System Calls: Practical

Objectives

1. Look at several unix/linux system calls.

Notes

• System calls are how the operating systems services are made available.
  • This is part of the OS's API
  • In the case of linux/unix they are c calls.
• The book discusses in theory a copy command.
  • I want to do this in practice.
  • See myCP.cpp.
    • I did this in about an hour.
    • And it most likely is not a very safe command.
    • But it is a basic version of the system call cp
    • Which is in /usr/bin/cp or /bin/cp
    • ls -i /usr/bin/cp /bin/cp (discuss hard link, inode number)
• Some explicit system calls
  • open, opens a file
  • close, closes a file
  • read, reads a set number of bytes from a file descriptor
  • write, writes a set number of bytes to a file descriptor
• In general in UNIX/Linux
  • System calls normally return -1 on error,
    • But this is not set in stone.
  • System calls set a global variable errno when an error occurs.
    • But this very much must be managed by the user
    • IE it is not reset after an error.
  • perror is a library call that looks at errno and
• Watch this with strace
  • This uses the ptrace system call
• I have another program
  • prio.cpp
  • This is somewhat equivelent to nice/renice
    • On Linux/UNIX processes have a "nice" level
      • -19 to 20
      • The lower, the more important the process is.
      • Users can normally raise their nice level (ie be more nice, let other processes have the cpu)
      • But they can't lower it.
  • This does two different types of requests.
  • getrlimit and getpriority request information
  • setpriority and nice set values.
  • run nice -n 4 prio