Unix is C—a preforking echo server inspired by tomayko.com and jacobian.org’s recent “* is Unix” posts

echo.c

/**
 * A simple preforking echo server in C.
 *
 * Building:
 *
 * $ gcc -Wall -o echo echo.c
 *
 * Usage:
 *
 * $ ./echo
 * 
 *   ~ then in another terminal ... ~
 *
 * $ echo 'Hello, world!' | nc localhost 4242  
 * 
 * Inspiration:
 *  http://tomayko.com/writings/unicorn-is-unix
 *  http://jacobian.org/writing/python-is-unix/
 */

#include <unistd.h> /* fork, close */
#include <stdlib.h> /* exit */
#include <string.h> /* strlen */
#include <stdio.h> /* perror, fdopen, fgets */
#include <sys/socket.h>
#include <sys/wait.h> /* waitpid */
#include <netdb.h> /* getaddrinfo */

#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

#define PORT "4242"
#define NUM_CHILDREN 3

#define MAXLEN 1024

int readline(int fd, char *buf, int maxlen); // forward declaration

int
main(int argc, char** argv)
{
    int i, n, sockfd, clientfd;
    int yes = 1; // used in setsockopt(2)
    struct addrinfo *ai;
    struct sockaddr_in *client;
    socklen_t client_t;
    pid_t cpid; // child pid 
    char line[MAXLEN];
    char cpid_s[32];
    char welcome[32];

    /* Create a socket and get its file descriptor -- socket(2) */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1) {
	die("Couldn't create a socket");
    }

    /* Prevents those dreaded "Address already in use" errors */
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&yes, sizeof(int)) == -1) {
	die("Couldn't setsockopt");
    }

    /* Fill the address info struct (host + port) -- getaddrinfo(3) */
    if (getaddrinfo(NULL, PORT, NULL, &ai) != 0) {
	die("Couldn't get address");
    }

    /* Assign address to this socket's fd */
    if (bind(sockfd, ai->ai_addr, ai->ai_addrlen) != 0) {
	die("Couldn't bind socket to address");
    }

    /* Free the memory used by our address info struct */
    freeaddrinfo(ai);

    /* Mark this socket as able to accept incoming connections */
    if (listen(sockfd, 10) == -1) {
	die("Couldn't make socket listen");
    }

    /* Fork you some child processes. */
    for (i = 0; i < NUM_CHILDREN; i++) {
	cpid = fork();
	if (cpid == -1) {
	    die("Couldn't fork");
	}

	if (cpid == 0) { // We're in the child ...
	    for (;;) { // Run forever ...
		/* Necessary initialization for accept(2) */
		client_t = sizeof client;

		/* Blocks! */
		clientfd = accept(sockfd, (struct sockaddr *)&client, &client_t);
		if (clientfd == -1) {
		    die("Couldn't accept a connection");
		}
		
		/* Send a welcome message/prompt */
		bzero(cpid_s, 32);
		bzero(welcome, 32);
		sprintf(cpid_s, "%d", getpid());
		sprintf(welcome, "Child %s echo> ", cpid_s);
		send(clientfd, welcome, strlen(welcome), 0);

		/* Read a line from the client socket ... */
		n = readline(clientfd, line, MAXLEN);
		if (n == -1) {
		    die("Couldn't read line from connection");
		}
		
		/* ... and echo it back */
		send(clientfd, line, n, 0);

		/* Clean up the client socket */
		close(clientfd);
	    }
	}
    }

    /* Sit back and wait for all child processes to exit */
    while (waitpid(-1, NULL, 0) > 0);

    /* Close up our socket */
    close(sockfd);

    return 0;
}

/**
 * Simple utility function that reads a line from a file descriptor fd, 
 * up to maxlen bytes -- ripped from Unix Network Programming, Stevens.
 */
int
readline(int fd, char *buf, int maxlen)
{
    int n, rc;
    char c;

    for (n = 1; n < maxlen; n++) {
	if ((rc = read(fd, &c, 1)) == 1) {
	    *buf++ = c;
	    if (c == '\n')
		break;
	} else if (rc == 0) {
	    if (n == 1)
		return 0; // EOF, no data read
	    else
		break; // EOF, read some data
	} else
	    return -1; // error
    }

    *buf = '\0'; // null-terminate
    return n;
}