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Tuesday 18 May 2010

Command Line Arguments

  • Getting the arguments from command prompt in c is known as command line arguments.
  • Command line is that it consists of a sequence of words,typically separated by whitespace.Main program can receive these words as an array of strings,one word per string.
  • main function will accept 2 parameters ,argv and argc
  • argc will be the count of the number of strings given on the command line.
  • argv will be the array of the arguments.since each word is a string argv is an array of pointers to char
  • Example:
        int main(int argc,char *argv[]){......}
  • The strings at the command line are stored in memory and address of the first string is stored in argv[0],address of the second string is stored in argv[1] and so on.
  • we can give any name instead of argv and argc
       main(int count,char*str[]){.....}
  • Example Program
    int main(int argc,char *argv[])
        int i;
        printf("Number of Arguments passed=%d\n",argc);
    sunitha@sunitha-laptop:~/ds$ ./a.out g h k
    Number of Arguments passed=4
Command Line Arguments  in Unix/Linux:
  • Most Unix/Linux applications lets the user to specify command-line arguments (parameters) in 2 forms

      * Short options
      consist of a - character followed by a single alphanumeric character
      for example for listing the files the command is ls -l
      * Long options (common in GNU applications)
      consists of two - characters (--)  followed by a string made up of letters, numbers, and hyphens.
  • Either type of option may be followed by an argument.
  • A space separates a short option from its arguments
  • Either a space or an = separates a long option from an argument.
  • GNU C provides 2 functions, getopt and getopt_long to parse the command line args specified in the above format
  • getopt - supports parsing only short options
  • getopt_long - supports parsing of both short and long options.

    In this blog, we would focus only on getopt_long function to parse both long and short options.
  • This function is defined in getopt.h

    int *getopt_long* (int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr)

     while (getopt_long (argc, argv, shortopts, longopts, index) != -1) {

    argc and argv are the command line args passed to main
    shortopts:  This is a string containing all the valid short option characters
               An option character can be followed by
                   a colon (:)  to indicate that it takes a required argument. (or)
                   2 colon (::) to indicate that its argument is optional                                                              shortopts can begin with a + to indicate that getopts_long should stop processing on sensing an unexpected arg
               Ex: If we have have a program that can accept the short args -a and -b, the option string would be "ab"
                   Now, if we have to force a mandatory required parameter for -a, but not for -b, the option string would be "a:b::"

    longopts: This is a array of structure, containing the following fields
                   char * , int , int* , int
    long option string  - The command line parameter in long format
                       type                - one of no_argument, required_argument or optional_argument
                       flag                - Set it to null for non-flag parameters
                                             For flag, set it to the address of an int, which should get set when the flag is mentioned in the cli
                       val                 - For flags, set it to the default value the flag should be initialized to
                                             For non-flags, set it to a number, that would uniquely identify this option
               The array has to be terminated with all zeros


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