Question

Please explain answer so I can understand.


1. Write a C program that prints out its command line arguments, one to a line.

Discussion: A C program starts at a function called main. main has two arguments, conventionally named argc and argv. argc is of type int; it’s the argument count, an integer that contains the number of command line arguments (including the program name). argv is the argument vector; it’s an array of pointers to characters, In C this is described as char *argv[] (or, equivalently and confusingly, char **argv - because C doesn’t really have arrays.)

Imagine that the command line is

% myprog pizza burrito kale

Then what’s actually happening is that somewhere in memory, when the program begins execution, the shell (working with the C runtime library) has arranged that the strings “myprog”, “pizza”, “burrito”, “kale” are stored somewhere in memory, AND that a table of four addresses - the addresses of the first characters of those strings - is also stored somewhere in memory. These addresses are pointers. In principle these strings can be anywhere but in practice they occupy consecutive memory locations. The diagram on the next page shows a picture in which I have arbitrarily chosen location 1128 as the address of the ‘m’ in “myprog” and 2020 as the address of the (nameless) table where the pointers to the strings are stored. In this example, the value of argc is 4 and the value of argv is 2020. The value of *argv is 1128. (What is its type?)The value of **argv is ‘m’. (What is its type? What is the value of argv[1]?)

2. Modify your program to print the arguments in reverse order.

Answer 1

The C program along with the comments is given below.

#include <stdio.h>

//arguments to the main function: argc and argv
int main(int argc, char** argv)
{
    //display the contents of argv using for loop
    printf("Arguments in the same order as given:-\n");
    for (int i = 0; i < argc; i++)
        printf("%s\n", argv[i]);
      
    //display the contents of argv in the reverse order using for loop [Modification]
    printf("\nArguments in the reverse order as given:-\n");
    for(int i=argc-1; i>=0; i--)
        printf("%s\n", argv[i]);
      
    return 0;
}

If the indendations are not clear, then please refer the screenshots of the code given below.

Command line arguments given were as follows:-

Output obtained is also given below.

Explanation:-

• Inorder to pass the command line arguments, the main function must be set to receive the two paramaters viz. argc and argv.
• The argc is an integer variable that stores the total number of arguments passed. The argv is a vector containing character pointers to the strings.
• The first argument i.e. argv[0] will be the program name. Rest of them will be followed by the given arguments.
• Using a for loop, iterate a variable i from 0 to argc-1 and print the content of argv[i]. This will display the contents of the argv array.
• To print the contents in the reverse order, iterate i from argc-1 to 0 using a for loop and print the argv[i].

Hope this helps. Doubts, if any, can be asked in the comments.