Pass arguments into C program from command line

Question

So I'm in Linux and I want to have a program accept arguments when you execute it from the command line.

For example,

./myprogram 42 -b -s

So then the program would store that number 42 as an int and execute certain parts of code depending on what arguments it gets like -b or -s.

Solution 1

You could use getopt.

 #include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>

 int
 main (int argc, char **argv)
 {
   int bflag = 0;
   int sflag = 0;
   int index;
   int c;

   opterr = 0;

   while ((c = getopt (argc, argv, "bs")) != -1)
     switch (c)
       {
       case 'b':
         bflag = 1;
         break;
       case 's':
         sflag = 1;
         break;
       case '?':
         if (isprint (optopt))
           fprintf (stderr, "Unknown option `-%c'.\n", optopt);
         else
           fprintf (stderr,
                    "Unknown option character `\\x%x'.\n",
                    optopt);
         return 1;
       default:
         abort ();
       }

   printf ("bflag = %d, sflag = %d\n", bflag, sflag);

   for (index = optind; index < argc; index++)
     printf ("Non-option argument %s\n", argv[index]);
   return 0;
 }

Solution 2

In C, this is done using arguments passed to your main() function:

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

More information can be found online such as this Arguments to main article.

Solution 3

Consider using getopt_long(). It allows both short and long options in any combination.

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

/* Flag set by `--verbose'. */
static int verbose_flag;

int
main (int argc, char *argv[])
{
  while (1)
    {
      static struct option long_options[] =
    {
      /* This option set a flag. */
      {"verbose", no_argument,       &verbose_flag, 1},
      /* These options don't set a flag.
         We distinguish them by their indices. */
      {"blip",    no_argument,       0, 'b'},
      {"slip",    no_argument,       0, 's'},
      {0,         0,                 0,  0}
    };
      /* getopt_long stores the option index here. */
      int option_index = 0;

      int c = getopt_long (argc, argv, "bs",
               long_options, &option_index);

      /* Detect the end of the options. */
      if (c == -1)
    break;

      switch (c)
    {
    case 0:
      /* If this option set a flag, do nothing else now. */
      if (long_options[option_index].flag != 0)
        break;
      printf ("option %s", long_options[option_index].name);
      if (optarg)
        printf (" with arg %s", optarg);
      printf ("\n");
      break;
    case 'b':
      puts ("option -b\n");
      break;
    case 's':
      puts ("option -s\n");
      break;
    case '?':
      /* getopt_long already printed an error message. */
      break;

    default:
      abort ();
    }
    }

  if (verbose_flag)
    puts ("verbose flag is set");

  /* Print any remaining command line arguments (not options). */
  if (optind < argc)
    {
      printf ("non-option ARGV-elements: ");
      while (optind < argc)
    printf ("%s ", argv[optind++]);
      putchar ('\n');
    }

  return 0;
}

Related:

• Which command line commands style do you prefer?
• What is the general syntax of a Unix shell command?

Solution 4

Take a look at the getopt library; it's pretty much the gold standard for this sort of thing.

Solution 5

Instead of getopt(), you may also consider using argp_parse() (an alternative interface to the same library).

From libc manual:

getopt is more standard (the short-option only version of it is a part of the POSIX standard), but using argp_parse is often easier, both for very simple and very complex option structures, because it does more of the dirty work for you.

But I was always happy with the standard getopt.

N.B. GNU getopt with getopt_long is GNU LGPL.

View more solutions

Comments

[OscarRyz]

Shouldn't He/She understand "main arguments passing" first? ;)

[Jonathan Leffler]

It will work on Linux because the getopt() function is GNU getopt() and you don't normally set POSIXLY_CORRECT in the environment, and GNU getopt() then processes option arguments before 'file' arguments, even when they follow a file argument as in the example. On POSIX platforms, it won't work....

[Jonathan Leffler]

....because the 42 will stop the -b and -s options from being interpreted as options. Of course, the design of the command line is bad too. I'll note that in the comments to the question.

[Jonathan Leffler]

Sorry - that's not a dreadfully good x-ref. There's a bug in the statement that "The declaration of the argv argument is often a novice programmer's first encounter with pointers to arrays of pointers and can prove intimidating" (argv is an array of pointers, not a pointer to an array of pointers).

[Jonathan Leffler]

Additionally, the next page shows an ad hoc option parser instead of using the standard getopt() or getopt_long() parsers - which is simply bad advice. No - it is not a good reference.

[jfs]

@Jonathan: The question does have the tag linux. Therefore GNU getopt() example is appropriate here.

[dmckee --- ex-moderator kitten]

"getopt is GNU LGPL": That depends on the getopt. It has been implemented several times. The one in Mac OS X is BSD licensed.

[Jonathan Leffler]

@J F Sebastian - yes, it does have a Linux tag; that's why it has an upvote from me. The structure of a command line and arguments ordering should not rely on that quirk; well, I still have to work across other platforms, and GNU getopt is not standard on other platforms (for all it is available).

[Jonathan Leffler]

...continued...As ever, if you do something consciously, knowing what is happening, it is usually OK. If you are unaware of it, you get nasty surprises sooner or later.

[Jonathan Leffler]

And AT&T released one in the mid-80s into the public domain, or something very close to the public domain. D McKee's point is very valid - GNU getopt() [and getopt_long()] are LGPL (or, older versions are GPL); not all versions of getopt() are GPL or LGPL.

[Shannon Nelson]

In C, a reference to an array is an address, just like a pointer is an address. Thus, argv can be referred to as both "an array" and "a pointer to an array". This is one of the beautiful simplicities of C as well as one of the points of confusions.