Chapter 3. The LLDB debugger

The LLDB debugger is a command-line tool for debugging C and C++ programs. Use LLDB to inspect memory within the code being debugged, control the execution state of the code, and detect the execution of particular sections of code.

LLVM Toolset is distributed with LLDB 12.0.1.

3.1. Prerequisites

3.2. Starting a debugging session

Use LLDB to start an interactive debugging session.

Procedure

  • To run LLDB on a program you want to debug, use the following command:

    • On Red Hat Enterprise Linux 7: 

      $ scl enable llvm-toolset-12.0 'lldb <binary_file>'

      • Replace <binary_file> with the name of your compiled program.

        You have started your LLDB debugging session in interactive mode. Your command-line terminal now displays the default prompt (lldb).

    • On Red Hat Enterprise Linux 8: 

      $ lldb <binary_file_name>

      • Replace <binary_file> with the name of your compiled program.

        You have started your LLDB debugging session in interactive mode. Your command-line terminal now displays the default prompt (lldb).

    • On Red Hat Enterprise Linux 9: 

      $ lldb <binary_file>

      • Replace <binary_file> with the name of your compiled program.

        You have started your LLDB debugging session in interactive mode. Your command-line terminal now displays the default prompt (lldb).

  • To quit the debugging session and return to the shell prompt, run the following command: 
(lldb) quit

3.3. Executing your program during a debugging session

Use LLDB to execute your program during your debugging session. The execution of your program stops when the first breakpoint is reached, when an error occurs, or when the program terminates.

Prerequisites

Procedure

  • To execute the program you are debugging, run: 

    (lldb) run

  • To execute the program you are debugging using a specific argument, run: 

    (lldb) run <argument>
    • Replace <argument> with the command-line argument you want to use.

3.4. Using breakpoints

Use breakpoints to pause the execution of your program at a set point in your source code.

Prerequisites

Procedure

  • To set a new breakpoint on a specific line, run the following command: 

    (lldb) breakpoint set --file <source_file_name> --line <line_number>
    • Replace <source_file_name> with the name of your source file and <line_number> with the line number you want to set your breakpoint at.

  • To set a breakpoint on a specific function, run the following command: 

    (lldb) breakpoint set --name <function_name>
    • Replace <function_name> with the name of the function you want to set your breakpoint at.

  • To display a list of currently set breakpoints, run the following command: 

    (lldb) breakpoint list

  • To delete a breakpoint, run: 

    (lldb) breakpoint clear -f <source_file_name> -l <line_number>
    • Replace <source_file_name> with the name of your source file and <line_number> with line number of the breakpoint you want to delete.

  • To resume the execution of your program after it reached a breakpoint, run: 

    (lldb) continue

  • To skip a specific number of breakpoints, run the following command: 

    (lldb) continue -i <breakpoints_to_skip>

    • Replace <breakpoints_to_skip> with the number of breakpoints you want to skip.

      Note

      To skip a loop, set the <breakpoints_to_skip> to match the loop iteration count.

3.5. Stepping through code

You can use LLDB to step through the code of your program to execute only one line of code after the line pointer.

Prerequisites

Procedure

  • To step through one line of code:

    1. Set your line pointer to the line you want to execute.

    2. Run the following command: 

      (lldb) step

  • To step through a specific number of lines of code:

    1. Set your line pointer to the line you want to execute.

    2. Run the following command: 

      (lldb) step -c <number>
      • Replace <number> with the number of lines you want to execute.

3.6. Listing source code

Before you execute the program you are debugging, the LLDB debugger automatically displays the first 10 lines of source code. Each time the execution of the program is stopped, LLDB displays the line of source code on which it stopped as well as its surrounding lines. You can use LLDB to manually trigger the display of source code during your debugging session.

Prerequisites

Procedure

  • To list the first 10 lines of the source code of the program you are debugging, run: 

    (lldb) list

  • To display the source code from a specific line, run: 

    (lldb) list <source_file_name>:<line_number>
    • Replace <source_file_name> with the name of your source file and <line_number> with the number of the line you want to display.

3.7. Displaying current program data

The LLDB debugger provides data on variables of any complexity, any valid expressions, and function call return values. You can use LLDB to display data relevant to the program state.

Prerequisites

Procedure

To display the current value of a certain variable, expression, or return value, run: 

(lldb) print <data_name>
  • Replace <data_name> with data you want to display.

3.8. Additional resources

  • For more information on the LLDB debugger, see the official LLDB documentation LLDB Tutorial.
  • For a list of GDB commands and their LLDB equivalents, see the GDB to LLDB Command Map.