Linux Programming – Part 10: gcc programming basics

Let’s start coding! But to do so, we need to prepare our coding environment. For a starter, use whatever text editor you like, either emacs or vim.

Environment settings

For the compiler, gcc (GNU Compiler Collection) will be our choice. To check if gcc is already installed on your machine, type the below code:

gcc -dumpversion

Just like in image 01, if the version number is shown, there’s nothing you have to do anymore. Congrats!

And if not, type the below command to install it manually. By the way, this is the Ubuntu environment.

sudo apt get install build-essential

For CentOS, type the below:

sudo  yum group install "Developer Tools"

gcc building processes

Here’s a sample code written in C language.

hello.c

#include<stdio.h>

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

In image 02, you can see there’s the hello.c file. by typing gcc hello.c, you can build the program. As you can see, there’s the file a.out.

In image02, the built file is executed, and it prints out ‘Hello World’ as being programmed.

In image03, I’ve actually named the compiled file name by specifying ‘hello’ when compiling the source code.

Command line arguments

Next, we need to talk about command line arguments. Command line arguments are characters you give when executing a program. In the example below, x,y, and z is the command line arguments and there are three of them.

$ echo x y z

argc and argv

Command line arguments are characters that are passed as main()’s arguments.

Take a look at hello.c again.

hello.c

#include<stdio.h>

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

The argc and argv are responsible for command line arguments. argc handles the command line argument’s number, and argv handles the actual command line arguments’ characters (more specifically, an array of characters). The argv’s type is “pointer array for char”.

image04 is argv’s structure and how it receives the arguments.

The first argv[0] actually receives the executed program’s name.

So, argc is always more than 1, and when you handle the command line arguments in your program, you always have to start it from argc[1].

Here’s a sample code:

args.c

#include<stdio.h>
#include<stdlib.h>

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

Here’s the result of the test execution of the code.

When you surround the arguments with ” “, it will be recognized as one argument.

Finally, it can read files inside a directory and shows their number.