getopt command line get values c++ - c++

Question

How do you get the values of the flags in getopt, I have tried googling it but all I got is on how to switch cases and set flags. I got the below code and what I am trying to do is to have three flags, -a, -b, -c but only two flags are passed like ./filename -a somevalue -c anothervalue or ./filename -b somevalue -c anothervalue any help?
int main(int argc, char *argv[])
{
int flagA = 0;
int flagB = 0;
while (1) {
char c;
c = getopt (argc, argv, "abc:");
if (c == -1) {
break;
}
switch (c) {
case 'a':
flagA = 1;
//cout<<optarg<<endl; //I tried printing the value but it only prints the second flag value
break;
case 'b':
flagB = 1;
cout<<optarg<<endl;
break;
case 'c':
cout<<optarg<<endl;
break;
case '?':
default:
cout<<"Usage: %s [-a] [-b <something>].\n", argv[0]<<endl;
}
if(flagA > 0){
//do something using the values of flagA and flagC
}
else if(flagB > 0){
//do something using the values of flagB and flagC
}
}
return 0;
}

Answer 1

If you intend to use optarg with a flag, the flag must have : after it in the option string.
c = getopt(argc, argv, "a:b:c:");
Your usage output statement should not have compiled for you. Also, you are attempting to mix C style format specifiers into a C++ output stream, which is incorrect.
cout << "Usage: " << argv[0] << " [-a] [-b <something>].\n";

Answer 2

There are multiple problems with the code, but the key one is in your invocation of getopt(). You have the option control string "abc:", which means that neither -a nor -b takes an option.
Assuming you're using a POSIX standard version of getopt() and not GNU getopt(), then when you run either:
./filename -a somevalue -c anothervalue
./filename -b somevalue -c anothervalue
the somevalue after the first flag means that options are finished and the remaining arguments are non-option arguments. GNU getopt(), in the absence of the environment variable POSIXLY_CORRECT, will permute the argument list so that -c anothervalue is also recognized (and the arguments are permuted such that somevalue ends up at the end).
Your code would be better written:
#define _XOPEN_SOURCE 600
#include <iostream>
#include <unistd.h>
using namespace std;
int main(int argc, char *argv[])
{
int flagA = 0;
int flagB = 0;
int opt;
char *c_opt = 0;
while ((opt = getopt(argc, argv, "abc:")) != -1)
{
switch (opt)
{
case 'a':
flagA = 1;
cout << "A: " << flagA << "\n";
break;
case 'b':
flagB = 1;
cout << "B: " << flagB << "\n";
break;
case 'c':
c_opt = optarg;
cout << "C: " << c_opt << "\n";
break;
default:
cerr << "Usage: " << argv[0] << " [-a] [-b] [-c <something>]\n";
return -1;
}
}
if (flagA)
{
// do something using the values of flagA and c_opt (if set)
}
else if (flagB)
{
// do something using the values of flagB and c_opt (if set)
}
else if (c_opt)
{
// do something using just c_opt (neither flagA nor flagB is set)
}
else
{
// do something if no options are specified (report error?)
}
for (int i = optind; i < argc; i++)
cout << "File: " << i << ": " << argv[i] << "\n";
return 0;
}
The code above (as amended) compiles cleanly with GCC 4.9.1 on Mac OS X 10.10 (Yosemite):
g++ -O3 -g -std=c++11 -Wall -Wextra -Werror opt.cpp -o opt
The BSD-ish getopt() hews close to the POSIX standard, so the result I get is:
$ ./opt -a 5 -c something
A: 1
File: 2: 5
File: 3: -c
File: 4: something
$
florw.wat commented:
I want the -a and -b options to take an argument. What I am trying to do is I have two functions which take two arguments, so if -a is set then I would like to call function a else if -b then call function b. I need the two values to call the functions.
Given that clarification, the code might look like:
#define _XOPEN_SOURCE 600
#include <cstdlib>
#include <iostream>
#include <unistd.h>
using namespace std;
static void usage(const char *arg0)
{
cerr << "Usage: " << arg0 << " [-a filename | -b filename] -c <something>\n";
exit(EXIT_FAILURE);
}
int main(int argc, char *argv[])
{
char *a_opt = 0;
char *b_opt = 0;
char *c_opt = 0;
int opt;
while ((opt = getopt(argc, argv, "a:b:c:")) != -1)
{
switch (opt)
{
case 'a':
a_opt = optarg;
cout << "A: " << a_opt << "\n";
break;
case 'b':
b_opt = optarg;
cout << "B: " << b_opt << "\n";
break;
case 'c':
c_opt = optarg;
cout << "C: " << c_opt << "\n";
break;
default:
cerr << "Unknown option: " << optopt << "\n";
usage(argv[0]);
break;
}
}
if (optind != argc || c_opt == 0 || (a_opt && b_opt) || (a_opt == 0 && b_opt == 0))
usage(argv[0]);
else if (a_opt)
cout << "A&C: " << a_opt << " " << c_opt << "\n";
else
cout << "B&C: " << b_opt << " " << c_opt << "\n";
return 0;
}
Sample run:
$ ./opt -a option -c something
A: option
C: something
A&C: option something
$