Errors and Error Handling

• Take a quick look at sethostname.
  • #include <unistd.h>.
    • This is a *nix thing.
  • It takes a string and a size of the string.
    • The string does not need to be null terminated.
    • Because you give the length in the second argument.
  • And if possible sets the host name to be the string.
• There are four error values possible
  • EFAULT - the string address is invalid.
  • EINVAL - the length is negative or larger than a maximum size.
  • ENAMETOOLONG - the name is longer than HOST_NAME_MAX
  • EPERM - permission denied.
• These values are not returned, they are stuck in a global variable errno.
• If you look in /usr/include/asm-generic/errno-base.h some of them are defined.
• This makes the output of bad.C make sense.
• Or even bad2.C.
• errno
  • Is set by the kernel when something goes wrong.
  • It is not reset to 0 when something goes right.
• In general, dealing with errno
  • Be aware that it only matches the last error.
  • A new error will overwrite it immediately
  • Including new errors in the error handling routine.
  • So save it.
  • What is worse, it might be changed by a successful function call
    • If is were an error in the function call that is caught and handled
  • GLOBAL VARIABLES ARE BAD
• The most standard way to deal with an error message is perror
  • This is defined in stdio.h
  • It takes a string and prints the string along with an appropriate error message based on errno
  • The man page for perror is very good and explains errno well.

  • if (0 != systemcall()) {
        perror("Call to systemcall failed:");
        // code to handle error if you can.
    }
    

• There is a deprecated array, char * sys_errlist[], but you should avoid that.
• There is a new function, strerror
  • In string.h
  • Takes errno
  • returns a pointer to a string
    • Just print or copy, don't mess with it.
    • Future calls to strerr will change this string.
  • Note LC_MESSAGES is possibly used to translate the string.
• In python the socket library
  • To start with, gethostname is in tye socket library
    • And it can't fail as it does not have the same interface as the c/c++ call.
    • It just returns a string.
  • sethostname is also in socket, but only in python 3.
    • It takes a string.
    • And raises an os exception if it fails.
    • see bad.py for an example.
      • Note, this is a python3
      • The print statements are different.
• bad.sh
  • Let's look at it.
  • The nature of the beast is somewhat different.
  • $? is the return value from the last command executed.
  • Someone ask me why we do a return 0 or exit 0 at the end of our programs.
  • Play with hostname.