gcc: wrong behaviour after new variable was added - c

Question

I am writing simple programs: server and client. You know, I am just learning all these stuff.
I added new variable (fileUp in server.c) and the client just crashed. I debugged it with gdb. The client can't read anything from the socket. Without that one variable works fine.
I did compile these programs with both gcc and g++ with -Wall. No errors, no warnings.
Programs are as simple as they can be. I don't understand what is wrong.
Any hint'll be appreciated.
server.c
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char **argv) {
struct sockaddr_in address, client;
int s = socket(AF_INET, SOCK_STREAM, 0);
memset(&address, 0, sizeof(address));
address.sin_family = AF_INET;
address.sin_addr.s_addr = htonl(INADDR_ANY);
#define PORT 54321
address.sin_port = htons(PORT);
if(bind(s, (struct sockaddr *)&address, sizeof(address))<0) {
perror("nie udał się bind");
exit(-1);
}
if(listen(s, 5)<0) {
perror("nie udał się listen");
exit(-1);
}
socklen_t client_len;
int c = accept(s, (struct sockaddr *)&client, &client_len);
int file = open("../data", O_RDONLY);
if(file<0) {
perror("nie udało się otworzyć pliku");
exit(-1);
}
#define MAX 1024
char buf[MAX];
int n = read(file, buf, MAX);
int fileUp = n;
do {
write(c, buf, MAX);
buf[n-1] = '\0';
printf("%d: %s\n", n, buf);
/*fileUp += n;
printf("pobrano: %d\n", fileUp);*/
n = read(file, buf, MAX);
getchar();
} while(n != 0);
close(c);
close(s);
return 0;
}
client.c
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char **argv) {
struct sockaddr_in address;
int s = socket(PF_INET, SOCK_STREAM, 0);
memset(&address, 0, sizeof(address));
address.sin_family = AF_INET;
#define PORT 54321
address.sin_port = htons(PORT);
if(inet_pton(AF_INET, argv[1], &address.sin_addr) <=0) {
perror("podano nieprawidłowy adres");
exit(-1);
}
if(connect(s, (struct sockaddr *)&address, sizeof(address))<0) {
perror("nie można się połączyć");
exit(-1);
}
#define MAX 1024
char buf[MAX];
int n = read(s, buf, MAX);
int fileDown = n;
do {
buf[n-1] = '\0';
printf("%d: %s\n", n, buf);
n = read(s, buf, MAX);
fileDown += n;
printf("pobrano: %d\n", fileDown);
} while(n != 0);
close(s);
return 0;
}

Answer 1

socklen_t client_len; should be socklen_t client_len = sizeof(client);
The stack layout will change when you add your new variable - so the uninitialized value in client_len just happened to work before, it doesn't after - most likely making your accept call fail, and then you're trying to write to an invalid FD.
You should of course also check the return value of accept