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