Infinite waiting during recvfrom() UDP server and client - c

Question

I'm trying to learn UDP on C.
My goal is to send a message in console, in the client consol to the server, and sending the exact same message from the server to the client.
When I send a message from client to server, I do receive it, but the opposit is not working, and the waiting is infinite.
I don't have any problem in console.
The weird thing is that i'm using the exact same methode to send both messages.
Here's my client, Thanks to everyone helping me :)
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#define PORT 8080
#define BUFSIZE 4096
// Driver code
int main(int argc, char *argv[]) {
char buffer[BUFSIZE];
struct sockaddr_in servaddr, cliaddr;
int len, n;
char *adresse= argv[1];
if (argc<1){
perror("nb args not enough");
exit(EXIT_FAILURE);
}
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = inet_addr(adresse);
servaddr.sin_port = htons(PORT);
// Socket()
int udp_server = socket(AF_INET, SOCK_DGRAM, 0) ;
if(udp_server<0){
perror("socket creation failed \n");
exit(EXIT_FAILURE);
}
// Bind())
int errBind = bind(udp_server, (const struct sockaddr *)&servaddr, sizeof(servaddr));
if(errBind<0){
perror("errBind \n" );
exit(EXIT_FAILURE);
}
// recvfrom()
n = recvfrom(udp_server, (char *)buffer, BUFSIZE,0, ( struct sockaddr *) &cliaddr,&len);
if (n<0){
perror("recvfrom error\n" );
exit(EXIT_FAILURE);
}
buffer[n] = '\0';
printf("CLIENT: %s\n", buffer);
char *message ="caca";
sendto(udp_server, (const char *)message, strlen(message),
0, (const struct sockaddr *) &cliaddr,
len);
close(udp_server);
//////////////////////////This part below is never reached
printf("end");
return 0;
}
Also here's my server:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#define PORT 8080
#define BUFSIZE 4096
// Driver code
int main(int argc, char *argv[]) {
char buffer[BUFSIZE];
// char *message = "AAAAAAAAAAAAAAAAAA";
struct sockaddr_in servaddr, cliaddr;
int len, n;
char *adresse= argv[1];
if (argc<1){
perror("nombre d'arguments insuffisents, veuillez entrer :\n ./serveur port_serveur \n");
exit(EXIT_FAILURE);
}
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = inet_addr(adresse);
// servaddr.sin_addr.s_addr = INADDR_ANY;
servaddr.sin_port = htons(PORT);
// Socket()
int udp_server = socket(AF_INET, SOCK_DGRAM, 0) ;
if(udp_server<0){
perror("socket creation failed \n");
exit(EXIT_FAILURE);
}
// Bind())
int errBind = bind(udp_server, (const struct sockaddr *)&servaddr, sizeof(servaddr));
if(errBind<0){
perror("errBind \n" );
exit(EXIT_FAILURE);
}
// recvfrom()
n = recvfrom(udp_server, (char *)buffer, BUFSIZE,0, ( struct sockaddr *) &cliaddr,&len);
if (n<0){
perror("recvfrom error\n" );
exit(EXIT_FAILURE);
}
buffer[n] = '\0';
printf("CLIENT: %s\n", buffer);
int l = sendto(udp_server, (const char *)buffer, strlen(buffer),
0, (const struct sockaddr *) &servaddr,
sizeof(servaddr));
close(udp_server);
printf("end");
//////////////// This is reached
return 0;
}

Answer 1

There are several issues. The main issue is that both the client and server first tries to receive, then send. Thus they cannot both receive the message from the other party. Also, the client need not explicitly bind the socket.
In principle, the client must know the server address, but the server does not know the client address. Hence, the flow of communication must be that the client first sends a message to the server which will make the client known to the server. The server can then send a message to the address of the client.
In summary, the server should:
Create socket
Bind socket
Wait for message (recvfrom())
Send message to sender of the received message (sendto())
Close socket
The client should:
Create socket
Send message to the server (sendto())
Wait for message (recvfrom())
Close socket