Question

I found on stackoverflow client-server code (using named pipes) and I modified it to let client send messages in while loop. Althought I get messages from server, I get “Illegal seek” on read/write and Im not sure if its an serious error, if something is really bad here?

server.cpp

#include <fcntl.h>
#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>
#include <string.h>

int main()
{
   int client_to_server;
   char *myfifo = "/tmp/client_to_server_fifo";

   int server_to_client;
   char *myfifo2 = "/tmp/server_to_client_fifo";

   char buf[BUFSIZ];

   /* create the FIFO (named pipe) */
   mkfifo(myfifo, 0666);
   mkfifo(myfifo2, 0666);

   /* open, read, and display the message from the FIFO */
   client_to_server = open(myfifo, O_RDONLY);
   server_to_client = open(myfifo2, O_WRONLY);

   printf("Server ON.\n");

   while (1)
   {
      read(client_to_server, buf, BUFSIZ);

      if (strcmp("exit",buf)==0)
      {
         printf("Server OFF.\n");
         break;
      }

      else if (strcmp("",buf)!=0)
      {
         printf("Received: %s\n", buf);
         printf("Sending back...\n");
         write(server_to_client,buf,BUFSIZ);
      }

      /* clean buf from any data */
      memset(buf, 0, sizeof(buf));
   }

   close(client_to_server);
   close(server_to_client);

   unlink(myfifo);
   unlink(myfifo2);
   return 0;
}

client.cpp

#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
   int client_to_server;
   char *myfifo = "/tmp/client_to_server_fifo";

   int server_to_client;
   char *myfifo2 = "/tmp/server_to_client_fifo";

   char str[BUFSIZ];


   //printf("Input message to serwer: ");
   //scanf("%s", str);


   /* write str to the FIFO */
   client_to_server = open(myfifo, O_WRONLY);
   server_to_client = open(myfifo2, O_RDONLY);

while(true)
{

   printf("> "); 
   scanf("%s", str);
   write(client_to_server, str, sizeof(str));

   perror("Write:"); //Very crude error check

   read(server_to_client,str,sizeof(str));

   perror("Read:"); // Very crude error check

   printf("From  server: %s\n",str);
}
   close(client_to_server);
   close(server_to_client);

   /* remove the FIFO */

   return 0;
}

Example output:

client :

> hello, server!
Write:: Success
Read:: Illegal seek
From  server: hello,
Write:: Illegal seek
Read:: Illegal seek
> From  server: server!
> its client here!
Write:: Illegal seek
Read:: Illegal seek
From  server: its
Write:: Illegal seek
Read:: Illegal seek
> From  server: client
Write:: Illegal seek
Read:: Illegal seek
> From  server: here!
> > hello, server!
// and so on ...

server :

Server ON.
Received: hello,
Sending back...
Received: server!
Sending back...
Received: its
Sending back...
Received: client
Sending back...
Received: here!
Sending back...
Received: >
Sending back...
Received: hello,
Sending back...
Received: server!
Sending back...
Received: Write::
Sending back...
Received: Success
Sending back...
Received: Read::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: From
Sending back...
Received: server:
Sending back...
Received: hello,
Sending back...
Received: Write::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: Read::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: >
Sending back...
Received: From
Sending back...
Received: server:
Sending back...
Received: server!
Sending back...
Received: >
Sending back...
Received: its
Sending back...
Received: client
Sending back...
Received: here!
Sending back...
Received: Write::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: Read::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: From
Sending back...
Received: server:
Sending back...
Received: its
Sending back...
Received: Write::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: Read::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: >
Sending back...
Received: From
Sending back...
Received: server:
Sending back...
Received: client
Sending back...
Received: Write::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: Read::
Sending back...
Received: Illegal
Sending back...
Received: seek
Sending back...
Received: >
Sending back...
Received: From
Sending back...
Received: server:
Sending back...
Received: here!
Sending back...
Received: >
Sending back...

Answer 1

Don’t call perror if there was no error.

You need to check if the previous function call failed before you can call perror or you’ll get meaningless error messages when no error actually happened. Functions (usually) aren’t required to set errno to anything particular if they don’t fail.