Getopt optional arguments? - c

Question

I have a program where you enter an option
-d
and then whether or not you supply a non-optional argument after the option, do something.
Heres my code:
#include <stdio.h>
#include <getopt.h>
#include <stdlib.h>
#define OPT_LIST "d::"
int main (int argc, char *argv[])
{
int c;
char string[] = "blah";
while ((c = getopt (argc, argv, OPT_LIST)) != -1)
{
switch (c)
{
case 'd':
printf("%s\n", optarg);
break;
case '?':
fprintf(stderr, "invalid option\n");
exit(EXIT_FAILURE);
}
}
}
So if you enter a non-optional argument after the option, it prints the argument. But I want it to print out the char "string" if the user doesn't supply a non-optional argument (this is why I put the double colon in the
OPT_LIST). But I'm not sure how to do this so any help would be greatly appreciated.
Heres what happens when I run the program:
user:desktop shaun$ ./arg -d hello
hello
user:desktop shaun$ ./arg -d
./arg: option requires an argument -- d
invalid option
I'm running a Mac with OS X using C language.

Answer 1

The "optional value of an option" feature is only a GNU libc extension, not required by POSIX, and is probably simply unimplemented by the libc shipped with Mac OS X.
The options argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (‘:’) to indicate that it takes a required argument. If an option character is followed by two colons (‘::’), its argument is optional; this is a GNU extension.
https://www.gnu.org/software/libc/manual/html_node/Using-Getopt.html
In fact, POSIX.1-2008, section 12.2, "Utility Syntax Guidelines", explicitly forbids this feature:
Guideline 7: Option-arguments should not be optional.
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02

Answer 2

According to the getopt documentation, it will return : if an option with an argument does not have one. It also sets optopt with the matching argument.
Therefore, use:
int main (int argc, char *argv[])
{
int c;
while ((c = getopt (argc, argv, ":d:f:")) != -1)
{
switch (c)
{
case 'd':
case 'f':
printf("option -%c with argument '%s'\n", c, optarg);
break;
case ':':
switch (optopt)
{
case 'd':
printf("option -%c with default argument value\n", optopt);
break;
default:
fprintf(stderr, "option -%c is missing a required argument\n", optopt);
return EXIT_FAILURE;
}
break;
case '?':
fprintf(stderr, "invalid option: -%c\n", optopt);
return EXIT_FAILURE;
}
}
return EXIT_SUCCESS;
}

Answer 3

Maybe I'm misunderstanding the question. I'm reading it as though the user wants to over ride getopt's default error handling. If that is the case, shouldn't there be a : in the beginning of their OPT_LIST? I think the code above is good, however, I think it will still print
./arg: option requires an argument -- d
To surpress that, don't we need a : at the beginning of the OPT_LIST? For example, changing this:
while ((c = getopt (argc, argv, "d:f:")) != -1)
to this:
while ((c = getopt (argc, argv, ":d:f:")) != -1)
Correct me if I'm wrong.

Answer 4

Try this solution. It works for me. Consider option 'z' as option with optional argument.
int c;
opterr = 0; //if (opterr != 0) (which it is by default), getopt() prints its own error messages for invalid options and for missing option arguments.
while ((c = getopt (argc, argv, "x:y:z:")) != -1)
switch (c)
{
case 'x':
case 'y':
case 'z':
printf("OK ... option -%c with argument '%s'\n", c, optarg);
break;
case '?':
if (optopt == 'x' || optopt == 'y')
fprintf (stderr, "ERR ... Option -%c requires an argument.\n", optopt);
else if(optopt == 'z' && isprint(optopt))
{
printf("OK ... option '-z' without argument \n");
break;
}
else if (isprint (optopt))
fprintf (stderr, "ERR ... Unknown option `-%c'.\n", optopt);
else
fprintf (stderr, "ERR ... Unknown option character `\\x%x'.\n", optopt);
return -1;
default: ;
}
Here are some examples:
./prog -x
ERR ... Option -x requires an argument.
./prog -y
ERR ... Option -x requires an argument.
./prog -z
OK ... option '-z' without argument
./prog -x aaa
OK ... option -x with argument 'aaa'
./prog -y bbb -x aaa
OK ... option -y with argument 'bbb'
OK ... option -x with argument 'aaa'
./prog -x aaa -y bbb -z
OK ... option -x with argument 'aaa'
OK ... option -y with argument 'bbb'
OK ... option '-z' without argument
./prog -x aaa -y bbb -z ccc
OK ... option -x with argument 'aaa'
OK ... option -y with argument 'bbb'
OK ... option -z with argument 'ccc'