Simple echo server in C++ and C in UNIX

Question

I wrote a server which gets messages from clients and responds to them with words it has in its dictionary (dictionary I have in my file). For example:

Client wrote: cat
Server got: cat, Server wrote: miauuu! (and client can see the massege 
from server in his window)

My dict.txt file looks like this:

cat
miauuu!
dog
raw,raw!
etc...

I have some problem with my code, cause my server gets messages from clients but still have problems to send a message to client... (in my client window i cant see a message from server). As a client I use telnet. Any ideas?

I mean, I run my server: ./server 9999 and telnet: telnet 127.0.0.1 9999 and can send a messages to server (and server can see them) but on telnets side I cant see any messages from server (also server seems to send an empty string (or nothing, I dont really know))

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <netdb.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <map>
#include <string>
#include <iostream>
#include <fstream>
using namespace std;

map <string, string> dict;

void* clients_service(void * arg)
{
    int client_fd = *((int*)&arg), buffsize = 1024, bytes_num = 0;
    char buffer[1024];
    char word[1024];

    while(1)
        {
            if ((bytes_num = read(client_fd, buffer, buffsize)) == -1)
            {
                perror("read error");
                exit(1);
            }
            else if (bytes_num == 0)
            {
                fprintf(stdout, "connection closed.\n");
                break;
            }

            buffer[bytes_num] = '\0';
            fprintf(stdout, "Got: %s", buffer);
            string from_client(buffer);
            string to_client = dict[from_client];

            char buff[1024];

            strncpy(buff, to_client.c_str(), strlen(buff));
            buff[strlen(buff) - 1] = '\0';

            if ((write(client_fd, buff, strlen(buff))) == -1)
            {
                fprintf(stderr, "write error\n");
                close(client_fd);
                break;
            }

            fprintf(stdout, "I wrote: %s", buff);

        }
        close(client_fd);
}

void loadDictionary(map <string, string> &dict)
{
    dict.clear();
    ifstream file("dict.txt");
    string s1, s2;

    if(file.is_open())
    {
        while(file.good())
        {
            getline(file, s1);
            getline(file, s2);
            dict.insert(pair<string, string>(s1, s2));
        }
    }

    file.close();
}

int main(int argc, char **argv)
{
    int server_soc_fd, port, buffsize = 1024;
    char buffer[buffsize];
    struct sockaddr_in addr;
    socklen_t addrlen = sizeof(addr);
    loadDictionary(dict);

    if (argc < 2)
    {
        fprintf(stderr, "Use : ./server {port number}\n");
        exit(1);
    }

    port = atoi(argv[1]);
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = INADDR_ANY;
    server_soc_fd = socket(AF_INET, SOCK_STREAM, 0);

    if(server_soc_fd < 0)
    {
        perror("socket");
        exit(1);
    }

    if(bind(server_soc_fd,(struct sockaddr *)&addr, addrlen) < 0)
    {
        perror("bind");
        exit(1);
    }

    if(listen(server_soc_fd, 10) < 0)
    {
        perror("listen");
        exit(1);
    }

    fprintf(stdout, "\nWorking...\n\n");

    while(true)
    {
        int client_soc_fd = accept(server_soc_fd, NULL, NULL);
        pthread_t thread;
        pthread_create(&thread, NULL, clients_service, (void*)client_soc_fd);
    }

    return 0;
}

Answer 1

This:

char buff[1024];

strncpy(buff, to_client.c_str(), strlen(buff));
buff[strlen(buff) - 1] = '\0';

will not fill buff .

char *strncpy(char *dest, const char *src, size_t n);

copies n bytes from src to dest .

So you better do:

const char * cstr = to_client.c_str();
size_t len = strlen(cstr);
strncpy(buff, cstr, max(len, sizeof(buff)-1));
buff[len] = 0;

or even better do:

const char * cstr = to_client.c_str();
size_t len = strlen(cstr);
if (!len)
  continue;

if ((write(client_fd, cstr, len) == -1)
{
  ...

Also you should not rely on write() writing all data at once.

So you might extend it, like so:

...
const char * cstr = to_client.c_str();
size_t len = strlen(cstr);

size_t lenWrittenTotal = 0;
while (lenWrittenTotal < len);
{
  ssize_t lenWritten = write(client_fd, cstr + lenWrittenTotal , len - lenWrittenTotal);

  if (lenWritten < 0)
  {
    perror("write");
    break;
  } 
  else if (lenWritten > 0)
  {
    lenWrittenTotal += lenWritten;
  }
  else
  {
    ... 
    break;
  }
};
...