unexpected multithreaded server reaction - C - c

Question

I'm working on a multithreaded server/client. The problem I have is that the server handling sometimes looks a little bit various. The message, which is send back is always correct, but the message the server prints out is a little bit weird. If it is a short word like "hello" everything works. If it is a long word or there are spaces in the string like "Binominalkoeffizient" the out-printed serversided message is:
Binomina
lkoeffiz
ient
fiz
Any idea where my mistake is?
PS: The server reaction is the same when I use telnet!
Server-Main:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <pthread.h>
#include "server.h"
int main(int argc, const char * argv[]) {
int sock;
struct sockaddr_in server;
sock = socket(AF_INET, SOCK_STREAM, 0);
socketStatusCheck(sock);
puts("[*] Starting Server ...");
puts("[*] Initialize Server ...");
initializeServer(&server, 8888);
bindServerToAddress(sock, server);
puts("[*] Waiting for incomming connections ... ");
puts("");
listen(sock, 3);
connectionSwitch(sock);
close(sock);
return 0;
}
Server-File
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <pthread.h>
#include "server.h"
void socketStatusCheck(int sock) {
if (sock == -1) {
perror("Error creating the socket: ");
exit(0);
}
}
void initializeServer(struct sockaddr_in *server, int port) {
server->sin_family = AF_INET;
server->sin_addr.s_addr = INADDR_ANY;
server->sin_port = htons(port);
}
void bindServerToAddress(int sock, struct sockaddr_in server) {
if (bind(sock, (struct sockaddr*) &server, sizeof(server)) < 0) {
perror("Error binding port: ");
}
}
void connectionSwitch(int sock) {
int nsock, lenbuf;
struct sockaddr_in client;
pthread_t pid = NULL;
lenbuf = sizeof(struct sockaddr_in);
while ((nsock = accept(sock, (struct sockaddr*) &client, (socklen_t*) &lenbuf))) {
puts("Client connected!");
if (pthread_create(&pid, NULL, connectionHandler, (void*) &nsock))
perror("Error creating thread: ");
}
if (nsock < 0) {
perror("Error accepting incomming client: ");
}
pthread_exit(pid);
}
void *connectionHandler(void *sockptr) {
int sock = *(int*) sockptr;
long isConnected;
char *smessage, *recvmessage;
smessage = "Hello! I am the server you just connected! \n";
write(sock, smessage, strlen(smessage));
recvmessage = malloc(5000 * sizeof(char)); // while ((isConnected = recv(sock, recvmessage, sizeof(recvmessage), 0)) > 0)
while ((isConnected = recv(sock, recvmessage, sizeof(recvmessage), 0)) > 0) {
//write(sock, recvmessage, sizeof(recvmessage));
send(sock, recvmessage, sizeof(recvmessage), 0);
puts(recvmessage);
}
if (isConnected == 0) {
perror("Client disconnected: ");
fflush(stdout);
}
free(recvmessage); recvmessage = NULL;
return 0;
}

Answer 1

This really has nothing to do with multithreading, and everything to do with the nature of SOCK_STREAM sockets.
Stream sockets are, as the name suggests, a stream of bytes; they do not preserve message boundaries such that what is sent with one call to send is received with one call to recv. A single send may be broken up across multiple recv calls, or multiple send calls may be coalesced into a single recv, or both. They do guarantee order, in that the bytes will be received in the same order they are sent.
You'll need to implement your own record marking, perhaps by inserting \0 characters to delimit words, or by using length prefixes.

Answer 2

This is normal behavior. When you use send you don't know how many bytes will be sent. It may happen that all the words , characters are sent.However there are ways to solve this problem. One way is to write a simple header to the string you send , which contains the length of the string you are sending . So you know when the string is ending . For example you can use a thread to look continuously for messages and because the header contains the length of the string you know when to print a \n.The behavior of the send cannot be altered , because it is the Kernel , that is doing this .

Answer 3

In addition to what the other answers say already:
Your code specifically asks to read 8 bytes at a time. recvmessage is a pointer, and pointers are 8 bytes on your system, so sizeof(recvmessage) is 8.