Client connects to server randomly - c

Question

I am trying to write a simple client/server program where the server first connects to the client, and sends the client a message. The client then echoes back the message to the server in uppercase.
The thing is, the connection is random; sometimes the client connects and sometimes it doesn't.
EDIT: when it doesn't connect, I get an "Address already in use" error. Is there some way to free the address on the server side?
SERVER.C
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
int main(){
int welcomeSocket, newSocket, portNum, clientLen, nBytes;
char buffer[1024];
struct sockaddr_in serverAddr;
struct sockaddr_storage serverStorage;
socklen_t addr_size;
int counter = 0;
welcomeSocket = socket(PF_INET, SOCK_STREAM, 0);
portNum = 7891;
serverAddr.sin_family = AF_INET;
serverAddr.sin_port = htons(portNum);
serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
memset(serverAddr.sin_zero, '\0', sizeof serverAddr.sin_zero);
bind(welcomeSocket, (struct sockaddr *) &serverAddr, sizeof(serverAddr));
/*---- Listen on the socket, with 5 max connection requests queued ----*/
if(listen(welcomeSocket,5)==0)
printf("Listening\n");
else
printf("Error\n");
/*---- Accept call creates a new socket for the incoming connection ----*/
addr_size = sizeof serverStorage;
while(1){
newSocket = accept(welcomeSocket, (struct sockaddr *) &serverStorage, &addr_size);
/* counter ++ ;*/
/* printf("client connected: %d\n",counter);*/
/*fork a child process to handle the new connection*/
if(!fork()){
/*---- Send message to the socket of the incoming connection ----*/
strcpy(buffer,"Hello World\n");
send(newSocket,buffer,13,0);
recv(newSocket,buffer,13,0);
/*---- Print the received message ----*/
printf("Data received: %s",buffer);
close(newSocket);
exit(0);
}
/*if parent, close the socket and go back to listening new requests*/
else{
close(newSocket);
}
}
return 0;
}
CLIENT.C
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <ctype.h>
int main(){
int clientSocket, portNum, nBytes;
char buffer[1024];
struct sockaddr_in serverAddr;
socklen_t addr_size;
clientSocket = socket(PF_INET, SOCK_STREAM, 0);
portNum = 7891;
serverAddr.sin_family = AF_INET;
serverAddr.sin_port = htons(portNum);
serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
memset(serverAddr.sin_zero, '\0', sizeof serverAddr.sin_zero);
/*---- Connect the socket to the server using the address struct ----*/
addr_size = sizeof serverAddr;
connect(clientSocket, (struct sockaddr *) &serverAddr, addr_size);
/*---- Read the message from the server into the buffer ----*/
nBytes = recv(clientSocket, buffer, 1024, 0);
/*---- Print the received message ----*/
printf("Data received: %s",buffer);
for (int i=0;i<nBytes-1;i++){
buffer[i] = toupper(buffer[i]);
}
send(clientSocket,buffer,nBytes,0);
return 0;
}

Answer 1

The "address already in use" error occurrs on a call to bind when a socket is already bound to that port. In the case of a listening TCP socket, that can happen when the program is restarted due to old connected sockets not being completely closed yet.
When binding a listening socket, you should set the SO_REUSEADDR socket option. This will allow you to bind a TCP listening socket in these situations.
int option = 1;
if (setsockopt(welcomeSocket, SOL_SOCKET, SO_REUSEADDR, &option, sizeof(option)) == -1) {
perror("setsockopt for SO_REUSEADDR failed");
exit(1);
}
This function should be called after socket but before bind.