A Basic example of socket programming in C

Hello,
I have a question about socket programming
The question was a homework of this university of past (2011?) course.
The server is simulating a sensor that provides readings of temperature, light and humidity ( temp.dat, light.dat, humid.dat ) each with single column of number, one per row.

temp.dat:
60
59
58
57
......
light.dat:
40
50
20
30
......
humid.dat:
1
2
3
4
......

The server will open each file, read in all sensor values, and serve every request from the client. If a client requests temperature, the server will return a certain temperature sample from temp.dat according to the time of the connection. Starting from the first value of temp.dat when a connection is established, every second the server should return the next value in temp.dat upon request. If EOF is reached, the server should wraps around and start from the beginning of the file again. Display readings to the screen:

Connected to server!
Data returned by the server:
TEMPERATURE = 60 HUMIDITY = 40 LIGHT = 1
Data returned by the server:
TEMPERATURE = 59 HUMIDITY = 50 LIGHT = 2
Data returned by the server:
TEMPERATURE = 58 HUMIDITY = 20 LIGHT = 3
......  ......  ......

I have attached the code in C for server and client parts, but I am quite obscure about reading the 3 files in the server side and display in the client side by request.
This is NOT my homework for any class, but for self study to understand basic server-client communication of socket programming in C.

Any suggestion is greatly appreciated and pseudo code is fine.

Thanks a lot!

Here's code which reads files in order when given as commandline arguments:

#include <stdio.h>

char line[4096];
char **files=NULL;
int filenum=0;
FILE *file=NULL;

// Close current file if any, open next from files[]
FILE *next_file() {
        if(file)
        {
                fclose(file);
                file=NULL;
        }

        if(files[filenum] == NULL) return(file=NULL); // out of files
        return(file=fopen(files[filenum++], "r"));
}

char *next_line(void) {
        // Keep trying to get a line until we get a line or run out of files
        while(1)
        {
                // If something is open, try and read it.
                // If we read it, return the line.
                if(file && fgets(line, 4096, file)) return(line);

                // If we're out of files, return error immediately.
                if(files[filenum] == NULL) return(NULL); // out of files

                // Close current and open next file
                next_file(files);
        }

        return(line);
}

void main(int argc, char *argv[]) {
        // get list of files from argv.
        // argv is organized like { "programname", "arg1", "arg2", "arg3", NULL }
        files=argv;
        filenum=1;      // valid arguments in argv[] start at 1

        while(next_line())
        {
                fputs(line, stdout);
                sleep(3);
        }
}

Being able to call next_line repeatedly without worrying about which file is open should make it easier.

Thanks!
Your code opens the file sequentially, but the output shows three values in a row, which seems to me the three files are opened simultaneously:

Connected to server! 
Data returned by the server: 
TEMPERATURE = 60 HUMIDITY = 40 LIGHT = 1 
Data returned by the server: 
TEMPERATURE = 59 HUMIDITY = 50 LIGHT = 2 
Data returned by the server: 
TEMPERATURE = 58 HUMIDITY = 20 LIGHT = 3

1) Is this right? I was thinking pthreads too, but not sure pthreads is the correct way.
2) I assumed this display part is piped in the client side. Can this part be embedded to the echoClient program, i.e. integrated this part into the echoClient program?

Thanks again!

1) No. It just reads lines.
2) No. It just prints lines.

Unless your current code is radically different from what it was before, that is. Show your code please.

Here is my code not using pthreads:

#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);
}

I gave it try with:

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

I had redundant output of the whole dataset instead of line by line.

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
......

The repeat number happened to be the same number of records. Looks like the while-loop is wrong. Spent a couple of hours debugging, no luck.

It sends humidity, temperature, and light as one line because you read from all three:

 while ((fgets(line1, MAXLINE, pF_temperature) != NULL) &&
        (fgets(line2, MAXLINE, pF_light) != NULL) &&
        (fgets(line3, MAXLINE, pF_humidity) != NULL))

...then remove newlines from all three:

    line1[strcspn(line1, "\r\n")] = 0;        //remove newline
    line2[strcspn(line2, "\r\n")] = 0;        //remove newline
    line3[strcspn(line3, "\r\n")] = 0;        //remove newline

...then concatenate all three with labels and tabs inbetween:

    strcat(sendline, "TEMPERATURE = ");
    strcat(sendline, line1);

    strcat(sendline, "\tLIGHT = ");
    strcat(sendline, line2);

    strcat(sendline, "\tHUMIDITY = ");
    strcat(sendline, line3);

Then send all three:

send(sockfd, sendline, strlen(sendline), 0);

You are not using my code at all.

I am not using yours as your code sends the three file sequentially, but the required output sends three columns in a row, one at a time. That's why I concatenate three lines into one.

Connected to server!  
Data returned by the server:  
TEMPERATURE = 60 HUMIDITY = 40 LIGHT = 1  
Data returned by the server:  
TEMPERATURE = 59 HUMIDITY = 50 LIGHT = 2  
Data returned by the server:  
TEMPERATURE = 58 HUMIDITY = 20 LIGHT = 3

Your code sends the files one after another.
If I treat the three items (temperature, light, humidity) as a single intact record, then your output seems different from the sample output format, whereas my way seems to be what is required, except the following problems:
1)The whole dataset is output all at once, instead of individual record (3 numbers in a row);
2)The whole dataset is output in the way of 1) 30 times in total (which is exactly the total record numbers), that's what should be fixed.
Not sure if I missed some of your points. Thanks.