Games: Processes and Inter-Process Communication

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Project 6 revised grades: range 46-100, mean 84
• Thursday we have a featured speaker from GM to speak to all undergrads (including first year students) about computing in automobiles
• Friday's quiz is postponed until Monday
• Reminder: project 7 due tomorrow night. Grading sheet is now online.
• Note that Project 8 (last one!) is online if you have finished #7.
• Final exam is scheduled for Wednesday May 4, noon-3pm in Packard 416
• Today: Processes and inter-process communication (IPC)

Processes

• Review: What is a process?

Processes

• Review: What is a process?
• A process is an executing instance of a program.
• The shell is a program designed to help you manage programs and processes.
  • User types a.out
  • Shell asks kernel for a new process
  • Shell asks the kernel to execute the program a.out in that new process
  • a.out runs

Running a program

• Q: How can a program run a program?

Running a program

• Q: How can a program run a program?
• A: The program calls execvp(2)
• Usage: execvp(const char *file, char *const argv[])
  • Program calls execvp()
  • Kernel loads program
  • Kernel puts arglist into program
  • Kernel calls main(argc, argv)

Example: execdemo.c

• Purpose: run a command

  #include <stdio.h>
  #include <unistd.h>
  
  main() {
    char *args[4] = {"ls", "-l", "/usr/bin", NULL};
    printf("before exec\n");
    fflush(0);
    execvp("ls", args);
    printf("after exec\n");
  }
  

• What is the result of running this program?

Getting a new process

• execvp() ran program, but never returned!
• We need to create a new process and have that process run the program.
• Q: How do we get a new process?

Getting a new process

• execvp() ran program, but never returned!
• We need to create a new process and have that process run the program.
• Q: How do we get a new process?
• A: Use fork() to clone an existing process
  • fork() takes no arguments
  • OS copies memory containing process
  • returns -1 on error, 0 for child, and pid of child in parent
  • Otherwise, exactly the same program, same place in program

Example: forkdemo3.c

• In forkdemo3.c we see exactly the PID for each process.
• So, using fork(), we can create a new process, which can call execvp() to run the program.
• Let's modify forkdemo3 to do so.
• But what should the parent do?

The Parent wait()s

• We know the child is executing the new process.
• The parent needs to call wait(2) for the child to die.
• Usage: p = wait(&from_child);
  • Returns -1 if no children, otherwise the pid of child that exitted
  • Exit code of child process is placed in from_child

Interprocess communication

• Exec()ing a program is like calling a function -- it has parameters (arguments) and can return a result (exit code).
• Not very satisfying way to have two processes talk to one another.
• What are other ways that two processes can communicate?

Interprocess communication

• UNIX supports a variety of interprocess communication methods.
• We know from our shell programming about UNIX pipes, in which the output of one process is fed into the input of another.
• We can build pipes ourselves, and will, when we cover chapters 10 and 11.
• Other options include:
  • Disk files
  • Shared memory
  • Named pipe
• We will explore each of them now.

Communication Using Files

• Using files
  • Easy to code
  • Supports multiple clients
  • Uses standard file permissions
  • Can span multiple machines (with NFS)
• Might need to worry about race conditions

Communication Using Shared Memory

• Shared memory is a segment of user space that multiple processes can access.
• The server creates a shared memory segment and then obtains a pointer to the segment.

  sd = shmget(id, size, IPC_CREAT|mode_flags);
  ptr = shmat(sd, NULL, 0);
  strcpy(ptr, "Wednesday 20 April");
  

• The client opens the segment and then obtains a pointer to it.

Communication Using Shared Memory

• The client opens the segment and then obtains a pointer to it.

  sd = shmget(id, 0, 0);
  ptr = shmat(sd, NULL, 0);
  printf("date is %s\n", ptr); 
  

• Features:
  • Supports multiple clients
  • Uses standard file permissions
  • Data are shared, not copied!
  • Transfers without system calls
• Still need to worry about race conditions

IPC Using Named Pipes

• A named pipe is pipe with a regular filename.
• Processes can open, read, and write to it. It acts as a FIFO queue.
• Any process can create a named pipe with mkfifo(3c)

  mkfifo(name, mode);
  

• Server opens fifo for writing and writes data to it.
• Client opens for reading, and reads data out.
• Open blocks if noone at other end.
• Features:
  • Pipes between unrelated processes
  • Uses standard file permissions
  • One client a time only

Example: Named Pipes

• mkfifo(1) is a system command to create named pipes.
• Use other programs to write and read from it.
   
  
• In p8, you'll need two such pipes to communicate between two copies of your program.