol);
+  if (socket_fd == -1) {
+    perror("socket()");
+    return EXIT_FAILURE;
+  }
+
+  // connect to the server socket as the address specified in server_info
+  rv = connect(socket_fd, server_info->ai_addr, server_info->ai_addrlen);
+  if (rv == -1) {
+    perror("connect()");
+    return EXIT_FAILURE;
+  }
+
+  char received_msg[1024];
+  char *line = NULL;
+  size_t line_cap = 0;
+  ssize_t line_len;
+
+  printf("> ");
+  while ((line_len = getline(&line, &line_cap, stdin)) > 0) {
+    line[line_len - 1] = '\0'; // to ignore the newline character at the end
+
+    // send the message to the server
+    int bytes_sent = send(socket_fd, line, line_len, 0);
+    if (bytes_sent == -1) {
+      perror("send()");
+      return EXIT_FAILURE;
+    }
+
+    // receive the message from the server
+    int bytes_read = recv(socket_fd, received_msg, sizeof received_msg, 0);
+    if (bytes_read == -1) {
+      perror("recv()");
+      return EXIT_FAILURE;
+    } else if (bytes_read == 0) {
+      fputs("recv(): server closed the connection\n", stderr);
+      return EXIT_FAILURE;
+    }
+
+    received_msg[bytes_read] = '\0'; // null-terminate the received message
+
+    printf("server says: %s\n", received_msg);
+    printf("> ");
+  }
+
+  return EXIT_SUCCESS;
+}
+
+void cleanup(void) {
+  freeaddrinfo(server_info); // free the addrinfo linked list
+}
+
+void handle_sigint(int sig) {
+  // call exit manually because atexit() registered the cleanup function
+  exit(EXIT_SUCCESS);
+}

src/server.c

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+#include <arpa/inet.h>
+#include <netdb.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "./shared.h"
+
+// the max number of connections that can wait in the queue
+#define BACKLOG 10
+
+void cleanup(void);
+void handle_sigint(int sig);
+void handle_client_message(int conn_fd);
+
+// to store the address information of the server
+struct addrinfo *server_info;
+
+int main(void) {
+  atexit(cleanup);
+  signal(SIGINT, handle_sigint);
+
+  // to store the return value of various function calls for error checking
+  int rv;
+
+  struct addrinfo hints = {0};     // make sure the struct is empty
+  hints.ai_family = AF_UNSPEC;     // don't care whether it's IPv4 or IPv6
+  hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
+  hints.ai_flags = AI_PASSIVE;     // fill in my IP for me
+
+  // NULL to assign the address of my local host to socket structures
+  rv = getaddrinfo(NULL, PORT, &hints, &server_info);
+  if (rv != 0) {
+    fprintf(stderr, "getaddrinfo(): %s\n", gai_strerror(rv));
+    return EXIT_FAILURE;
+  }
+
+  // create a socket, which apparently is no good by itself because it's not
+  // bound to an address and port number
+  int socket_fd = socket(server_info->ai_family, server_info->ai_socktype,
+                         server_info->ai_protocol);
+  if (socket_fd == -1) {
+    perror("socket()");
+    return EXIT_FAILURE;
+  }
+
+  // lose the "Address already in use" error message. why this happens
+  // in the first place? well even after the server is closed, the port
+  // will still be hanging around for a while, and if you try to restart the
+  // server, you'll get an "Address already in use" error message
+  int yes = 1;
+  if (setsockopt(socket_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof yes) == -1) {
+    perror("setsockopt()");
+    exit(1);
+  }
+
+  // bind the socket to the address and port number specified in server_info
+  rv = bind(socket_fd, server_info->ai_addr, server_info->ai_addrlen);
+  if (rv == -1) {
+    perror("bind()");
+    return EXIT_FAILURE;
+  }
+
+  // time to listen for incoming connections
+  // BACKLOG is the max number of connections that can wait in the queue
+  rv = listen(socket_fd, BACKLOG);
+  if (rv == -1) {
+    perror("listen()");
+    return EXIT_FAILURE;
+  }
+
+  printf("server is listening on port %s...\n", PORT);
+
+  struct sockaddr_storage their_addr;
+  socklen_t addr_size = sizeof their_addr;
+  while (true) {
+    int conn_fd = accept(socket_fd, (struct sockaddr *)&their_addr, &addr_size);
+    if (conn_fd == -1) {
+      perror("accept()");
+      return EXIT_FAILURE;
+    }
+
+    printf("client %d connected\n", conn_fd);
+
+    // since this is a blocking function, we'll only be able to handle one
+    // client at a time
+    handle_client_message(conn_fd);
+  }
+
+  return EXIT_SUCCESS;
+}
+
+/**
+ * this is a blocking fuction that handles the messages from the client
+ */
+void handle_client_message(int conn_fd) {
+  while (true) {
+    char received_msg[1024];
+    int bytes_read = recv(conn_fd, received_msg, sizeof received_msg - 1, 0);
+    if (bytes_read == -1) {
+      perror("handle_client_message(): recv()");
+      return;
+    } else if (bytes_read == 0) {
+      puts("handle_client_message(): Client disconnected");
+      return;
+    }
+
+    received_msg[bytes_read] = '\0';
+
+    printf("client %d says: %s\n", conn_fd, received_msg);
+
+    char reply[1024] = "you said: \0";
+
+    // concat the received message to the reply message
+    strncat(reply, received_msg, sizeof reply - strlen(reply) - 1);
+
+    int bytes_sent = send(conn_fd, reply, strlen(reply), 0);
+    if (bytes_sent == -1) {
+      perror("handle_client_message(): send()");
+      return;
+    }
+  }
+
+  close(conn_fd);
+}
+
+void cleanup(void) {
+  freeaddrinfo(server_info); // free the addrinfo linked list
+}
+
+void handle_sigint(int sig) {
+  // call exit manually because atexit() registered the cleanup function
+  exit(EXIT_SUCCESS);
+}

src/shared.h

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+#ifndef SHARED_H
+#define SHARED_H
+
+// the PORT number that the server will be listening on and string because,
+// well, getaddrinfo() wants a string as the service parameter
+const char *PORT = "3000";
+
+#endif