error: invalid conversion from ‘void*’ to ‘arguments*’ [-fpermissive] - c

Question

I am trying to study the examples of GNU C Library Code,to be specific, the usage of Argp and try it, but when I try
Argp-Example-3
Gcc hints me there is an error, but I do just copy the code from the web site. Here's the message?
argex.C: In function ‘error_t parse_opt(int, char*, argp_state*)’:
argex.C:92:45: error: invalid conversion from ‘void*’ to ‘arguments*’ [-fpermissive]
struct arguments *arguments = state->input;
^
argex.C: In function ‘int main(int, char**)’:
argex.C:138:30: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
arguments.output_file = "-";
^
argex.C is the file name.
It seems that I am converting 'void' to 'arguments'??
Thank you!
Heres the souce code
/* This program uses the same features as example 2, and uses options and
arguments.
We now use the first four fields in ARGP, so here's a description of them:
OPTIONS -- A pointer to a vector of struct argp_option (see below)
PARSER -- A function to parse a single option, called by argp
ARGS_DOC -- A string describing how the non-option arguments should look
DOC -- A descriptive string about this program; if it contains a
vertical tab character (\v), the part after it will be
printed *following* the options
The function PARSER takes the following arguments:
KEY -- An integer specifying which option this is (taken
from the KEY field in each struct argp_option), or
a special key specifying something else; the only
special keys we use here are ARGP_KEY_ARG, meaning
a non-option argument, and ARGP_KEY_END, meaning
that all arguments have been parsed
ARG -- For an option KEY, the string value of its
argument, or NULL if it has none
STATE-- A pointer to a struct argp_state, containing
various useful information about the parsing state; used here
are the INPUT field, which reflects the INPUT argument to
argp_parse, and the ARG_NUM field, which is the number of the
current non-option argument being parsed
It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the
given KEY wasn't recognized, or an errno value indicating some other
error.
Note that in this example, main uses a structure to communicate with the
parse_opt function, a pointer to which it passes in the INPUT argument to
argp_parse. Of course, it's also possible to use global variables
instead, but this is somewhat more flexible.
The OPTIONS field contains a pointer to a vector of struct argp_option's;
that structure has the following fields (if you assign your option
structures using array initialization like this example, unspecified
fields will be defaulted to 0, and need not be specified):
NAME -- The name of this option's long option (may be zero)
KEY -- The KEY to pass to the PARSER function when parsing this option,
*and* the name of this option's short option, if it is a
printable ascii character
ARG -- The name of this option's argument, if any
FLAGS -- Flags describing this option; some of them are:
OPTION_ARG_OPTIONAL -- The argument to this option is optional
OPTION_ALIAS -- This option is an alias for the
previous option
OPTION_HIDDEN -- Don't show this option in --help output
DOC -- A documentation string for this option, shown in --help output
An options vector should be terminated by an option with all fields zero. */
#include <stdlib.h>
#include <argp.h>
const char *argp_program_version =
"argp-ex3 1.0";
const char *argp_program_bug_address =
"<bug-gnu-utils#gnu.org>";
/* Program documentation. */
static char doc[] =
"Argp example #3 -- a program with options and arguments using argp";
/* A description of the arguments we accept. */
static char args_doc[] = "ARG1 ARG2";
/* The options we understand. */
static struct argp_option options[] = {
{"verbose", 'v', 0, 0, "Produce verbose output" },
{"quiet", 'q', 0, 0, "Don't produce any output" },
{"silent", 's', 0, OPTION_ALIAS },
{"output", 'o', "FILE", 0,
"Output to FILE instead of standard output" },
{ 0 }
};
/* Used by main to communicate with parse_opt. */
struct arguments
{
char *args[2]; /* arg1 & arg2 */
int silent, verbose;
char *output_file;
};
/* Parse a single option. */
static error_t
parse_opt (int key, char *arg, struct argp_state *state)
{
/* Get the input argument from argp_parse, which we
know is a pointer to our arguments structure. */
struct arguments *arguments = state->input;
switch (key)
{
case 'q': case 's':
arguments->silent = 1;
break;
case 'v':
arguments->verbose = 1;
break;
case 'o':
arguments->output_file = arg;
break;
case ARGP_KEY_ARG:
if (state->arg_num >= 2)
/* Too many arguments. */
argp_usage (state);
arguments->args[state->arg_num] = arg;
break;
case ARGP_KEY_END:
if (state->arg_num < 2)
/* Not enough arguments. */
argp_usage (state);
break;
default:
return ARGP_ERR_UNKNOWN;
}
return 0;
}
/* Our argp parser. */
static struct argp argp = { options, parse_opt, args_doc, doc };
int
main (int argc, char **argv)
{
struct arguments arguments;
/* Default values. */
arguments.silent = 0;
arguments.verbose = 0;
arguments.output_file = "-";
/* Parse our arguments; every option seen by parse_opt will
be reflected in arguments. */
argp_parse (&argp, argc, argv, 0, 0, &arguments);
printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
"VERBOSE = %s\nSILENT = %s\n",
arguments.args[0], arguments.args[1],
arguments.output_file,
arguments.verbose ? "yes" : "no",
arguments.silent ? "yes" : "no");
exit (0);
}

Answer 1

You should use a lowercase "c" for c files, i.e. argex.c, not argex.C. The uppercase C indicates a c++ file, and the implicit void* to anything conversion doesn't work in c++.
You could also add -x c to the command line (before the file name), but you really should use the correct extension.

Answer 2

To fix the implicit cast, if your compiling for c++, you also need to:
...
parse_opt (int key, char *arg, struct argp_state *state)
{
/* Get the input argument from argp_parse, which we
know is a pointer to our arguments structure. */
struct arguments *arguments = static_cast<struct arguments*>(state->input);
...

Answer 3

I am using the wrong file name. I should use ".c" but I missed using ".C".