Linux g++ Command with Practical Examples

Introduction

In this lab, we will explore the Linux g++ command and its practical applications. We will start by understanding the basics of the g++ command, which is the GNU C++ compiler used to compile C++ source code into executable programs. Then, we will compile a simple C++ program using g++ and explore various compiler flags and optimization options. This lab aims to provide a comprehensive understanding of the g++ command and its usage in software development.

Linux Commands Cheat Sheet

Skills Graph

Understand the Basics of g++ Command

In this step, we will learn the basics of the g++ command, which is the GNU C++ compiler. The g++ command is used to compile C++ source code into an executable program.

First, let's check the version of g++ installed in our Ubuntu 22.04 Docker container:

g++ --version

Example output:

g++ (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The g++ command has several important options that you can use to control the compilation process. Some of the commonly used options are:

• -c: Compile or assemble the source files, but do not link.
• -o <output>: Name the output file.
• -g: Produce debugging information in the output file.
• -Wall: Enable all warning messages.
• -Wextra: Enable extra warning messages.
• -std=c++11 (or c++14, c++17, etc.): Specify the C++ standard to use.
• -O0, -O1, -O2, -O3: Set the optimization level.

Now, let's try compiling a simple C++ program using the g++ command:

cd ~/project
nano hello.cpp

Add the following code to the hello.cpp file:

#include <iostream>

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

Save the file and exit the text editor. Now, let's compile the program:

g++ -o hello hello.cpp

This will create an executable file named hello in the current directory. You can run the program like this:

./hello

Example output:

Hello, World!

Compile a Simple C++ Program with g++

In this step, we will learn how to compile a simple C++ program using the g++ command.

First, let's create a new C++ file in the ~/project directory:

cd ~/project
nano simple.cpp

Add the following code to the simple.cpp file:

#include <iostream>

int main() {
    int a = 10, b = 20;
    std::cout << "a + b = " << a + b << std::endl;
    return 0;
}

Save the file and exit the text editor.

Now, let's compile the program using the g++ command:

g++ -o simple simple.cpp

This will create an executable file named simple in the current directory. You can run the program like this:

./simple

Example output:

a + b = 30

The -o option specifies the name of the output file. If you don't use this option, g++ will create an executable file named a.out by default.

You can also add additional compiler flags to control the compilation process. For example, the -g flag will include debugging information in the compiled binary, which can be useful for troubleshooting:

g++ -g -o simple simple.cpp

And the -Wall and -Wextra flags will enable additional warning messages, which can help you identify potential issues in your code:

g++ -Wall -Wextra -o simple simple.cpp

Explore g++ Compiler Flags and Optimization

In this step, we will explore more g++ compiler flags and learn how to optimize the performance of our C++ programs.

First, let's create a new C++ file that performs a simple calculation:

cd ~/project
nano optimize.cpp

Add the following code to the optimize.cpp file:

#include <iostream>

int main() {
    int sum = 0;
    for (int i = 0; i < 1000000000; i++) {
        sum += i;
    }
    std::cout << "Sum: " << sum << std::endl;
    return 0;
}

This program calculates the sum of the first 1 billion integers.

Now, let's compile the program without any optimization flags:

g++ -o optimize optimize.cpp
time ./optimize

Example output:

Sum: 499999999500000000
real    0m1.123s
user    0m1.120s
sys     0m0.003s

Next, let's try compiling the program with the -O2 optimization flag:

g++ -O2 -o optimize optimize.cpp
time ./optimize

Example output:

Sum: 499999999500000000
real    0m0.189s
user    0m0.185s
sys     0m0.003s

As you can see, the -O2 optimization flag significantly improves the performance of the program.

You can also try other optimization levels, such as -O0 (no optimization), -O1, -O3, and -Ofast. Each level provides different trade-offs between compilation time, program size, and execution speed.

Another useful flag is -march=native, which tells the compiler to generate code optimized for the specific CPU architecture of the host machine. This can provide additional performance improvements:

g++ -O2 -march=native -o optimize optimize.cpp
time ./optimize

Example output:

Sum: 499999999500000000
real    0m0.174s
user    0m0.170s
sys     0m0.003s

Finally, let's explore the -ffast-math flag, which enables aggressive floating-point optimizations. This flag can provide significant performance improvements for programs that heavily use floating-point operations, but it may also introduce some loss of precision:

g++ -O2 -ffast-math -o optimize optimize.cpp
time ./optimize

Example output:

Sum: 499999999500000000
real    0m0.159s
user    0m0.155s
sys     0m0.003s

As you can see, the various compiler flags can have a significant impact on the performance of your C++ programs. It's important to experiment and find the right balance of optimization, performance, and precision for your specific use case.

Summary

In this lab, we learned the basics of the g++ command, which is the GNU C++ compiler. We explored how to compile a simple C++ program using g++ and examined various compiler flags and optimization options. We covered compiling a "Hello, World!" program and a more complex C++ program, as well as understanding the purpose of different compiler flags such as -c, -o, -g, -Wall, -Wextra, -std=c++11, and the optimization levels -O0 to -O3. These steps provided a solid foundation for working with the g++ command and compiling C++ programs in a Linux environment.

Linux Commands Cheat Sheet