Using g++ and gdb

G++ is the C++ compiler under Unix/Linux, and gdb is a powerful debugging tool under Unix/Linux. Most of the time, the C/C++ code we write does not run on Windows but on a Linux system. Here I record some usage of g++ and gdb in case they are needed.

Using g++

When g++ performs compilation work, it needs 4 steps in total:

• Preprocessing, generating a .i file, mainly expanding header files and macro definitions: g++ -E test.cpp -o test.i
• Compilation, converting the preprocessed file into assembly language and generating a .s file: g++ -S test.i -o test.s
• Assembly, turning the assembly into object code (machine code) and generating a .o file, mainly converting the .s file into a binary file: g++ -c test.s -o test.o
• Linking, linking the object code to generate an executable program: g++ test.o -o test

The steps above can also be done with a single command:

g++ test.cpp -o test

When compiling multiple files, there are two approaches:

$ g++ a.cpp b.cpp c.cpp -o test  #compile all files at once

$ g++ a.cpp -o a.o  #compile separately
$ g++ b.cpp -o b.o
$ g++ c.cpp -o c.o
$ g++ a.o b.o c.o -o test

Here the separate compilation approach is recommended, since when you modify one or more of the files you only need to compile that file or those files, rather than compiling from scratch.

Commonly used g++ options:

Under Linux the file type does not depend on the suffix, but generally speaking: .o is an object file, equivalent to a .obj file in windows .so is a shared library, that is, a shared object used for dynamic linking, roughly the same as .dll .a is a static library, several .o files combined together, used for static linking .la are some shared libraries automatically generated by libtool, mainly recording some configuration information.

Using gdb

The .cpp must be compiled with the -g option for the generated executable to be debuggable at source level with gdb

g++ -g test.cpp -o test

It is worth noting that -g has 4 levels:

• g0 is the same as not adding -g. That is, no information is included
• g1 includes only minimal information; generally it is used only when you do not need to debug but only need backtrace information, and you really care about program size or have some other confidentiality/special requirement.
• g2 is the gdb default level and includes most of the information you need.
• g3 includes some extra information, for example macro definition information. When you need to debug macro definitions, use -g3

Generally speaking, gdb mainly helps you accomplish the following four things:

1. Start the program, and run the program according to your own requirements.

2. Let the debugged program stop at a breakpoint at a specified position.

3. When the program stops, inspect what is happening in the program at that moment.

4. Dynamically change the execution environment of the program.

Several ways to run a program under gdb:

• gdb ${program} then, after entering gdb, type run(short form r) ${arg1} ${arg2} … ${argN}
• gdb --args ${program} ${arg1} ${arg2} … ${argN}, then after entering gdb run run
• After entering gdb, type file ${program}. Then use set args ${arg1} ${arg2} … ${argN} to set the program arguments, and then run run

Commonly used gdb debugging commands:

Reference

macky0668 — basic usage of the g++ C++ compiler under linux

Xi Shan Ju Shi who loves tinkering — a series on commonly used GDB commands

chenyu who is changing himself — a summary of basic gdb debugging commands and usage under linux

Getting started with gdb debugging, a high-quality guide written by an expert