Error in recvfrom in C socket programming? - c

Question

Sorry for my not perfet english
I have to implement reliable communication using UDP protocol; for start, i'm tring realize a simple program; a client send a message to server with NULL in buffer; the servers understands it as a request, then sends to client response, which is a number;
code client:
/*
* newClient.c
*
* Created on: 22 lug 2017
* Author: claudio
*/
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SERV_PORT 5193
#define MAXLINE 1024
void err_exit(char* str)
{
perror(str);
exit(EXIT_FAILURE);
}
int request_to_server(int sockfd,int* x,struct sockaddr_in addr)
{
int n;
if(sendto(sockfd, NULL, 0, 0, (struct sockaddr *) &addr, sizeof(addr)) < 0)
err_exit("sendto\n");
n = recvfrom(sockfd,(char*)x,sizeof(int),0,NULL,NULL);
if (n < 0) {
perror("errore in recvfrom");
exit(1);
}
if(n > 0) {
printf("client received: %d\n",*(int*)x);
}
return 1;
}
int main(int argc, char *argv[ ]) {
int sockfd;
struct sockaddr_in servaddr;
int x;
if (argc != 2) {
fprintf(stderr, "utilizzo: daytime_clientUDP <indirizzo IP server>\n");
exit(1);
}
if ((sockfd = socket(PF_INET, SOCK_DGRAM, 0)) < 0) { /* crea il socket */
perror("errore in socket");
exit(1);
}
memset((void *)&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(SERV_PORT);
if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) <= 0) {
err_exit("error in inet_pton for %s");
}
if(request_to_server(sockfd,&x,servaddr))
printf("client received %d from server\n",x);
exit(EXIT_SUCCESS);
}
this is server code:
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#define SERV_PORT 5193
#define MAXLINE 1024
void err_exit(char* str)
{
perror(str);
exit(EXIT_FAILURE);
}
int generate_casual()
{
int x = random()%1000 + 1; //number between 1 and 1000
return x;
}
void listen_request(int sockfd,char* buff,struct sockaddr_in* addr,socklen_t* len)
{
struct sockaddr_in servaddr = *addr;
socklen_t l = *len;
printf("listening request\n");
if ( (recvfrom(sockfd, buff, MAXLINE, 0, (struct sockaddr *)&servaddr, &l)) < 0){
printf("errno code: %d\n",errno);
err_exit("recvfrom\n");
}
*addr = servaddr;
*len = l;
return;
}
int main(int argc, char **argv)
{
(void) argc;
(void) argv;
int sockfd;
socklen_t len;
struct sockaddr_in addr;
char buff[MAXLINE];
srand(time(NULL));
memset((void *)&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(SERV_PORT); /* numero di porta del server */
if ((sockfd = socket(PF_INET, SOCK_DGRAM, 0)) < 0)
err_exit("errore in socket");
if (bind(sockfd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
perror("errore in bind");
exit(1);
}
listen_request(sockfd,buff,&addr,&len);
int n_ack = generate_casual();
//char* p =(char*)&n_ack;
if (sendto(sockfd, (char*)&n_ack, sizeof(int), 0, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
err_exit("sendto");
}
printf("server: client is connected\n");
return 0;
}
when i run, sometimes client receives correctly number; but sometimes i have an error on recvfrom with msg "Invalid argument" ed errno code is 22;
why this?I have no idea in which case it runs and when it doesn't work, code is the same..