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Mead's Guide to getopt

Quick links:

Introduction
Using getopt
Option arguments
Unknown and missing option arguments
Non-option arguments

Optional option arguments
Long options

Long options and flags

Summary

Other uses for command line arguments

Introduction

Most command line programs can have their behavior altered by supplying options to the command when it is invoked. Consider a typical command line to compile a C program:

gcc -Wall -Wextra main.c foo.c bar.c -O -o program -ansi -pedantic -Werror

Before gcc can even begin compiling the source files, it first must parse the entire command line so as to understand exactly how the programmer expects the compiler to behave. Imagine having to deal with hundreds of options like the gcc command options. This summary of options shows hundreds of options alone. And don't believe for a minute that they are only for command-line apps. All "real" games have hundreds of them:

Valve's command line options
Quake's options One of the best games of all time!
Unreal options

It isn't hard to imagine that a program that accepts a lot of arguments, options, and option arguments will need to include a lot of code just to parse the command line. Writing the code yourself is certainly possible, but having to do that for every program is not only tedious, but is very repetitive, inefficient, and error-prone.

Fortunately, there are many libraries and APIs (Application Programming Interfaces) that are designed specifically for that task. One of those APIs is a function called getopt.

Before looking at getopt, a brief review of command line arguments is in order.

Usually, we see main prototyped as:

int main(void);

However, main is sort of overloaded to take parameters as well:

   /* These prototypes are the same */ 	
int main(int argc, char *argv[]);
int main(int argc, char **argv);

We may even see it like this:

   /* These prototypes are the same */   
int main(int argc, char *argv[], char *env[]);

As we've seen with arrays as parameters, the declarations above are equivalent.

argc - The number of arguments passed to main (the program).
argv - An array of pointers to NUL-terminated strings (the arguments).
The caller passes the arguments; the caller of main is the C runtime system.
The first element in argv (argv[0]) is the name of the program.

This trivial program simply prints out each argument:

int main(int argc, char *argv[])
{
  int i;
  
  for (i = 0; i < argc; i++)
    printf("arg%i = %s\n", i, argv[i]);
    
  return 0;
}

If our program was named foo and we were to invoke the program like this:

foo one two three 911

we would see something like this printed out:

foo
one
two
three
911

Another example:

foo one "two three" four 911

foo
one
two three
four
911

Another way of printing the arguments using pointers instead of subscripts:

int main(int argc, char **argv)
{
  while (*argv)
    printf("%s\n", *argv++);

  return 0;
}

Diagram of the arguments when invoked as:

foo one "two three" four 911

The character between the strings Two and Three in the diagram above is the space character, ASCII 32. Also, notice that the double-quote characters are not passed to the program.

Note: Because argv is an array of pointers to characters (strings), you can only pass strings to a program. If you want to use the parameter as a number, you will have to convert it from a string to a number yourself. See the Data Conversion section in the C Runtime Library, specifically the atoi function.

tablen for 64-bit Linux.

dumpit for 64-bit Linux.

Using getopt

The getopt function is prototyped like this (in getopt.h):

int getopt(int argc, char *const argv[], const char *optstring);

The first two parameters are exactly like main and are usually just passed from main to getopt as-is.

According to the documentation for getopt, you are supposed to include unistd.h. However, I've recently discovered that this doesn't work on all systems. Using getopt.h instead of unistd.h appears to fix the problem.

There are also a few global variables defined within the API:

extern char *optarg;
extern int optind, opterr, optopt;

The interesting part of the getopt function is the last parameter:

const char *optstring

This string is an encoding (of sorts) that contains all of the single-letter options that a program wants to accept. For example, if the program want's to accept these options:

-a   -b   -X

then optstring would simply contain the string: "abX" (The order doesn't matter, although the characters are case-sensitive. The hyphens are not present, either.) The program would contain code similar to this:

#include <stdio.h>  /* printf     */
#include <getopt.h> /* getopt API */

int main(int argc, char *argv[])
{
  int opt;

  while ((opt = getopt(argc, argv, "abX")) != -1) 
  {
     switch (opt) 
     {
      case 'a':
        printf("Option a was provided\n");
        break;
      case 'b':
        printf("Option b was provided\n");
        break;
      case 'X':
        printf("Option X was provided\n");
        break;
     }
  }
  
  return 0;
}

Sample runs (assume the program has been compiled to a.out):

Command line options	Output
./a.out -b	Option b was provided
./a.out -b -X -a	Option b was provided Option X was provided Option a was provided
./a.out -bXa	Option b was provided Option X was provided Option a was provided
./a.out -a b X	Option a was provided
./a.out -t	./a.out: invalid option -- 't'
./a.out a b c

Of course, in a real program, the programmer would actually do something with the options rather than just print out the information. However, this demonstrates how the getopt function behaves.

Option Arguments

Below is a simple gcc compile command:

gcc foo.c -o bar.o -c

This command will compile only (-c) the file foo.c into bar.o (-o bar.o). The -o option is different than the -c option in that it requires an argument itself. If you want your options to accept an argument, you must provide getopt with that information.

Let's assume we want the a and b options to accept an argument. This is how optstring would look now:

"a:b:X"

The colon after the letter tells getopt to expect an argument after the option. Now the code looks like this (partial):

while ((opt = getopt(argc, argv, "a:b:X")) != -1) 
{
   switch (opt) 
   {
    case 'a':
      printf("Option a has arg: %s\n", optarg);
      break;
    case 'b':
      printf("Option b has arg: %s\n", optarg);
      break;
    case 'X':
      printf("Option X was provided\n");
      break;
   }
}

If the option requires an argument, the external variable optarg is a pointer to the argument. Recall these global variables defined in getopt.h:

extern char *optarg;
extern int optind, opterr, optopt;

Sample runs (assume the program has been compiled to a.out):

Command line options	Output
./a.out -a one -b two -X	Option a has arg: one Option b has arg: two Option X was provided
./a.out -aone -btwo	Option a has arg: one Option b has arg: two
./a.out -X -a	Option X was provided ./a.out: option requires an argument -- 'a'
./a.out -a -X	Option a has arg: -X

When using single-letter options that require an argument, as in -a and -b above, white space between the option and the argument is optional. The first string following the option will be used as the argument (regardless of whether or not it starts with a minus sign).

Unknown Options and Missing Option Arguments

By default, getopt prints errors that it encounters. This is sometimes helpful, but we'd rather deal with the errors ourselves. To disable the automatic error printing, simply put a colon as the first character in optstring:

":a:b:X"

Now, we need to handle two error conditions:

The user has provided an unknown option, e.g.: ./a.out -t
The user has failed to provide a required argument for an option, e.g.: ./a.out -a

The code (partial) looks like this:

while ((opt = getopt(argc, argv, ":a:b:X")) != -1) 
{
   switch (opt) 
   {
    case 'a':
      printf("Option a has arg: %s\n", optarg);
      break;
    case 'b':
      printf("Option b has arg: %s\n", optarg);
      break;
    case 'X':
      printf("Option X was provided\n");
      break;
    case '?':
      printf("Unknown option: %c\n", optopt);
      break;
    case ':':
      printf("Missing arg for %c\n", optopt);
      break;
   }
}

Recall these global variables from getopt.h:

extern char *optarg;
extern int optind, opterr, optopt;

Sample runs (assume the program has been compiled to a.out):

Command line options	Output
./a.out -a	Missing arg for a
./a.out -t	Unknown option: t
./a.out -a one -t -X -b	Option a has arg: one Unknown option: t Option X was provided Missing arg for b
./a.out -a one,two,three	Option a has arg: one,two,three
./a.out -a "one two three"	Option a has arg: one two three

A note about the last two examples above:

If you want the option to have multiple arguments, you must not have any spaces between them.
If you do have spaces, you must enclose them in quotes.
It is up to the programmer to then parse the multiple arguments, as getopt does not have that capability.

Non-Option Arguments

Again, a sample gcc compile command:

gcc -Wall -Wextra main.c foo.c bar.c -O -o program -ansi -pedantic -Werror

This command line has:

1 command
```
gcc
```

7 options

-Wall  -Wextra  -O  -o  -ansi  -pedantic  -Werror

1 option argument for the -o option (which is the output file name):

program

3 non-option arguments (or simply arguments):
```
main.c  foo.c  bar.c
```
which are the files that the compiler will actually compile.

Before going any further, make sure that you really understand what every string in the example above means. In other words, which strings are commands, options, arguments, or option arguments and why? This will tell you if you understand how the command line works.

Up until now, we've only focused on the options. What about the real arguments to gcc? getopt behaves in two different ways, as this code demonstrates:

#include <stdio.h>  /* printf */
#include <getopt.h> /* getopt */

int main(int argc, char *argv[])
{
  int opt;

  while ((opt = getopt(argc, argv, ":a:b:X")) != -1) 
  {
     switch (opt) 
     {
      case 'a':
        printf("Option a has arg: %s\n", optarg);
        break;
      case 'b':
        printf("Option b has arg: %s\n", optarg);
        break;
      case 'X':
        printf("Option X was provided\n");
        break;
      case '?':
        printf("Unknown option: %c\n", optopt);
        break;
      case ':':
        printf("Missing arg for %c\n", optopt);
        break;
     }
  }

    /* Get all of the non-option arguments */
  if (optind < argc) 
  {
    printf("Non-option args: ");
    while (optind < argc)
      printf("%s ", argv[optind++]);
    printf("\n");
  }
  
  return 0;
}

Again, the global variables from getopt.h:

extern char *optarg;
extern int optind, opterr, optopt;

Sample runs (assume the program has been compiled to a.out):

Command line options	Output
./a.out x -a one y -X z	Option a has arg: one Option X was provided Non-option args: x y z
./a.out x y z -a one -b two	Option a has arg: one Option b has arg: two Non-option args: x y z

As you can see, the default behavior for getopt is to move all of the non-option arguments to the end of the array. When getopt has no more options to parse, it returns -1 and the while loop ends. The external variable optind is used as an index into argv so we can retrieve the remaining arguments.

If you want to have getopt parse and return the non-option arguments in the while loop (in the order specified), you must direct it to do so by putting a minus (-) in front of the optstring:

"-:a:b:X"

Note: When supplying both - and : at the front of the string, the minus must come first.

Sometimes, having getopt rearrange the non-option arguments is problematic, especially when some of the options apply only to specific non-option arguments.

Sample code:

#include <stdio.h>  /* printf */
#include <getopt.h> /* getopt */

int main(int argc, char *argv[])
{
  int opt;

  while ((opt = getopt(argc, argv, "-:a:b:X")) != -1) 
  {
     switch (opt) 
     {
      case 'a':
        printf("Option a has arg: %s\n", optarg);
        break;
      case 'b':
        printf("Option b has arg: %s\n", optarg);
        break;
      case 'X':
        printf("Option X was provided\n");
        break;
      case '?':
        printf("Unknown option: %c\n", optopt);
        break;
      case ':':
        printf("Missing arg for %c\n", optopt);
        break;
      case 1:
        printf("Non-option arg: %s\n", optarg);
        break;
     }
  }
  
  return 0;
}

Sample runs:

Command line options	Output
./a.out x y z -a foo	Non-option arg: x Non-option arg: y Non-option arg: z Option a has arg: foo
./a.out x -a foo y -b bar z -X w	Non-option arg: x Option a has arg: foo Non-option arg: y Option b has arg: bar Non-option arg: z Option X was provided Non-option arg: w

Command line options

Output

./a.out x y z -a foo

Non-option arg: x
Non-option arg: y
Non-option arg: z
Option a has arg: foo

./a.out x -a foo y -b bar z -X w

Non-option arg: x
Option a has arg: foo
Non-option arg: y
Option b has arg: bar
Non-option arg: z
Option X was provided
Non-option arg: w

Putting it all together using this call:

getopt(argc, argv, "-:a:b:X")

and running the program:

./a.out -t x -a foo -M y -b bar z -X w -b

Output:

Unknown option: t
Non-option arg: x
Option a has arg: foo
Unknown option: M
Non-option arg: y
Option b has arg: bar
Non-option arg: z
Option X was provided
Non-option arg: w
Missing arg for b

Optional Option Arguments

Sometimes an option may not require an argument, but it allows an optional argument. There is a syntax for that, as well: Two colons (::) must follow the letter in optstring:

":a::b:X"

In the string above, option a will accept an optional argument. (Option b requires an argument) Usage:

Option syntax Meaning
-a
OK, No argument provided (optional).
-afoo
OK, argument is foo
-a foo
Wrong, no space allowed with optional arguments.
foo is considered a non-option argument.
-bfoo
OK, argument is foo (required).
-b foo
OK, argument is foo (required).
-b
Wrong, option b requires an argument.

Option syntax	Meaning
-a	OK, No argument provided (optional).
-afoo	OK, argument is `foo`
-a foo	Wrong, no space allowed with optional arguments. `foo` is considered a non-option argument.
-bfoo	OK, argument is `foo` (required).
-b foo	OK, argument is `foo` (required).
-b	Wrong, option `b` requires an argument.

Since the argument is optional, you will have to check the value of optarg to see if it is a valid pointer (otherwise, it's NULL).

Code sample:

while ((opt = getopt(argc, argv, "-:a::b:X")) != -1) 
{
   switch (opt) 
   {
    case 'a':
      printf("Option a has arg: %s\n", optarg ? optarg : "(none)");
      break;
    case 'b':
      printf("Option b has arg: %s\n", optarg);
      break;
    case 'X':
      printf("Option X was provided\n");
      break;
    case '?':
      printf("Unknown option: %c\n", optopt);
      break;
    case ':':
      printf("Missing arg for %c\n", optopt);
      break;
    case 1:
      printf("Non-option arg: %s\n", optarg);
      break;
   }
}

Sample output:

Command line	Output
./a.out -a -b bar -X	Option a has arg: (none) Option b has arg: bar Option X was provided
./a.out -afoo -b bar -X	Option a has arg: foo Option b has arg: bar Option X was provided
./a.out -a foo -b bar -X	Option a has arg: (none) Non-option arg: foo Option b has arg: bar Option X was provided

Command line

Output

./a.out -a -b bar -X

Option a has arg: (none)
Option b has arg: bar
Option X was provided

./a.out -afoo -b bar -X

Option a has arg: foo
Option b has arg: bar
Option X was provided

./a.out -a foo -b bar -X

Option a has arg: (none)
Non-option arg: foo
Option b has arg: bar
Option X was provided

Long Options

There are some problems with the short, single-character options:

There are a limited number of single-characters. For small programs, this is probably not a problem.
It can be very difficult to remember what each option does. Sure, if -a is for add and -d is for delete, what about options for altering, assigning, aardvarking, etc?

Just look at the number of options for rsync and wget. They have tons of options. Having the ability to use more than just a single character gives us virtually unlimited options. An example of a long option in diff:

diff file1.txt file2.txt --strip-trailing-cr

There are many options for diff. So, not only do we have unlimited option names with longer names, but they can be self-explanatory. Consider this:

diff file1.txt file2.txt -Z

Do you know what -Z does? Probably not, unless you are an expert with diff. How about this:

diff file1.txt file2.txt --ignore-trailing-space

I'm pretty sure that most programmers know what this does, or have a good idea of what it does. (-Z is short for --ignore-trailing-space. Who knew?)

This is the prototype for the long option function:

int getopt_long(int argc, char * const argv[], const char *optstring, 
                const struct option *longopts, int *longindex);

The first three options are the same as the short version. The last two parameters are new. This is what an option struct looks like:

struct option 
{
  const char *name;    /* name without -- in front                                  */
  int         has_arg; /* one of: no_argument, required_argument, optional_argument */
  int        *flag;    /* how the results are returned                              */
  int         val;     /* the value to return or to put in flag                     */
};

An example (from the man page):

#include <getopt.h> /* getopt */
#include <stdlib.h> /* exit   */
#include <stdio.h>  /* printf */

int main(int argc, char **argv)
{
  int c;

  while (1) 
  {
      int option_index = 0;
      static struct option long_options[] = 
      {
          {"add",     required_argument, NULL,  0 },
          {"append",  no_argument,       NULL,  0 },
          {"delete",  required_argument, NULL,  0 },
          {"verbose", no_argument,       NULL,  0 },
          {"create",  required_argument, NULL,  0 },
          {"file",    optional_argument, NULL,  0 },
          {NULL,      0,                 NULL,  0 }
      };

      c = getopt_long(argc, argv, "-:abc:d::", long_options, &option_index);
      if (c == -1)
           break;

      switch (c) 
      {
        case 0:
          printf("long option %s", long_options[option_index].name);
          if (optarg)
             printf(" with arg %s", optarg);
          printf("\n");
          break;

        case 1:
          printf("regular argument '%s'\n", optarg); /* non-option arg */
          break;

        case 'a':
          printf("option a\n");
          break;

       case 'b':
          printf("option b\n");
          break;

        case 'c':
          printf("option c with value '%s'\n", optarg);
          break;

        case 'd':
          printf("option d with value '%s'\n", optarg ? optarg : "NULL");
          break;

        case '?':
          printf("Unknown option %c\n", optopt);
          break;

        case ':':
          printf("Missing option for %c\n", optopt);
          break;

        default:
          printf("?? getopt returned character code 0%o ??\n", c);
       }
  }

  return 0;
}

Sample output:

Command line	Output
./a.out --delete=foo -c5 --add=yes --append	long option delete with arg foo option c with value '5' long option add with arg yes long option append
./a.out --d=foo --ad=yes --ap --a	long option delete with arg foo long option add with arg yes long option append Unknown option
./a.out --create=5 --create 6 --c=7 --c 8	long option create with arg 5 long option create with arg 6 long option create with arg 7 long option create with arg 8
./a.out --file=5 --file 6 --file7	long option file with arg 5 long option file regular argument '6' Unknown option

With the long option, you don't have to provide the entire string. If getopt can deduce what the option is with only a few characters, it will match that option. In the example above, --d matches --delete because it is the only option that begins with --d. In the case of --add and --append, two characters are necessary to disambiguate between them because they both begin with --a.

Another example (partial). This example shows how to associate a short option with the long option. Note the fourth field of the struct is not 0, but is the associated short option:

while (1) 
{
    int option_index = 0;
    static struct option long_options[] = 
    {
        {"add",     required_argument, NULL,  'a'},
        {"append",  no_argument,       NULL,  'p'},
        {"delete",  required_argument, NULL,  'd'},
        {"verbose", no_argument,       NULL,  'v'},
        {"create",  required_argument, NULL,  'c'},
        {"file",    optional_argument, NULL,  'f'},
        {NULL,      0,                 NULL,    0}
    };

    c = getopt_long(argc, argv, "-:a:pd:vc:f::", long_options, &option_index);
    if (c == -1)
      break;

    switch (c) 
    {
      case 0:
        printf("long option %s", long_options[option_index].name);
        if (optarg)
           printf(" with arg %s", optarg);
        printf("\n");
        break;

      case 1:
        printf("regular argument '%s'\n", optarg);
        break;

      case 'a':
        printf("option a with value '%s'\n", optarg);
        break;

     case 'p':
        printf("option p\n");
        break;

      case 'd':
        printf("option d with value '%s'\n", optarg);
        break;

     case 'v':
        printf("option v\n");
        break;

      case 'c':
        printf("option c with value '%s'\n", optarg);
        break;

      case 'f':
        printf("option f with value '%s'\n", optarg ? optarg : "NULL");
        break;

      case '?':
        printf("Unknown option %c\n", optopt);
        break;

      case ':':
        printf("Missing option for %c\n", optopt);
        break;

      default:
        printf("?? getopt returned character code 0%o ??\n", c);
     }
}

Sample output. Notice that there are no longer any long options returned from getopt as they are all short options, even if the user provided a long option:

Command line	Output
./a.out --delete=foo -c5 --add=yes --append	option d with value 'foo' option c with value '5' option a with value 'yes' option p
./a.out --d=foo --ad=yes --ap	option d with value 'foo' option a with value 'yes' option p
./a.out --create=5 --create 6 --c=7 --c 8	option c with value '5' option c with value '6' option c with value '7' option c with value '8'
./a.out --file=5 --file 6 --file7	option f with value '5' option f with value 'NULL' regular argument '6' Unknown option

Suppose we remove the v from the string, changing this:

"-:a:pd:vc:f::"

to this:

"-:a:pd:c:f::"

We still have this in our long options structure:

{"verbose", no_argument,       NULL,  'v'},

Running this:

./a.out -v

produces this:

Unknown option v

Running this:

./a.out --verbose

produces this:

?? getopt returned character code 0166 ??

The character code is the ASCII code (shown here in octal) for the letter v.

You should always provide a default case in the switch statement. That will help you to find bugs (like above) in your code. Not doing so will cause you a lot of wasted time trying to figure out what's going wrong.

Long Options and Flags

Sometimes, we just want a true or false value. We want to enable or disable something.

When a variable acts like a boolean (.e.g. a value of 0 or 1), we call that kind of variable a flag.
Some command line options are just flags to the program, meaning simply enable or disable some functionality.
Flags generally default to 0, meaning "off".
The mere presence of a flag on the command line (without any arguments) sets the flag to 1, meaning "on".
We've seen this quite often with gcc:
```
gcc -c foo.c
```

-c

flag

gcc

-c

true

false

and

The getopt_long provides a short-hand notation for setting flags as this example shows:

#include <getopt.h> /* getopt */
#include <stdio.h>  /* printf */

/* File scope flags, all default to 0 */
static int f_add;
static int f_append;
static int f_create;
static int f_delete;
static int f_verbose;

int main(int argc, char **argv)
{
  int c;

  while (1) 
  {
    int option_index = 0;
    static struct option long_options[] = 
    {
      {"add",     no_argument, &f_add,     1},
      {"append",  no_argument, &f_append,  1},
      {"create",  no_argument, &f_create,  1},
      {"delete",  no_argument, &f_delete,  1},
      {"verbose", no_argument, &f_verbose, 1},
      {NULL,      0,           NULL,       0}
    };

    c = getopt_long(argc, argv, "-:", long_options, &option_index);
    if (c == -1)
      break;

    switch (c) 
    {
      case 1:
        printf("regular argument '%s'\n", optarg);
        break;

      case '?':
        printf("Unknown option %c\n", optopt);
        break;
    }
  }

  printf("    f_add: %i\n", f_add);
  printf(" f_append: %i\n", f_append);
  printf(" f_delete: %i\n", f_delete);
  printf(" f_create: %i\n", f_create);
  printf("f_verbose: %i\n", f_verbose);

  return 0;
}

Sample output:

Command line	Output
./a.out --verbose --create	f_add: 0 f_append: 0 f_delete: 0 f_create: 1 f_verbose: 1
./a.out --verbose --append --create --add --delete	f_add: 1 f_append: 1 f_delete: 1 f_create: 1 f_verbose: 1
./a.out --v --c --ap --ad --d	f_add: 1 f_append: 1 f_delete: 1 f_create: 1 f_verbose: 1
./a.out -v -c -d -a	Unknown option v Unknown option c Unknown option d Unknown option a f_add: 0 f_append: 0 f_delete: 0 f_create: 0 f_verbose: 0

Recall the option structure:

struct option 
{
  const char *name;    /* name without -- in front                                  */
  int         has_arg; /* one of: no_argument, required_argument, optional_argument */
  int        *flag;    /* how the results are returned                              */
  int         val;     /* the value to return or to put in flag                     */
};

Notes:

If the flag field is not NULL, then the variable represented by the pointer will be used to store the value of the val field. This makes it convenient to set flags, without having to do it in a case label in the switch statement.
If you still want to handle the short options, you will have to include them in optstring and handle them in the switch statement yourself.
The flag variables must be outside of the function, either global or (preferably) file scope. (They could be static inside of the function, but would be less useful.) This is a limitation of the initialization of the structure fields in C.
Using global or file scope variables is generally not a problem because the flags will likely be used in other parts of the program, so they can't be local to this function.
The value of the val field can be any integer, but for flags, it's generally 1.

Summary

Behavior:

Default behavior is to act like POSIX and require all options before args. Using a minus sign (-) as the first character disables this.

Without the minus sign (-), all non-option args are moved to the end of the array and will be processed outside of the while loop. (See code above.) getopt ignores them during the parsing.
With the minus sign (-), the non-options are parsed as they are found. getopt returns 1 when a non-option arg is found.

By default, you can't distinguish between an unknown option and a missing (required) option arg. getopt returns a question mark (?) in both cases. Using a colon (:) as the first character causes getopt to:
return ? for an unknown option
return : for a missing required option arg
When using both the minus sign (-) and colon (:) in the option string, the minus sign (-) must come before the colon (:) at the front of the string.

Summary for short options:

If a short option has a required arg, you can use either of these first two:

-ofoo   (OK, space is not required)
-o foo  (OK, space is allowed)
-o=foo  (incorrect, the arg will be interpreted as '=foo')

If a short option has an optional arg, you can use either of these first two:

-o      (OK, no arg supplied, optarg is NULL)
-ofoo   (OK, optional arg supplied, optarg points to 'foo')
-o foo  (incorrect, foo is just a regular arg to the command)

Summary for long options:

If a long option has a required arg, you can use either of these first two:

--option=arg    (OK, equal sign with no spaces)
--option arg    (OK, space is allowed)
--option = arg  (incorrect, no spaces allowed with equal sign)
--optionarg     (incorrect, must have a space or an equal sign)

If a long option has an optional arg, you can use either of these first two:

--option      (OK, no arg supplied, optarg is NULL)
--option=arg  (OK, must use equal sign with no spaces)
--option arg  (incorrect, must use equal sign with no spaces)
--optionarg   (incorrect, must use equal sign with no spaces)

Other points:

Sometimes, you need to process the command line more than once. To do so, you must reset the global variable optind to 1 before calling getopt. Resetting this variable causes getopt to start over at the beginning of the command line.
Using the long options, you can add some shortcuts by setting flag to the address of a variable. Look at the end of section 2.3.3.1 in this document.
Some shells (like bash) will do auto-complete for long options. For example, at a command prompt type 'diff --str' (without the quotes) and then press the TAB key. Nice. Very Nice.

Other Uses For Command Line Arguments

Even if you think that you aren't going to use the command line or that no one else is going to run your program from the command line, there are other reasons why supporting command line arguments will make your program much nicer to use.

Uses in GUI Environments

Have you ever dragged and dropped a file in Windows, Mac, Linux, or any other graphical environment? (Of course you have.) Well, a lot of the functionality of drag and drop is implemented via command line arguments. That's right, command line arguments.

If you drag and drop a file onto an executable (or shortcut) in a graphical environment, you will notice that the executable is launched (run) and it opens the file that was dropped. So, if you drop a file onto Notepad.exe in Windows, Notepad will run and open the file that was dropped. This doesn't happen automatically. It works because the people that programmed Notepad provided a command line interface, which can also be used by GUI environments. (It's actually just another way to communicate with the program.)

If you dropped a file named foo.txt onto Notepad.exe, the GUI environment is really doing something like this:

Notepad.exe foo.txt

which is very similar to how you would run Notepad.exe from the command line and pass the name of the file, foo.txt, to the program.

Launching a Program from a Running Program

Most students have learned a simple trick that executing a program from within their C/C++ code can be done using the system function. If your program was running and you wanted to execute Notepad.exe (Windows, of course) from your program and have it edit foo.txt, you would do this:

system("Notepad foo.txt");

This will load Notepad.exe and pass the filename, foo.txt. to Notepad. Notepad.exe will parse the command line and realize that it is being asked to load the file foo.txt.

It is often a good idea to have your programs support command line arguments, even if you don't think that you or anyone else will ever actually execute the program from a command line. Command line arguments are a very simple way for one process to communicate with another and are supported on all operating systems in pretty much the same way.

Additional Information:

man page for getopt .
getoptgetopt on Wikipedia.
An introduction to getopt from Informit. (PDF file).
A version for Windows can be found here.