Posted 2022-04-05KnowledgeBase8 minutes read (About 1206 words)

More on g++

Introduction

g++ is a tool for preprocessing, compilation, assembly and linking of source code to generate an executable file or library.

As easy as:

sudo apt update

sudo apt install g++

General syntax to run g++ compiler is:

g++ <flags_with_params> <source_files>
The above command takes a bunch of source files along with some flags and generates an executable file or a library file.
Flags are very important because they can control the way compilation occurs.
For example, if we have a single source file main.cpp and need to generate an executable file named out, the basic syntax for this task is g++ main.cpp -o out
Here, main.cpp is a source file and -o is a flag to specify the output filename. out is the name of the output executable file.
Command can be as complex as g++ -Wall -std=c++14 -I/some/path/to/include/dir -O3 -c -fpermissive -fPIE -o objectfile.o sourcefile1.cpp sourcefile2.cpp sourcefile2.cpp -L/path/to/library/dir -lsomelibrary1 lsomelibrary2
Here, -W -std -O -c -f -o -L -l are the flags along with their parameters and sourcefile1.cpp sourcefile2.cpp sourcefile2.cpp are three source files.

flags and source files can be written in any order. (with some exceptions while linking libraries, discussed below)

Take a simple C++ code main.cpp

#include <iostream>

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

Any of the following commands can be used to generate the executable file main

g++ main.cpp -o main

g++ -o main main.cpp

g++ main.cpp -omain

g++ -omain main.cpp
As you can see, it doesn’t matter in what order you keep -o flag and source file main.cpp
Moreover, it doen’t matter if you keep a space after -o flag or not.
Now, lets add some more flags: -Wall to show all the warnings, -g to enable debugging, and -std to specify C++ standard.
All the commands below produces identical output:

g++ -Wall -g -std=c++14 main.cpp -o main

g++ -Wall main.cpp -g -omain -std=c++14

g++ main.cpp -Wall -omain -g -std=c++14
Now, lets try this with more than one source file

Multiple files

Executable file is a standalone file, which can be executed i.e run as a program. Whereas a library is a collection of stuff that can be invoked from the executable files.
All the examples above show how to create an executable file.
There are two types of library:
- Static Library
- Dynamic Library

Static library is nothing but a precompiled source file that can be linked to generate an executable file or other library.

Static Library

First flow chart shows how an executable is generated by processing all the source files via the whole pipeline.
Second flow chart shows object files are generated in intermediate steps and are linked later to generate an executable file.
These object files can be viewed as static libraries that are linked to main.o to produce an executable file.
This can be done using following commands:

g++ -c main.cpp add.cpp subtract.cpp (Generate Object files)

g++ main.o add.o subtract.o -o main (Generate executable from object files)
Moreover, we can mix up source files and object files while generating executable file:

g++ -c add.cpp subtract.cpp

g++ main.cpp add.o subtract.o -o main
Here, add.o and subtract.o are the object files acting as library for main.cpp
Now, lets try to bundle up these object files into a single file, which we can call Static Library

g++ -c add.cpp subtract.cpp (first generate add.o and subtract.o)

ar rc my_first_library add.o subtract.o (Bundle/Archive the object files to a single file my_first_library)

g++ main.cpp my_first_library -o main (Now, use this library to produce the executable file)
Thats it! You have sucessfully created and used your first Static Library.
ar is called archiver. A tool used to create, modify, and extract from archives.
You can see the list of files in archive using: ar -t my_first_library

Still some work left …

Well, now let’s look into some naming conventions and standard ways of linking a static library.
Generally, static library is named as: lib<library_name>.a
And is linked as:

g++ <source_files> -o <output_filename> -L<path/to/lib> -l<name_of_lib>
So, correct way of linking a static library in above example is:

g++ -c add.cpp subtract.cpp (first generate add.o and subtract.o)

ar rc libmy_first_library.a add.o subtract.o (Bundle/Archive the object files to a single file)

g++ main.cpp -o main -L. -lmy_first_library (Now, use this library to produce the executable file)

Dynamic library are not linked with the executable file during compilation process. They are linked during execution. i.e. linked dynamically.
Dynamic library is named as: lib<library_name>.so
Commands to create dynamic library for above example are:

g++ -c add.cpp subtract.cpp (First generate object files as before)

g++ -shared add.o subtract.o -o libmy_first_shared_library.so (generate a shared library form the object files)

g++ main.cpp -o main -L. -lmy_first_shared_library (Now create an executable by linking the dynamic library)
Now, if you run ./main, it will probably give an error.
That’s because the executable main needs to know where to search for library libmy_first_shared_library.so.
We can add the search path to the environment variable LD_LIBRARY_PATH so that main finds it.

export LD_LIBRARY_PATH=/path/to/dynamic_library
Now, when you run ./main, it should work perfectly.
You can also see all the dynamic dependencies/libraries linked to main by using command ldd main. i.e. List Dynamic Dependencies of executable file main

While generating the executable main by linking static or dynamic library in above example, we have used:

g++ main.cpp -o main -L. -lname_of_lib
But as the flags are position independent, we may try to compile with:

g++ -o main -L. -lname_of_lib main.cpp
But this time, it shows error stating undefined reference
The source file main.cpp uses the functions add and subtract, which is found inside the linked library.
g++ is designed in such a way that dependent stuff must be mentioned beforehand.
As main.cpp is dependent on -lname_of_lib , main.cpp must be written before -lname_of_lib
Other flags can be kept anywhere.