Instruction

PIPE Links

The purpose of this lab is to learn the principle of creating unnamed links and the principles of communication between two related processes.

1. Introduction
   
   • We can only use unnamed (PIPE) communication links between two related processes.
   • We create it using function:
     int pipe(int *filedes);
     
     • filedes - two-element array
     • filedes[0] - is the file descriptor open for reading
     • filedes[1] - is the file descriptor open for writing
   • We usually use communication links to communicate between two processes, but the example below will be on one:
     

     main()
     {
     int pipefd[2],n;
     char buff[100];
     if (pipe(pipefd) <0)
         perror("pipe error\n");
     printf("read fd = %d, write fd = %d\n",pipefd[0],pipefd[1]);
     if (write(pipefd[1],"hello world\n",12) !=12)
         perror("write error\n");
     if ((n = read(pipefd[0],buff,sizeof(buff))) <=0)
         perror("read error\n");
     write(1, buff, n); /* file desc=1 stdout */
     exit(0);
     }

2. Aim
   Write a client-server program that:
   • gets the file name from the user 0,5 point
   • creates an unnamed link PIPE 0,5 point
   • creates a new process using the fork function(not popen!) 0,5 points
   • passes the file name to the child process. 0,5pkt
   • the child process reads the file with the given name and the content (text) or information about possible errors (e.g. no file) is sent to the parent process (0.5 point)
   • parent process displays on standard output what it received from the child (correct file transfer up to 2 points).
     
3. It can be usefull:
   • gets or fgets
   • perror, strerror
   • read, write
   • fork, wait
   • pipe, close
   • strcat, strlen, strcpy
     
     
4. Final remark
   One should remember about the memory allocation for transmitted messages. It is also important that the pipe link is used only for one-way data transfer. If we want to have two-way communication, we need to create 2 links. 1 close a read desc in child and a write desc in parent, and the other close a write desc in child and read esc in parent. The file can be of any length, it can be read and sent in a loop (not a single reading). Do not thoughtlessly use memset , do not use single-character reading, don't combine with sending special bytes to end the transmission. The read function is a synchronous function.
   
   
5. Report
   Of course, don't forget the report.