GNU/Linux C language: Command Options Parsing

1.GNU/Linux Command-Line Conventions

Almost all GNU/Linux programs obey some conventions about how command-line arguments are interpreted.The arguments that programs expect fall into two categories: options (or flags) and other arguments. Options modify how the program behaves, while other arguments provide inputs (for instance, the names of input files).

Options come in two forms:

● Short options consist of a single hyphen and a single character (usually a lowercase or uppercase letter). Short options are quicker to type.

● Long options consist of two hyphens, followed by a name made of lowercase and uppercase letters and hyphens. Long options are easier to remember and easier to read (in shell scripts, for instance).

Usually, a program provides both a short form and a long form for most options it supports, the former for brevity and the latter for clarity. For example, most programs understand the options -h and --help, and treat them identically. Normally, when a program is invoked from the shell, any desired options follow the program name immediately. Some options expect an argument immediately following. Many programs, for example, interpret the option --output foo to specify that output of the program should be placed in a file named foo. After the options, there may follow other command-line arguments, typically input files or input data.

For example, the command ls -s / displays the contents of the root directory.The -s option modifies the default behavior of ls by instructing it to display the size (in kilobytes) of each entry.The / argument tells ls which directory to list.The --size option is synonymous with -s, so the same command could have been invoked as ls --size /.

The GNU Coding Standards list the names of some commonly used command-line options. If you plan to provide any options similar to these, it’s a good idea to use the names specified in the coding standards.Your program will behave more like other programs and will be easier for users to learn.

2.Using getopt_long

Parsing command-line options is a tedious chore. Luckily, the GNU C library provides a function that you can use in C and C++ programs to make this job somewhat easier (although still a bit annoying).This function, getopt_long, understands both short and long options. If you use this function, include the header file <getopt.h>.

Suppose, for example, that you are writing a program that is to accept the three options shown in Table 2.1.

Table 2.1 Example Program Options

Short Form	Long Form	Purpose
-h	--help	Display usage summary and exit
-o filename	--output filename	Specify output filename
-v	--verbose	Print verbose messages

In addition, the program is to accept zero or more additional command-line arguments, which are the names of input files.

To use getopt_long, you must provide two data structures.The first is a character string containing the valid short options, each a single letter. An option that requires an argument is followed by a colon. For your program, the string ho:v indicates that the valid options are -h, -o, and -v, with the second of these options followed by an argument.

To specify the available long options, you construct an array of struct option elements. Each element corresponds to one long option and has four fields. In normal circumstances, the first field is the name of the long option (as a character string, without the two hyphens); the second is 1 if the option takes an argument, or 0 otherwise; the third is NULL; and the fourth is a character constant specifying the short option synonym for that long option.The last element of the array should be all zeros.You could construct the array like this:

const struct option long_options[] ={      { "help", 0, NULL, 'h'},      {"output", 1, NULL, 'o'},      { "verbose", 0, NULL, 'v'},      {NULL, 0, NULL, 0} };

 

You invoke the getopt_long function, passing it the argc and argv arguments to main, the character string describing short options, and the array of struct option elements describing the long options.

● Each time you call getopt_long, it parses a single option, returning the shortoption letter for that option, or –1 if no more options are found.

● Typically, you’ll call getopt_long in a loop, to process all the options the user has specified, and you’ll handle the specific options in a switch statement.

● If getopt_long encounters an invalid option (an option that you didn’t specify as a valid short or long option), it prints an error message and returns the character ? (a question mark). Most programs will exit in response to this, possibly after displaying usage information.

● When handling an option that takes an argument, the global variable optarg points to the text of that argument.

● After getopt_long has finished parsing all the options, the global variable optind contains the index (into argv) of the first nonoption argument. Listing 2.2 shows an example of how you might use getopt_long to process your arguments.

Listing 2.2 (getopt_long.c) Using getopt_long

#include <getopt.h> #include <stdio.h> #include <stdlib.h> /* The name of this program. */ const char* program_name; /* Prints usage information for this program to STREAM (typically  stdout or stderr), and exit the program with EXIT_CODE. Does not  return. */ void print_usage(FILE* stream, int exit_code) {      fprintf(stream, "Usage: %s options [ inputfile ... ]\n", program_name);      fprintf(stream, " -h --help Display this usage information.\n"          " -o --output filename Write output to file.\n"          " -v --verbose Print verbose messages.\n");      exit(exit_code); } /* Main program entry point. ARGC contains number of argument list  elements; ARGV is an array of pointers to them. */ int main(int argc, char* argv[]) {      int next_option;      /* A string listing valid short options letters. */      const char* const short_options = "ho:v";      /* An array describing valid long options. */      const struct option long_options[] = { { "help", 0, NULL, 'h' }, {               "output", 1, NULL, 'o' }, { "verbose", 0, NULL, 'v' }, { NULL, 0,               NULL, 0 } /* Required at end of array. */      };      /* The name of the file to receive program output, or NULL for       standard output. */      const char* output_filename = NULL;      /* Whether to display verbose messages. */      int verbose = 0;      /* Remember the name of the program, to incorporate in messages.       The name is stored in argv[0]. */      program_name = argv[0];      do {          next_option                    = getopt_long(argc, argv, short_options, long_options, NULL);          switch (next_option) {          case 'h': /* -h or --help */               /* User has requested usage information. Print it to standard                output, and exit with exit code zero (normal termination). */               print_usage(stdout, 0);          case 'o': /* -o or --output */               /* This option takes an argument, the name of the output file. */               output_filename = optarg;               break;          case 'v': /* -v or --verbose */               verbose = 1;               break;          case '?': /* The user specified an invalid option. */               /* Print usage information to standard error, and exit with exit                code one (indicating abnormal termination). */               print_usage(stderr, 1);          case -1: /* Done with options. */               break;          default: /* Something else: unexpected. */               abort();          }      } while (next_option != -1);      /* Done with options. OPTIND points to first nonoption argument.       For demonstration purposes, print them if the verbose option was       specified. */      if (verbose) {          int i;          for (i = optind; i < argc; ++i)               printf("Argument: %s\n", argv[i]);      }      /* The main program goes here. */      return 0; }

Using getopt_long may seem like a lot of work, but writing code to parse the command-line options yourself would take even longer.The getopt_long function is very sophisticated and allows great flexibility in specifying what kind of options to accept. However, it’s a good idea to stay away from the more advanced features and stick with the basic option structure described.