Argc & Argv Demo

All programs have an entry point. In C++, the entry point to our program is the main function.

#include <iostream>

using namespace std;

int main(int argc, char const *argv[]) {
  cout << "Hello World!\n"
  return 0;
}

In this hello world program, the main function has the prototype int main(int argc, char const *argv[]). This means that the function's name is main, it's return type is int, and it has a named and typed parameter list.

The parameter list is (int argc, char const *argv[]). There are two parameters. We can also call the parameters arguments. The first parameter is named argc and it is an int type. The second parameter is named argv and it is a char const* [] type.

It may appear that argv is very complicated yet it is just a list of words. If you type in the command ./argc_argv_demo cat dog rhino bird sheep, then argv will be created by the operating system and initialized for you to contain the list of words "cat", "dog", "rhino", "bird", "sheep". This means that argv will have 5 items in it and they will be numbered from 0 through 4. (Remember in C++, we start counting from 0.)

To access something that is entered on the command line, we just need to walk from the start of the list at location 0 all the way to the end. The way we know the length of argv is by using argc. The variable argc is a count of how many strings (or words) are stored in argv.

With some practice, you can write programs that will read in arguments from the command line.

How to Build

This example program has a Makefile which you may use to build the program argc_argv_demo. The Makefile has the following targets:

• all: builds the project
• clean: removes object and dependency files
• spotless: removes everything the clean target removes and all binaries
• format: outputs a diff showing where your formatting differes from the Google C++ style guide
• tidy: output of the linter to give you tips on how to improve your code
• headercheck: check to make sure your files have the appropriate header

To build the program use the make command. The Makefile's default target is to build all.

Example Output

Below is an example of what to expect when using the program argc_argv_demo.

$ make
set -e; clang++ -MM -g -Wall -pipe -std=c++14  argc_argv_demo.cc \
	| sed 's/\(argc_argv_demo\)\.o[ :]*/\1.o argc_argv_demo.d : /g' > argc_argv_demo.d; \
	[ -s argc_argv_demo.d ] || rm -f argc_argv_demo.d
clang++ -g -Wall -pipe -std=c++14  -c argc_argv_demo.cc
clang++ -g -Wall -pipe -std=c++14 -o argc_argv_demo argc_argv_demo.o 
$ ./argc_argv_demo 
The command that was executed is: "./argc_argv_demo"
The argument count (argc) is 1
Argv stores the command line arguments as strings...
argv[0] is "./argc_argv_demo"
$ ./argc_argv_demo cat dog rhino bird sheep
The command that was executed is: "./argc_argv_demo cat dog rhino bird sheep"
The argument count (argc) is 6
Argv stores the command line arguments as strings...
argv[0] is "./argc_argv_demo"
argv[1] is "cat"
argv[2] is "dog"
argv[3] is "rhino"
argv[4] is "bird"
argv[5] is "sheep"
$ ./argc_argv_demo cat blue 1.123 orange 4 yellow 0.123 red -4.87
The command that was executed is: "./argc_argv_demo cat blue 1.123 orange 4 yellow 0.123 red -4.87"
The argument count (argc) is 10
Argv stores the command line arguments as strings...
argv[0] is "./argc_argv_demo"
argv[1] is "cat"
argv[2] is "blue"
argv[3] is "1.123"
argv[4] is "orange"
argv[5] is "4"
argv[6] is "yellow"
argv[7] is "0.123"
argv[8] is "red"
argv[9] is "-4.87"
$