Getting Started Tutorial on GDB in Linux

Preface

gdb is a very useful debugging tool under Linux. Although it is a debugging tool in command line mode, its functions are more powerful than you can imagine. Here is a brief introduction to the use of gdb. Friends in need can take a look at the detailed introduction below.

Breakpoints

Break at a specified location in the code, causing the program to break there.

• break <function> Stop when entering the specified function
• break <linenum> Stop at the specified line number.
• break +/-offset Stop at offset lines before or after the current line number. offiset is a natural number.
• break filename:linenum Stop at line linenum in source file filename.
• break ... if <condition> ... can be the above parameters, condition represents the condition, and the program stops when the condition is met. For example, in a loop body, you can set break if i=100, which means that the program stops when i is 100.

Examples:

(gdb) break sc_smartcontract_handler.cpp:45
Breakpoint 4 at 0x424d42: file sc_smartcontract_handler.cpp, line 45.

Breakpoint related operations

• delete Delete all breakpoints
• delete breakpoint [n] Delete a breakpoint
• disable breakpoint [n] Disable a breakpoint
• enable breakpoint [n] Enable a breakpoint
• info breakpoints [n] View current breakpoint information

Observation Point

Capture points are used to capture some events when the program is running. Such as: loading shared libraries (dynamic link libraries), C++ exceptions, etc. It is often used to locate bugs.

The command format for capturing a point is: catch <event>, where event can be the following:

• watch <expr> Interrupt when variable changes
• rwatch <expr> Interrupt when variable is read
• awatch <expr> Interrupt when variable value is read or written

You can use the info watchpoints [n] command to view the current watchpoint information.

View variables

The most common way to view variables is

(gdb) print {variable name}

(gdb) print argc
$1 = 1

If you print an array, the tail of the printed array may not be displayed due to the default settings. You can set the maximum length of the printed array with the following command:

(gdb) set print elements 300

View code during debugging

• list function, such as list main: displays the code near the main function
• list file:function For example, list main.c:main: displays the code near the main function in main.c
• list n1,n2, such as list 10,20, displays lines 10 to 20 of the current file

Although the list is very convenient, it is still not satisfactory. It would be nice if the code can be displayed while running. The answer is yes.

Use the following command to start gdb: gdb -tui project name or after starting gdb, enter the command focus, as shown in the figure:

Resume program execution and single-step debugging

In gdb, the commands related to debugging stepping are mainly as follows:

• continue Continue running the program until the next breakpoint (similar to F5 in VS)
• next Steps through the process without entering the sub-function (similar to F10 in VS)
• setp step by statement, will enter the sub-function (similar to F11 in VS)
• until runs until the current statement block ends
• finish Run to the end of the function and jump out, and print the return value of the function (similar to VS Shift+F11)

Interrupt in a specific thread

You can define whether your breakpoint should be on all threads, or on a specific thread. GDB can easily help you do this.

• break <linespec> thread <threadno>
• break <linespec> thread <threadno> if ...

linespec specifies the source line number where the breakpoint is set. threadno specifies the thread ID. Note that this ID is assigned by GDB. You can view thread information in the running program through the "info threads" command. If you do not specify thread <threadno>, it means your breakpoint is set on all threads. You can also specify breakpoint conditions for a thread. like:

(gdb) break frik.c:13 thread 28 if bartab > lim

When your program is stopped by GDB, all running threads are stopped as well. This allows you to see the overall status of the running program. When you resume the program, all threads will be resumed. Even when the main process is being single-step debugged.

Summarize

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