UDP client /server program to transfer a string in reverse

/* UDP server code  to reverse a string */

#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h> /* close() */
#include <string.h> /* memset() */

#define LOCAL_SERVER_PORT 1500
#define MAX_MSG 100

int main(int argc, char *argv[]) {
int l,x,y;
char swp;
int sd, rc, n, cliLen, flags;
struct sockaddr_in cliAddr, servAddr;
char msg[MAX_MSG];

/* socket creation */
sd=socket(AF_INET, SOCK_DGRAM, 0);
if(sd<0) {
printf("%s: cannot open socket \n",argv[0]);
exit(1);
}

/* bind local server port */
servAddr.sin_family = AF_INET;
servAddr.sin_addr.s_addr = htonl(INADDR_ANY);
servAddr.sin_port = htons(LOCAL_SERVER_PORT);
rc = bind (sd, (struct sockaddr *) &servAddr,sizeof(servAddr));
if(rc<0) {
printf("%s: cannot bind port number %d \n",
argv[0], LOCAL_SERVER_PORT);
exit(1);
}

printf("%s: waiting for data on port UDP %u\n",
argv[0],LOCAL_SERVER_PORT);


flags = 0;


/* server infinite loop */
while(1) {

/* init buffer */
memset(msg,0x0,MAX_MSG);

/* receive message */
cliLen = sizeof(cliAddr);
n = recvfrom(sd, msg, MAX_MSG, flags,
(struct sockaddr *) &cliAddr, &cliLen);

if(n<0) {
printf("%s: cannot receive data \n",argv[0]);
continue;
}

/*reversing string */
l=strlen(msg);

for(x=0,y=l-1;x<l/2;x++,y--)
{
swp=msg[x];
msg[x]=msg[y];
msg[y]=swp;
}
/* print received message */
printf("%s: from %s:UDP%u : %s \n",
argv[0],inet_ntoa(cliAddr.sin_addr),
ntohs(cliAddr.sin_port),msg);


sleep(1);
sendto(sd,msg,n,flags,(struct sockaddr *)&cliAddr,cliLen);


}/* end of server infinite loop */

return 0;

}



/* UDP client code  */


#include <stdlib.h> /* for exit() */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h> /* memset() */
#include <sys/time.h> /* select() */

#define REMOTE_SERVER_PORT 1500
#define MAX_MSG 100


#define SOCKET_ERROR -1

int isReadable(int sd,int * error,int timeOut) { // milliseconds
fd_set socketReadSet;
FD_ZERO(&socketReadSet);
FD_SET(sd,&socketReadSet);
struct timeval tv;
if (timeOut) {
tv.tv_sec = timeOut / 1000;
tv.tv_usec = (timeOut % 1000) * 1000;
} else {
tv.tv_sec = 0;
tv.tv_usec = 0;
} // if
if (select(sd+1,&socketReadSet,0,0,&tv) == SOCKET_ERROR) {
*error = 1;
return 0;
} // if
*error = 0;
return FD_ISSET(sd,&socketReadSet) != 0;
} /* isReadable */



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

int sd, rc, i, n, echoLen, flags, error, timeOut;
struct sockaddr_in cliAddr, remoteServAddr, echoServAddr;
struct hostent *h;
char msg[MAX_MSG];


/* check command line args */
if(argc<3) {
printf("usage : %s <server> <data1> ... <dataN> \n", argv[0]);
exit(1);
}

/* get server IP address (no check if input is IP address or DNS name */
h = gethostbyname(argv[1]);
if(h==NULL) {
printf("%s: unknown host '%s' \n", argv[0], argv[1]);
exit(1);
}

printf("%s: sending data to '%s' (IP : %s) \n", argv[0], h->h_name,
inet_ntoa(*(struct in_addr *)h->h_addr_list[0]));

remoteServAddr.sin_family = h->h_addrtype;
memcpy((char *) &remoteServAddr.sin_addr.s_addr,
h->h_addr_list[0], h->h_length);
remoteServAddr.sin_port = htons(REMOTE_SERVER_PORT);

/* socket creation */
sd = socket(AF_INET,SOCK_DGRAM,0);
if(sd<0) {
printf("%s: cannot open socket \n",argv[0]);
exit(1);
}

/* bind any port */
cliAddr.sin_family = AF_INET;
cliAddr.sin_addr.s_addr = htonl(INADDR_ANY);
cliAddr.sin_port = htons(0);

rc = bind(sd, (struct sockaddr *) &cliAddr, sizeof(cliAddr));
if(rc<0) {
printf("%s: cannot bind port\n", argv[0]);
exit(1);
}


flags = 0;

timeOut = 100; // ms


/* send data */
for(i=2;i<argc;i++) {
rc = sendto(sd, argv[i], strlen(argv[i])+1, flags,
(struct sockaddr *) &remoteServAddr,
sizeof(remoteServAddr));

if(rc<0) {
printf("%s: cannot send data %d \n",argv[0],i-1);
close(sd);
exit(1);
}


/* init buffer */
memset(msg,0x0,MAX_MSG);

while (!isReadable(sd,&error,timeOut)) printf(".");
printf("\n");

/* receive echoed message */
echoLen = sizeof(echoServAddr);
n = recvfrom(sd, msg, MAX_MSG, flags,
(struct sockaddr *) &echoServAddr, &echoLen);

if(n<0) {
printf("%s: cannot receive data \n",argv[0]);
continue;
}

/* print received message */
printf("%s: echo from %s:UDP%u : %s \n",
argv[0],inet_ntoa(echoServAddr.sin_addr),
ntohs(echoServAddr.sin_port),msg);



}

return 1;

}