[Unit 3] Command Line Argument

int main(int argc, char* argv[]) or int main(int argc, char** argv)

1. Overview

• argc : argument count
• argv : argument value which is an array of string
• In C++ we have std::string, equivalent in C we have char*
• A string “abc” is an array of characters {‘a’, ’b’, ’c’}
• An array is a pointer which points to the 1st character in the array thus char[] equal char*
• As argv is an array of “string”, or we can say is an array of “array of characters”, so it’s a 2 dimensional pointer

// we could use both ways
int main(int argc, char** argv)
int main(int argc, char *argv[])

2. Code

• Create the “test.cpp” file

/*
#include <stdio.h>  : is the header file in the C standard library. It is used for input/output printf(), scanf() (in C++, stdio.h became cstdio)
#include <iostream> : is the input output class in C++ and objects (std::cout, std::cin...).
So if you're using C++ just use #include <iostream>
*/
#include <iostream>

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

• Compile the source

// std::cout, ... is present in the C++ standard library, which would need explicit linking with -lstdc++ when using gcc; g++ links the standard library by default.
gcc test.cpp -lstdc++ -o test

// or we could use g++ which is preferred over gcc
g++ -Wall -Wextra -Werror -c test.cpp -o tes

• Testing

btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$ gcc test.cpp -lstdc++ -o test
btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$ ./test
Argument[0] : ./test
btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$ ./test a b 1 2 hello
Argument[0] : ./test
Argument[1] : a
Argument[2] : b
Argument[3] : 1
Argument[4] : 2
Argument[5] : hello
btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$ echo $?
0
btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$

NOTE

• The first argument value is the path of your program
• In the older C++, we could define a void main(), but in the later it requires to define a int main() which should return a value
• The returned value could be any number, but usually 0 which means no error
• The returned value will be read by the OS, we also could see the returned value by echo $?

btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$ echo $?
0
btnguyen@DESKTOP-UAIA29B:/mnt/h/DEVELOPER/Linux/prog$

3. Bonus

• Instead of opening a terminal then typing the command, we could do it directly in code C.

int system(const char* command)

• The system()will fork a child process to execute your command.
• The cons of system() is slow, and we can’t control the error because the returned value of system() is not the status of a child process but just means it executed the command.