GetOpt: Older Version

There is a library function that allows us to semim automatically parse the command line options.
man 3 getopt
- man -k getopt
- Note the code at the bottom
- Note the see also
This is for old style arguments.
int getopt(int argc, char *argv[], const char *optstring);
- You need to #include <unistd.h>
- argc and argv are the parameters to main.
- optstring is a string containing the flags
  - a letter means there is just a flag, no arguments
  - a letter followed by a : means that there will be an argument
  - a letter followed by 2 :: means there might be an argument.
    - This is not standard, it is a GNU extension.
There are some external variables involved with this function
- They are global, so you don't need to declare them.
- ```
extern char *optarg;
extern int optind, opterr, optopt; 
```
- optarg is for argument options
  - "G:" in the optstring.
  - -G hello, optarg will hold hello
- optind is the index into argv for the current action.
  - On an error, it will point to the item that caused a problem.
  - On -1, it is the first argv value that was not an option.
  - It is used internally by getopt, so if you mess with it, you will cause problmes? Be warned.
- int optopt.
  - This is actually a char, but c...
  - If there is an unrecognized flag,
    - The function returns a ?
    - And optopt contains the bad character.
- int opterr
  - This is probably a bool.
  - This controls error output by getopt
    - If it is non-zero (true) and there is an error, it is prited to stderr. (discuss this).
    - If it is zero (false) and there is an error, nothing is printed.
    - but the return value is still ?
    - and optopt contins the bad character.
Take a look at getOptZamp.cpp.
There are versions to parse the new GNU (long) flags as well
int getopt_long(int argc, char *argv[], const char *optstring, const struct option *longopts, int *longindex);
There is also a boost library extension to do this.
As well as other libraries.