CPP SMTP Client (See related posts)

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

#define HELO "HELO\n"
#define DATA "DATA\n"
#define QUIT "QUIT\n"

FILE *fin;
int sock;
struct sockaddr_in server;
struct hostent *hp, *gethostbyname();
char buf[BUFSIZ+1];
int len;
char *host_id;
char *from_id;
char *to_id;
char *file_id;
char wkstr[100];

/*=====Send a string to the socket=====*/

send_socket(char *s)
{
	write(sock,s,strlen(s));
	write(1,s,strlen(s));
}

/*=====Read a string from the socket=====*/

read_socket()
{
	len = read(sock,buf,BUFSIZ);
	write(1,buf,len);
}

/*=====MAIN=====*/
int main(int argc, char* argv[])
{

if(argc != 5)
{
 printf("USAGE: %s <host> <from> <to> <filename>\n\n", argv[0]);
 exit(1);
}

host_id=argv[1];
from_id=argv[2];
to_id=argv[3];
file_id=argv[4];

/*=====Create Socket=====*/
sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock==-1)
{
 perror("opening stream socket");
 exit(1);
}

/*=====Verify host=====*/
server.sin_family = AF_INET;
hp = gethostbyname(host_id);
if (hp==(struct hostent *) 0)
{
 fprintf(stderr, "%s: unknown host\n", host_id);
 exit(2);
}

/*=====Connect to port 25 on remote host=====*/
printf ("hostent %s\n", hp->h_addr_list[0]);
memcpy((char *) &server.sin_addr, (char *) hp->h_addr, hp->h_length);

server.sin_port=htons(25); /* SMTP PORT */

if (connect(sock, (struct sockaddr *) &server, sizeof server)==-1)
{
 perror("connecting stream socket");
 exit(1);
}

/*=====Write some data then read some =====*/

read_socket(); /* SMTP Server logon string */

send_socket(HELO); /* introduce ourselves */
read_socket(); /*Read reply */

send_socket("MAIL from: "); /* Mail from us */
send_socket(from_id);
send_socket("\n");
read_socket(); /* Sender OK */

send_socket("RCPT To: "); /*Mail to*/
send_socket(to_id);
send_socket("\n");
read_socket(); /*Recipient OK*/

send_socket(DATA);/*body to follow*/
read_socket(); /*ok to send */

fin=fopen(file_id, "r"); /* open file */
while(1)
{
 if(fgets(wkstr, 100, fin)==NULL) break; /* exit on EOF */
 send_socket(wkstr);
}
fclose(fin); /* close file */

send_socket(fin); /*send file*/
send_socket(".\n");

read_socket(); /* OK*/
send_socket(QUIT); /* quit */
read_socket(); /* log off */

/*=====Close socket and finish=====*/
close(sock);
exit(0);
}