System Calls

• This is chapter 3, you should read it.
• As we discussed previously, the linux API is through system calls.
  • 
  • Some of these are wrapped by glibc to give us library functions that are easier to use.
  • The difference between the two really doesn't matter to me.
• Let's write some code.
  • A simple program that allows the user to move around the file system.
  • Perhaps the basics of a shell
  • The program prints the current directory.
  • Then reads in the user's response.
  • on QUIT the program exits.
  • Otherwise it attempts to change to the directory given.
  • Print errors when a problem occurs.
• Start by looking at the man page for chdir
  • Notice the include file listed.
  • Notice that there are two different forms
    • One that takes a const char *
    • This is either a relative or absolute path
    • But not a ~ path, the shell does this.
    • The second takes an int
    • This is for a file descriptor, not a path
  • Read the descriptions, make sure you understand what they do.
  • Notice the return value
    • 0 on success, -1 on fail
    • This is very common, but not universal
    • Pay attention to this.
  • errno
    • man errno
    • This is set by the kernel if an error occurs
    • It is NEVER unset.
    • Library calls such as cout, could change this as ell.
    • Best to do the following

    • if (test for fail on system call) {
          do something with errno
      }
      	      

    • Back to this in a bit.
  • Back to chdir, look at the errors
    • Some of these make sense (EACCESS)
    • Some of them are scary (ENOMEM)
    • Some will make sense later (ELOOP)
  • Last thing to look at here is See Also
    • getcwd looks like it might be a system call we want.
    • path_resolution is probably some theory, man 7 usually is.
    • chroot is more advanced.
• errors
  • What do you do when you have an error?
    • The errno might give you a way to recover
    • Or at least print an error message and exit.
  • chdir is nice in that it will not change anything on error. No recovery.
  • But it is nice to know what the problem is.
  • There are several library functions which help
    • perror (char *)
      • Takes a user supplied message
      • And prints the message and the error string.
      • Needs <stdio.h> for c
        • but I also got away with <string.h>
    • strerror
      • Will return the error string given the error number.
      • <cstring> and <cerrno> for c++
      • Use this string immediately or copy it.
      • It will be overwritten.
      • Do not free this string.
• read the man page for getcwd
• Make
  • I use make.
  • You should too.
  • You should ask questions about it now, or any time in the future.