variable length message over tcp - c

Question

I am trying to send variable length message in TCP but the message length that
is printed is always 0. I am using beej guide code for send and receive and pack and unpack to send the header length.Can someone please point out the mistake.
server code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define MAX_SIZE 50
void error(const char *msg)
{
perror(msg);
exit(1);
}
void packi32(unsigned char *buf, unsigned long i)//from beej guide
{
*buf++ = i>>24; *buf++ = i>>16;
*buf++ = i>>8; *buf++ = i;
}
/* sends all data - thanks to Beej's Guide to Network Programming */
int sendall(int s, char *buf, int *len)
{
int total=0;
int bytesleft=*len;
int n=0;
/* send all the data */
while(total<*len){
/* send some data */
n=send(s,buf+total,bytesleft,0);
/* break on error */
if(n==-1)
break;
/* apply bytes we sent */
total+=n;
bytesleft-=n;
}
/* return number of bytes actually send here */
*len=total;
/* return -1 on failure, 0 on success */
return n==-1?-1:0;
}
int sendus(int s,char *msg)
{
char buf[4];//here 4 is header length
int len = strlen(msg);
packi32(buf,len);
int ll = 4;// ll is header length containing message length
int x;
if((x = sendall(s,buf,&ll))< 0)
printf("value of sent x %d\n",x);
int y = sendall(s,msg,&len);
return y;
}
int main(int argc, char *argv[])
{
int sockfd, newsockfd, portno;
socklen_t clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
if (argc < 2)
{
fprintf(stderr,"ERROR, no port provided\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0)
error("ERROR on binding");
listen(sockfd,5);
clilen = sizeof(cli_addr);
newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
if (newsockfd < 0)
error("ERROR on accept");
char msg[1024] = "hello";
int xx = sendus(sockfd,(char*)msg);
close(newsockfd);
close(sockfd);
return 0;
}
client code
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include<errno.h>
void error(const char *msg)
{
perror(msg);
exit(0);
}
/** unpacki32() -- unpack a 32-bit int from a char buffer (like ntohl())
*/
unsigned long unpacki32(unsigned char *buf)
{
return (buf[0]<<24) | (buf[1]<<16) | (buf[2]<<8) | buf[3];
}
/* receives all data - modelled after sendall() */
int recvall(int s, char *buf, int *len, int timeout){
int total=0;
int bytesleft=*len;
int n=0;
time_t start_time;
time_t current_time;
/* clear the receive buffer */
bzero(buf,*len);
time(&start_time);
/* receive all data */
while(total<*len){
/* receive some data */
n=recv(s,buf+total,bytesleft,0);
/* no data has arrived yet (non-blocking socket) */
if(n==-1 && errno==EAGAIN){
time(&current_time);
if(current_time-start_time>timeout)
break;
sleep(1);
continue;
}
/* receive error or client disconnect */
else if(n<=0)
break;
/* apply bytes we received */
total+=n;
bytesleft-=n;
}
/* return number of bytes actually received here */
*len=total;
/* return <=0 on failure, bytes received on success */
return (n<=0)?n:total;
}
int recvme(int s, char *buf)
{
char len[4];
int l = 4;
int n = recvall(s,len,&l,10);
int msg_len = unpacki32(len);
printf("msg_length :%d",msg_len);
int z ;
z = recvall(s,buf,&msg_len,10);
return z;
}
int main(int argc, char *argv[])
{
int sockfd, portno, n;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[1024];
if (argc < 3)
{
fprintf(stderr,"usage %s hostname port\n", argv[0]);
exit(0);
}
portno = atoi(argv[2]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
server = gethostbyname(argv[1]);
if (server == NULL)
{
fprintf(stderr,"ERROR, no such host\n");
exit(0);
}
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr,
(char *)&serv_addr.sin_addr.s_addr,
server->h_length);
serv_addr.sin_port = htons(portno);
if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0)
error("ERROR connecting");
n = recvme(sockfd, buffer);
if (n < 0)
error("ERROR reading from socket");
printf("%s\n",buffer);
close(sockfd);
return 0;
}

Answer 1

The basic cause for the failure is in the server's line
int xx = sendus(sockfd,(char*)msg);
- sockfd is the listen() socket, but we have to use the newsockfd returned from accept(), i. e.
sendus(newsockfd, msg);