udp client server program c - c

Question

I am writing a simple client and server program in C. I am able to send date from client to server. But, I am not able to send acknowledge from server to client.
/*******************udpserver.c*****************/
int main(int argc, char *argv[])
{
/* Variable and structure definitions. */
int sd, rc;
struct sockaddr_in serveraddr, clientaddr;
clientaddrlen = sizeof(clientaddr);
int serveraddrlen = sizeof(serveraddr);
char buffer[100];
char *bufptr = buffer;
int buflen = sizeof(buffer);
if((sd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
perror("UDP server - socket() error");
exit(-1);
}
printf("UDP server - socket() is OK\n");
memset(&serveraddr, 0x00, serveraddrlen);
serveraddr.sin_family = AF_INET;
serveraddr.sin_port = htons(0);
serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
if((rc = bind(sd, (struct sockaddr *)&serveraddr, serveraddrlen)) < 0) {
perror("UDP server - bind() error");
close(sd);
exit(-1);
}
int addr_len = sizeof(serveraddr);
if (getsockname(sd, (struct sockaddr *) &serveraddr, &addr_len)<0) {
perror("Error getting socket name.\n");
return -1;
}
printf("Using IP %s and port %d\n", inet_ntoa(serveraddr.sin_addr), ntohs(serveraddr.sin_port));
printf("UDP server - Listening...\n");
rc = recvfrom(sd, bufptr, buflen, 0, (struct sockaddr *)&clientaddr, &clientaddrlen);
if(rc < 0) {
perror("UDP Server - recvfrom() error");
close(sd);
exit(-1);
}
printf("UDP Server received the following:\n \"%s\" message\n", bufptr);
printf("UDP Server replying to the UDP client...\n");
rc = sendto(sd, bufptr, buflen, 0, (struct sockaddr *)&clientaddr, clientaddrlen);
if(rc < 0) {
perror("UDP server - sendto() error");
close(sd);
exit(-1);
}
printf("UDP Server - sendto() is OK...\n");
close(sd);
exit(0);
}
My UDPClient program:
/****************udpclient.c********************/
int main(int argc, char *argv[])
{
/* Variable and structure definitions. */
int sd, rc;
struct sockaddr_in serveraddr, clientaddr;
int serveraddrlen = sizeof(serveraddr);
char server[255];
char buffer[100];
char *bufptr = buffer;
int buflen = sizeof(buffer);
struct hostent *hostp;
memset(buffer, 0x00, sizeof(buffer));
/* 36 characters + terminating NULL */
memcpy(buffer, "Hello! A client request message lol!", 37);
if((sd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
perror("UDP Client - socket() error");
exit(-1);
}
else
printf("UDP Client - socket() is OK!\n");
if(argc != 3) {
/*Use default hostname or IP*/
printf("UDP Client - Usage <Server hostname or IP>\n");
exit(0);
}
memset(&serveraddr, 0x00, sizeof(struct sockaddr_in));
serveraddr.sin_family = AF_INET;
serveraddr.sin_port = htons(atoi(argv[2]));
hostp = gethostbyname(argv[1]);
if(hostp == (struct hostent *)NULL) {
printf("HOST NOT FOUND --> ");
printf("h_errno = %d\n", h_errno);
exit(-1);
}
else {
printf("UDP Client - gethostname() of the server is OK... \n");
printf("Connected to UDP server\n");
}
memcpy(&serveraddr.sin_addr, hostp->h_addr, sizeof(serveraddr.sin_addr));
rc = sendto(sd, bufptr, buflen, 0, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
if(rc < 0) {
perror("UDP Client - sendto() error");
close(sd);
exit(-1);
}
else
printf("UDP Client - sendto() is OK!\n");
printf("Waiting a reply from UDP server...\n");
rc = recvfrom(sd, bufptr, buflen, 0, (struct sockaddr *)&serveraddr, &serveraddrlen);
if(rc < 0) {
perror("UDP Client - recvfrom() error");
close(sd);
exit(-1);
} else {
printf("UDP client received the following: \"%s\" message\n", bufptr);
}
close(sd);
exit(0);
}
When running the two programs, I am getting the following output:
UdpServer:
$ ./UdpServer
UDP server - socket() is OK
Using IP 0.0.0.0 and port 49932
UDP server - Listening...
UDP Server received the following:
"Hello! A client request message lol!" message
UDP Server replying to the UDP client...
UDP Server - sendto() is OK...
UdpClient:
$ ./UdpClient MyPC 49932
UDP Client - socket() is OK!
UDP Client - gethostname() of the server is OK...
Connected to UDP server
UDP Client - sendto() is OK!
Waiting a reply from UDP server...
UdpClient program is stuck at this point. Could anyone please explain what the problem is?

Answer 1

You might like to use select() to make the process wait until data is avaibale for reading:
...
{
fd_set rfds;
int retval;
FD_ZERO(&rfds);
FD_SET(sd, &rfds);
retval = select(sd+1, &rfds, NULL, NULL, NULL);
if (retval == -1)
perror("select()");
else
printf("Data is available for reading now.\n");
...
}
...

Answer 2

If the server and client are running on the same machine, give
$ ./UdpClient localhost 49932
instead of
$ ./UdpClient MyPC 49932
else
$ ./UdpClient <server-IP-address> 49932
Also in the server code,
clientaddrlen = sizeof(clientaddr);
should be
int clientaddrlen = sizeof(clientaddr);
But I guess that's just a copy-paste mistake.