simple.c

// $ make simple && ./simple &
// $ telnet localhost 6969

#include <netdb.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

unsigned char buf[BUFSIZ];

int handleConn(int conn);

int main(void)
{
	// socket
	int sock = socket(PF_INET, SOCK_STREAM, 0);
	if (sock < 0) {
		perror("socket");
		exit(1);
	}

	// (address)
	struct sockaddr_in name = {
		.sin_family = AF_INET,
		.sin_port = htons(6969),
		.sin_addr = { .s_addr = INADDR_ANY },
	};

	// bind
	if (bind(sock, (const struct sockaddr *)&name, sizeof name) < 0) {
		perror("bind");
		exit(1);
	}

	// listen
	if (listen(sock, 1) < 0) {
		perror("listen");
		exit(1);
	}

	// accept
	for (;;) {
		int conn = accept(sock, 0, 0);
		if (conn < 0) {
			perror("accept");
			exit(1);
		}

		handleConn(conn);
	}

	return 0;
}

int handleConn(int conn)
{
	int nread = 0;
	int nwritten = 0;

	printf("handleConn() conn=%d\n", conn);

	while (nread = read(conn, buf, BUFSIZ), nread > 0) {
		buf[nread] = '\0';
		printf("nread=%d buf=%s\n", nread, buf);

		nwritten = write(STDOUT_FILENO, buf, nread);
		if (nwritten < 0) {
			perror("write");
			return -2;
		}
		nwritten = write(conn, buf, nread);
		if (nwritten < 0) {
			perror("write");
			return -2;
		}
	}

	if (nread < 0) {
		perror("read");
		return -1;
	}

	printf("closing conn=%d\n", conn);
	if (close(conn) < 0) {
		perror("close");
		exit(1);
	}

	return 0;
}