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argp example

Here is a code example that uses argp to parse command-line options. Remember, to compile this example, copy it to a file called something like argex.c, then compile it with the command gcc -o argex argex.c and run the resulting binary with the command ./argex.

#include <stdio.h>
#include <argp.h>

const char *argp_program_version =
"argex 1.0";

const char *argp_program_bug_address =
"<bug-gnu-utils@gnu.org>";

/* This structure is used by main to communicate with parse_opt. */
struct arguments
{
  char *args[2];            /* ARG1 and ARG2 */
  int verbose;              /* The -v flag */
  char *outfile;            /* Argument for -o */
  char *string1, *string2;  /* Arguments for -a and -b */
};

/*
   OPTIONS.  Field 1 in ARGP.
   Order of fields: {NAME, KEY, ARG, FLAGS, DOC}.
*/
static struct argp_option options[] =
{
  {"verbose", 'v', 0, 0, "Produce verbose output"},
  {"alpha",   'a', "STRING1", 0,
   "Do something with STRING1 related to the letter A"},
  {"bravo",   'b', "STRING2", 0,
   "Do something with STRING2 related to the letter B"},
  {"output",  'o', "OUTFILE", 0,
   "Output to OUTFILE instead of to standard output"},
  {0}
};


/*
   PARSER. Field 2 in ARGP.
   Order of parameters: KEY, ARG, STATE.
*/
static error_t
parse_opt (int key, char *arg, struct argp_state *state)
{
  struct arguments *arguments = state->input;

  switch (key)
    {
    case 'v':
      arguments->verbose = 1;
      break;
    case 'a':
      arguments->string1 = arg;
      break;
    case 'b':
      arguments->string2 = arg;
      break;
    case 'o':
      arguments->outfile = arg;
      break;
    case ARGP_KEY_ARG:
      if (state->arg_num >= 2)
	{
	  argp_usage(state);
	}
      arguments->args[state->arg_num] = arg;
      break;
    case ARGP_KEY_END:
      if (state->arg_num < 2)
	{
	  argp_usage (state);
	}
      break;
    default:
      return ARGP_ERR_UNKNOWN;
    }
  return 0;
}

/*
   ARGS_DOC. Field 3 in ARGP.
   A description of the non-option command-line arguments
     that we accept.
*/
static char args_doc[] = "ARG1 ARG2";

/*
  DOC.  Field 4 in ARGP.
  Program documentation.
*/
static char doc[] =
"argex -- A program to demonstrate how to code command-line options
and arguments.\vFrom the GNU C Tutorial.";

/*
   The ARGP structure itself.
*/
static struct argp argp = {options, parse_opt, args_doc, doc};

/*
   The main function.
   Notice how now the only function call needed to process
   all command-line options and arguments nicely
   is argp_parse.
*/
int main (int argc, char **argv)
{
  struct arguments arguments;
  FILE *outstream;

  char waters[] =
"a place to stay
enough to eat
somewhere old heroes shuffle safely down the street
where you can speak out loud
about your doubts and fears
and what's more no-one ever disappears
you never hear their standard issue kicking in your door
you can relax on both sides of the tracks
and maniacs don't blow holes in bandsmen by remote control
and everyone has recourse to the law
and no-one kills the children anymore
and no-one kills the children anymore
  --\"the gunners dream\", Roger Waters, 1983\n";

  /* Set argument defaults */
  arguments.outfile = NULL;
  arguments.string1 = "";
  arguments.string2 = "";
  arguments.verbose = 0;

  /* Where the magic happens */
  argp_parse (&argp, argc, argv, 0, 0, &arguments);

  /* Where do we send output? */
  if (arguments.outfile)
    outstream = fopen (arguments.outfile, "w");
  else
    outstream = stdout;

  /* Print argument values */
  fprintf (outstream, "alpha = %s\nbravo = %s\n\n",
	   arguments.string1, arguments.string2);
  fprintf (outstream, "ARG1 = %s\nARG2 = %s\n\n",
	   arguments.args[0],
	   arguments.args[1]);

  /* If in verbose mode, print song stanza */
  if (arguments.verbose)
    fprintf (outstream, waters);

  return 0;
}

Compile the code, then experiment! For example, here is the program output if you simply type argex:

Usage: argex [OPTION...] ARG1 ARG2
Try `argex --help' or `argex --usage' for more information.

Here is the output from argex --usage:

Usage: argex [-v?V] [-a STRING1] [-b STRING2] [-o OUTFILE] [--alpha=STRING1]
            [--bravo=STRING2] [--output=OUTFILE] [--verbose] [--help] [--usage]
            [--version] ARG1 ARG2

Here is the output from argex --help:

Usage: argex [OPTION...] ARG1 ARG2
argex -- A program to demonstrate how to code command-line options
and arguments.

  -a, --alpha=STRING1        Do something with STRING1 related to the letter A
  -b, --bravo=STRING2        Do something with STRING2 related to the letter B
  -o, --output=OUTFILE       Output to OUTFILE instead of to standard output
  -v, --verbose              Produce verbose output
  -?, --help                 Give this help list
      --usage                Give a short usage message
  -V, --version              Print program version

Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.

From the GNU C Tutorial.

Report bugs to <bug-gnu-utils@gnu.org>.

Here is the output from argex Foo Bar:

alpha =
bravo =

ARG1 = Foo
ARG2 = Bar

And finally, here is the output from argex --verbose -a 123 --bravo=456 Foo Bar:

alpha = 123
bravo = 456

ARG1 = Foo
ARG2 = Bar

a place to stay
enough to eat
somewhere old heroes shuffle safely down the street
where you can speak out loud
about your doubts and fears
and what's more no-one ever disappears
you never hear their standard issue kicking in your door
you can relax on both sides of the tracks
and maniacs don't blow holes in bandsmen by remote control
and everyone has recourse to the law
and no-one kills the children anymore
and no-one kills the children anymore
  --"the gunners dream", Roger Waters, 1983

You can of course also send the output to a text file with the -o or --output option.