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======Exercise 45: A Simple TCP/IP Client======

   I'm going to use the RingBuffer to create a very simplistic little
   network testing tool called netclient. To do this I have to add some
   stuff to the Makefile to handle little programs in the bin/ directory.

======Augment The Makefile======

   First, add a variable for the programs just like the unit tests TESTS
   and TEST_SRC variables:
======PROGRAMS_SRC=$(wildcard bin/*.c)======
======PROGRAMS=$(patsubst %.c,%,$(PROGRAMS_SRC))======

   Then you want to add the PROGRAMS to the all target:
all: $(TARGET) $(SO_TARGET) tests $(PROGRAMS)

   Then add PROGRAMS to the rm line in the clean target:
rm -rf build $(OBJECTS) $(TESTS) $(PROGRAMS)

   Finally you just need a target at the end to build them all:
$(PROGRAMS): CFLAGS += $(TARGET)

   With these changes you can drop simple .c files into bin and make will
   build them and link them to the library just like the tests are done.

======The netclient Code======

   The code for the little netclient looks like this:
#undef NDEBUG
#include <stdlib.h>
#include <sys/select.h>
#include <stdio.h>
#include <lcthw/ringbuffer.h>
#include <lcthw/dbg.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>
#include <fcntl.h>

struct tagbstring NL = bsStatic("\n");
struct tagbstring CRLF = bsStatic("\r\n");

int nonblock(int fd) {
    int flags = fcntl(fd, F_GETFL, 0);
    check(flags >= 0, "Invalid flags on nonblock.");

    int rc = fcntl(fd, F_SETFL, flags | O_NONBLOCK);
    check(rc == 0, "Can't set nonblocking.");

    return 0;
error:
    return -1;
}

int client_connect(char *host, char *port)
{
    int rc = 0;
    struct addrinfo *addr = NULL;

    rc = getaddrinfo(host, port, NULL, &addr);
    check(rc == 0, "Failed to lookup %s:%s", host, port);

    int sock = socket(AF_INET, SOCK_STREAM, 0);
    check(sock >= 0, "Cannot create a socket.");

    rc = connect(sock, addr->ai_addr, addr->ai_addrlen);
    check(rc == 0, "Connect failed.");

    rc = nonblock(sock);
    check(rc == 0, "Can't set nonblocking.");

    freeaddrinfo(addr);
    return sock;

error:
    freeaddrinfo(addr);
    return -1;
}

int read_some(RingBuffer *buffer, int fd, int is_socket)
{
    int rc = 0;

    if(RingBuffer_available_data(buffer) == 0) {
        buffer->start = buffer->end = 0;
    }

    if(is_socket) {
        rc = recv(fd, RingBuffer_starts_at(buffer), RingBuffer_available_space(b
uffer), 0);
    } else {
        rc = read(fd, RingBuffer_starts_at(buffer), RingBuffer_available_space(b
uffer));
    }

    check(rc >= 0, "Failed to read from fd: %d", fd);

    RingBuffer_commit_write(buffer, rc);

    return rc;

error:
    return -1;
}


int write_some(RingBuffer *buffer, int fd, int is_socket)
{
    int rc = 0;
    bstring data = RingBuffer_get_all(buffer);

    check(data != NULL, "Failed to get from the buffer.");
    check(bfindreplace(data, &NL, &CRLF, 0) == BSTR_OK, "Failed to replace NL.")
;

    if(is_socket) {
        rc = send(fd, bdata(data), blength(data), 0);
    } else {
        rc = write(fd, bdata(data), blength(data));
    }

    check(rc == blength(data), "Failed to write everything to fd: %d.", fd);
    bdestroy(data);

    return rc;

error:
    return -1;
}


int main(int argc, char *argv[])
{
    fd_set allreads;
    fd_set readmask;

    int socket = 0;
    int rc = 0;
    RingBuffer *in_rb = RingBuffer_create(1024 * 10);
    RingBuffer *sock_rb = RingBuffer_create(1024 * 10);

    check(argc == 3, "USAGE: netclient host port");

    socket = client_connect(argv[1], argv[2]);
    check(socket >= 0, "connect to %s:%s failed.", argv[1], argv[2]);

    FD_ZERO(&allreads);
    FD_SET(socket, &allreads);
    FD_SET(0, &allreads);

    while(1) {
        readmask = allreads;
        rc = select(socket + 1, &readmask, NULL, NULL, NULL);
        check(rc >= 0, "select failed.");

        if(FD_ISSET(0, &readmask)) {
            rc = read_some(in_rb, 0, 0);
            check_debug(rc != -1, "Failed to read from stdin.");
        }

        if(FD_ISSET(socket, &readmask)) {
            rc = read_some(sock_rb, socket, 0);
            check_debug(rc != -1, "Failed to read from socket.");
        }

        while(!RingBuffer_empty(sock_rb)) {
            rc = write_some(sock_rb, 1, 0);
            check_debug(rc != -1, "Failed to write to stdout.");
        }

        while(!RingBuffer_empty(in_rb)) {
            rc = write_some(in_rb, socket, 1);
            check_debug(rc != -1, "Failed to write to socket.");
        }
    }

    return 0;

error:
    return -1;
}

   This code uses select to handle events from both stdin (file descriptor
   0) and from the socket it uses to talk to a server. It uses RingBuffers
   to store the data and copy it around, and you can consider the
   functions read_some and write_some early prototypes for similar
   functions in the RingBuffer library.

   In this little bit of code are quite a few networking functions you may
   not know. As you hit a function you don't know, look it up in the man
   pages and make sure you understand it. This one little file could
   actually get you to research all the APIs required to write a little
   server in C.

======What You Should See======

   If you have everything building then the quickest way to test it is see
   if you can get a special file off learncodethehardway.org:
$
$ ./bin/netclient learncodethehardway.org 80
======GET /ex45.txt HTTP/1.1======
======Host: learncodethehardway.org======

======HTTP/1.1 200 OK======
======Date: Fri, 27 Apr 2012 00:41:25 GMT======
======Content-Type: text/plain======
======Content-Length: 41======
======Last-Modified: Fri, 27 Apr 2012 00:42:11 GMT======
======ETag: 4f99eb63-29======
======Server: Mongrel2/1.7.5======

======Learn C The Hard Way, Exercise 45 works.======
^C
$

   What I did there is I type in the syntax needed to make the HTTP
   request for the file /ex45.txt, then the Host: header line, then hit
   ENTER to get an empty line. I then get the response, with headers and
   the content. After that I just hit CTRL-c to exit.

======How To Break It======

   This code definitely could have bugs, and currently in the draft of the
   book I'm going to have to keep working on this. In the meantime, try
   analyzing the code I have here and thrashing it against other servers.
   There's a tool called netcat that is great for setting up these kinds
   of servers. Another thing to do is use a language like Python or Ruby
   to create a simple "junk server" that spews out junk and bad data,
   closes connections randomly, and other nasty things.

   If you find bugs, report them in the comments and I'll fix them up.

======Extra Credit======

     * As I mentioned, there's quite a few functions you may not know, so
       look them up. In fact, look them all up even if you think you know
       them.
     * Run this under valgrind and look for errors.
     * Go back through and add various defensive programming checks to the
       functions to improve them.
     * Use the getopt function to allow the user to give this the option
       to not translate \n to \r\n. This is only needed on protocols that
       require it for line endings, like HTTP. Sometimes you don't want
       the translation, so give the user an option.


   Copyright (C) 2010 Zed. A. Shaw

   Credits