#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <arpa/inet.h>

#define MAXLINE 4096   /*max text line length*/
#define SERV_PORT 3000 /*port*/

FILE *pF_temperature = NULL;
FILE *pF_light = NULL;
FILE *pF_humidity = NULL;

int main(int argc, char **argv) 
{
 int sockfd;
 struct sockaddr_in servaddr;
  char sendline[MAXLINE], recvline[MAXLINE];

  char line1[MAXLINE];
  char line2[MAXLINE];
  char line3[MAXLINE];

 //basic check of the arguments
 //additional checks can be inserted
 if (argc != 5) {
  perror("Usage: TCPClient <IP address of the server"); 
  exit(1);
 }
    
 //Create a socket for the client
 //If sockfd<0 there was an error in the creation of the socket
 if ((sockfd = socket (AF_INET, SOCK_STREAM, 0)) <0) {
  perror("Problem in creating the socket");
  exit(2);
 }
    
 //Creation of the socket
 memset(&servaddr, 0, sizeof(servaddr));
 servaddr.sin_family = AF_INET;
 servaddr.sin_addr.s_addr= inet_addr(argv[1]);
 servaddr.sin_port =  htons(SERV_PORT); //convert to big-endian order
    
 //Connection of the client to the socket 
 if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr))<0) {
  perror("Problem in connecting to the server");
  exit(3);
 }

    pF_temperature = fopen(argv[2], "r");
    pF_light = fopen(argv[3], "r");
    pF_humidity = fopen(argv[4], "r");

    if ((pF_temperature == NULL) || (pF_light == NULL) || (pF_humidity == NULL))
    {
      perror("Error: Can not open the three files!");
      return -1;
    }

/*while (fgets(sendline, MAXLINE, stdin) != NULL) {         */
 while ((fgets(line1, MAXLINE, pF_temperature) != NULL) &&
        (fgets(line2, MAXLINE, pF_light) != NULL) &&
        (fgets(line3, MAXLINE, pF_humidity) != NULL)) 
    {
    line1[strcspn(line1, "\r\n")] = 0;        //remove newline
    line2[strcspn(line2, "\r\n")] = 0;        //remove newline
    line3[strcspn(line3, "\r\n")] = 0;        //remove newline

    strcat(sendline, "TEMPERATURE = ");
    strcat(sendline, line1);
    
    strcat(sendline, "\tLIGHT = ");
    strcat(sendline, line2);
    
    strcat(sendline, "\tHUMIDITY = ");
    strcat(sendline, line3);
    
    strcat(sendline, "\n");        //add extra newline for one row
    
    send(sockfd, sendline, strlen(sendline), 0);
        
  if (recv(sockfd, recvline, MAXLINE, 0) == 0){
   //error: server terminated prematurely
   perror("the server terminated prematurely"); 
   exit(4);
  }

  printf("%s\n", "String received from the server: ");
  fputs(recvline, stdout);

} 
 exit(0);
}

$./a.out 192.168.1.34 temperature.dat light.dat humidity.dat

String received from the server: 
TEMPERATURE = 60    LIGHT = 1    HUMIDITY = 100
TEMPERATURE = 59    LIGHT = 2    HUMIDITY = 150
TEMPERATURE = 58    LIGHT = 3    HUMIDITY = 120
TEMPERATURE = 57    LIGHT = 4    HUMIDITY = 130
TEMPERATURE = 56    LIGHT = 5    HUMIDITY = 140
TEMPERATURE = 55    LIGHT = 6    HUMIDITY = 120
......