Thread: Arguments to main

The arguments to main come from the command line you enter to run the program...

If, for example, you had a program called weather and entered the command line:

weather Monday Rain Light

Inside weather main would receive...

argc = 3;
argv[1] = Monday
argv[2] = Rain
argv[3] = Light

It's a way of passing options and startup info into a program.

Programs need parameters passed to them, just like our functions do sometimes. This is how it's done.

What else is there to say about it? Google and read up, maybe?

Originally Posted by codecaine_21

Still a tad bit confused, sorry! Why am i passing arguments to main anyway? If you may provide a more elaborate definition/explination. And may you give me a working example to run so i can see it work?

Well... you're learning C... here's a little project for you...

Write a program that accepts and displays command line arguments.

Some examples where command line arguments are extremely helpful can be found by opening a windows CMD shell (Win+r -> CMD -> ok) then type help. Then type help command for each of the programs listed... The whole thing is 100% driven by command line arguments.

Linux is the same only bigger... just about everything on Linux is CLA driven.

Why would you use CLA in your own programs... well that's entirely a matter of what you are trying to accomplish. One of the most common uses is to pass in a filename to be opened when the program starts. Every windows program needs this feature.

That's what I'm getting at -- there isn't much "insight" on this matter, to share. *Argv[] is how parameters are passed to main().

Just like an *array[] is passed to a function.

Read your books chapter on this, and work the exercises. You'll get it. Zee insight barrel, she is emp-ty.

Thank you CommonTater. And Adak, i didnt understand why it was used. so i asked!

Code:

gcc
gcc: no input files

Code:

grep
Usage: grep [OPTION]... PATTERN [FILE]...
Try `grep --help' for more information.

Code:

cp
cp: missing file operand
Try `cp --help' for more information.

Code:

sudo
usage: sudo -h | -K | -k | -L | -V
usage: sudo -v [-AknS] [-p prompt]
usage: sudo -l[l] [-AknS] [-g groupname|#gid] [-p prompt] [-U username] [-u
            username|#uid] [-g groupname|#gid] [command]
usage: sudo [-AbEHknPS] [-C fd] [-g groupname|#gid] [-p prompt] [-u
            username|#uid] [-g groupname|#gid] [VAR=value] [-i|-s] [<command>]
usage: sudo -e [-AknS] [-C fd] [-g groupname|#gid] [-p prompt] [-u
            username|#uid] file ...

...you now see the value in passing arguments to programs.

That was a very nice break down and helps a whole lot
QuadraticFighte!!!!! So when i pass an argument(s) like lets say sudo apt-get install. apt-get is an argument, install and sudo also. So when i write the code do i make or need a function for the first second and third arg??

Originally Posted by codecaine_21

That was a very nice break down and helps a whole lot
QuadraticFighte!!!!! So when i pass an argument(s) like lets say sudo apt-get install. apt-get is an argument, install and sudo also. So when i write the code do i make or need a function for the first second and third arg??

Yes, you will have to write internal program code do deal with what comes in from the command line. C is pretty clever, but it's not a mind reader.

Originally Posted by codecaine_21

That was a very nice break down and helps a whole lot
QuadraticFighte!!!!! So when i pass an argument(s) like lets say sudo apt-get install. apt-get is an argument, install and sudo also. So when i write the code do i make or need a function for the first second and third arg??

Well ya, if you want your program to process command line arguments, you need to program it to handle such arguments.

In your specific example, sudo is a program that runs other programs with root privileges, and the argument is the program to run with ITS own arguments: "apt-get install". Sudo just prompts for a password, and then, if we enter the right one, it executes apt-get install. Finally, install is an argument to apt-get.

If you have a program foo, you can pass it arguments, as in

Code:

$foo Hello World

So in your main function:

Code:

int main(int argc, char** argv)

The values of argc and argv are:

Code:

argc = 3
argv[0] = "foo"
argv[1] = "Hello"
argv[2] = "World"

Originally Posted by whiteflags

I'd like to see a precipitous decline in use of CLI in general, as well.

That would end file associations... sending all these point and click file browsers back to the stone age.