Difference between revisions of "Debugging with Valgrind"

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*VGDB
 
*VGDB
 
*Valgrind executing YourProgram
 
*Valgrind executing YourProgram
 +
  
 
Though GDB can connect to VGDB directly (process - to - process), Lazarus cannot use this mode. You have to use Remote debugging. In this setup VGDB plays role of GDBServer listening on a TCP port, and Lazarus via GDB connects to it.
 
Though GDB can connect to VGDB directly (process - to - process), Lazarus cannot use this mode. You have to use Remote debugging. In this setup VGDB plays role of GDBServer listening on a TCP port, and Lazarus via GDB connects to it.
Line 25: Line 26:
 
*VGDB
 
*VGDB
 
*Valgrind executing YourProgram
 
*Valgrind executing YourProgram
 +
  
 
Usually you will run all parts on same computer. But there is possibility to run them on 2 computers via network.
 
Usually you will run all parts on same computer. But there is possibility to run them on 2 computers via network.
Line 34: Line 36:
 
1- Install valgrind
 
1- Install valgrind
 
Refer to Valgrind resources to install it.
 
Refer to Valgrind resources to install it.
 +
  
 
2- Compile your program with Valgrind option
 
2- Compile your program with Valgrind option
 
[[File:valgrind_project_options.png]]
 
[[File:valgrind_project_options.png]]
 +
  
 
3- Configure Lazarus debugger
 
3- Configure Lazarus debugger
 
[[File:valgrind_tools_options.png]]
 
[[File:valgrind_tools_options.png]]
 +
  
 
4- Start Valgrind and VGDB in a separate shell
 
4- Start Valgrind and VGDB in a separate shell
 
<syntaxhighlight lang=shell>
 
<syntaxhighlight lang=shell>
valgrind --tool=memcheck --leak-check=yes --vgdb-error=0 ./myprogram 2>valgrind.trc &
+
valgrind --tool=memcheck --leak-check=yes --log-file=valgrind.trc --vgdb-error=0 ./myprogram &
 
sleep 2; vgdb --port=33333
 
sleep 2; vgdb --port=33333
 
</syntaxhighlight>
 
</syntaxhighlight>
 +
  
 
5- Run your program in Lazarus
 
5- Run your program in Lazarus

Revision as of 04:11, 4 March 2020

English (en)

!!!WORK IN PROGRESS!!!

Introduction

Valgrind is an instrumentation framework for building dynamic analysis tools. There are Valgrind tools that can automatically detect many memory management and threading bugs, and profile your programs in detail. You can read about Valgrind here https://valgrind.org/

A program running under Valgrind is not executed directly by the CPU. Instead it runs on a synthetic CPU provided by Valgrind.

The debugging applications stack is following:

  • GDB
  • VGDB
  • Valgrind executing YourProgram


Though GDB can connect to VGDB directly (process - to - process), Lazarus cannot use this mode. You have to use Remote debugging. In this setup VGDB plays role of GDBServer listening on a TCP port, and Lazarus via GDB connects to it.

So the debugging applications stack with Lazarus will be:

  • Lazarus IDE
  • GDB
  • tcp/ip
  • VGDB
  • Valgrind executing YourProgram


Usually you will run all parts on same computer. But there is possibility to run them on 2 computers via network.

In the following example everything will be on same computer. Network interface will be localhost and TCP port will be 333333.

Configuration

1- Install valgrind Refer to Valgrind resources to install it.


2- Compile your program with Valgrind option valgrind project options.png


3- Configure Lazarus debugger valgrind tools options.png


4- Start Valgrind and VGDB in a separate shell

valgrind --tool=memcheck --leak-check=yes --log-file=valgrind.trc --vgdb-error=0 ./myprogram &
sleep 2; vgdb --port=33333


5- Run your program in Lazarus

Lazarus via VGDB communicates with the program in Valgrind. You can set breakpoints, inspect variable values and so on, like in regular debugger. Also Lasarus will stop on lines where Valgrind throws exceptions. Valgrind tracks memory usage very carefully and issues exceptions where FPC, RTL and a memory manager may ignore memory misuse. Like at line 4 below.

  mo:=TMyObj.Create;
  mo.Field:=12345;
  mo.Free;
  if mo.Field = 0 // Mistakenly accessing released memory block 
  then exit;

Please note Valgrind is accurate but it is not a fastest debugger. Be patient. When Valgrind issues an exeption Lazarus will show a message. valgrind error.png After you close it Lazasarus will position you on a source row where the exception has happened. Lazarus provides with very basic information on what exacly happened. Valgrind gives you much more details. To get more details you can either look into Valgrind trace file. But more convenien is to load the trace file into IDE. Select menu View -> Leaks and Traces Open valgrind.trc valgrind_trace.png

Here especially helpful is the section "Block was allocated at". Select a line where you see your program source, double click and Lazarus will position cursor at a line where the memory in question was allocated.