Chapter 8. GNU Debugger (GDB) The GNU Debugger , commonly abbreviated as GDB , is a command line tool that can be used to debug programs written in various programming languages. It allows you to inspect memory within the code being debugged, control the execution state of the code, detect the execution of particular sections of code, and much more. Red Hat Developer Toolset is distributed with GDB 11.2 . This version is more recent than the version included in Red Hat Enterprise Linux and the release of Red Hat Developer Toolset and provides some enhancements and numerous bug fixes. 8.1. Installing the GNU Debugger In Red Hat Developer Toolset, the GNU Debugger is provided by the devtoolset-12-gdb package and is automatically installed with devtoolset-12-toolchain as described in Section 1.5, "Installing Red Hat Developer Toolset" . 8.2. Preparing a Program for Debugging Compiling Programs with Debugging Information To compile a C program with debugging information that can be read by the GNU Debugger , make sure the gcc compiler is run with the -g option: Similarly, to compile a C++ program with debugging information: Example 8.1. Compiling a C Program With Debugging Information Consider a source file named fibonacci.c that has the following contents: #include <stdio.h> #include <limits.h> int main (int argc, char *argv[]) { unsigned long int a = 0; unsigned long int b = 1; unsigned long int sum; while (b < LONG_MAX) { printf("%ld ", b); sum = a + b; a = b; b = sum; } return 0; } Compile this program on the command line using GCC from Red Hat Developer Toolset with debugging information for the GNU Debugger : This creates a new binary file called fibonacci in the current working directory. Installing Debugging Information for Existing Packages To install debugging information for a package that is already installed on the system: Note that the yum-utils package must be installed for the debuginfo-install utility to be available on your system. Example 8.2. Installing Debugging Information for the glibc Package Install debugging information for the glibc package: 8.3. Running the GNU Debugger To run the GNU Debugger on a program you want to debug: This starts the gdb debugger in interactive mode and displays the default prompt, (gdb) . To quit the debugging session and return to the shell prompt, run the following command at any time: Note that you can execute any command using the scl utility, causing it to be run with the Red Hat Developer Toolset binaries used in preference to the Red Hat Enterprise Linux system equivalent. This allows you to run a shell session with Red Hat Developer Toolset gdb as default: Note To verify the version of gdb you are using at any point: Red Hat Developer Toolset's gdb executable path will begin with /opt . Alternatively, you can use the following command to confirm that the version number matches that for Red Hat Developer Toolset gdb : Example 8.3. Running the gdb Utility on the fibonacci Binary File This example assumes that you have successfully compiled the fibonacci binary file as shown in Example 8.1, "Compiling a C Program With Debugging Information" . Start debugging fibonacci with gdb : 8.4. Listing Source Code To view the source code of the program you are debugging: Before you start the execution of the program you are debugging, gdb displays the first ten lines of the source code, and any subsequent use of this command lists another ten lines. Once you start the execution, gdb displays the lines that are surrounding the line on which the execution stops, typically when you set a breakpoint. You can also display the code that is surrounding a particular line: Similarly, to display the code that is surrounding the beginning of a particular function: Note that you can change the number of lines the list command displays: Example 8.4. Listing the Source Code of the fibonacci Binary File The fibonacci.c file listed in Example 8.1, "Compiling a C Program With Debugging Information" has exactly 17 lines. Assuming that you have compiled it with debugging information and you want the gdb utility to be capable of listing the entire source code, you can run the following command to change the number of listed lines to 20: You can now display the entire source code of the file you are debugging by running the list command with no additional arguments: 8.5. Setting Breakpoints Setting a New Breakpoint To set a new breakpoint at a certain line: You can also set a breakpoint on a certain function: Example 8.5. Setting a New Breakpoint This example assumes that you have compiled the fibonacci.c file listed in Example 8.1, "Compiling a C Program With Debugging Information" with debugging information. Set a new breakpoint at line 10: Listing Breakpoints To display a list of currently set breakpoints: Example 8.6. Listing Breakpoints This example assumes that you have followed the instructions in Example 8.5, "Setting a New Breakpoint" . Display the list of currently set breakpoints: Deleting Existing Breakpoints To delete a breakpoint that is set at a certain line: Similarly, to delete a breakpoint that is set on a certain function: Example 8.7. Deleting an Existing Breakpoint This example assumes that you have compiled the fibonacci.c file listed in Example 8.1, "Compiling a C Program With Debugging Information" with debugging information. Set a new breakpoint at line 7: Remove this breakpoint: 8.6. Starting Execution To start an execution of the program you are debugging: If the program accepts any command line arguments, you can provide them as arguments to the run command: The execution stops when the first breakpoint (if any) is reached, when an error occurs, or when the program terminates. Example 8.8. Executing the fibonacci Binary File This example assumes that you have followed the instructions in Example 8.5, "Setting a New Breakpoint" . Execute the fibonacci binary file: 8.7. Displaying Current Values The gdb utility allows you to display the value of almost anything that is relevant to the program, from a variable of any complexity to a valid expression or even a library function. However, the most common task is to display the value of a variable. To display the current value of a certain variable: Example 8.9. Displaying the Current Values of Variables This example assumes that you have followed the instructions in Example 8.8, "Executing the fibonacci Binary File" and the execution of the fibonacci binary stopped after reaching the breakpoint at line 10. Display the current values of variables a and b : 8.8. Continuing Execution To resume the execution of the program you are debugging after it reached a breakpoint: The execution stops again when another breakpoint is reached. To skip a certain number of breakpoints (typically when you are debugging a loop): The gdb utility also allows you to stop the execution after executing a single line of code: Finally, you can execute a certain number of lines: Example 8.10. Continuing the Execution of the fibonacci Binary File This example assumes that you have followed the instructions in Example 8.8, "Executing the fibonacci Binary File" , and the execution of the fibonacci binary stopped after reaching the breakpoint at line 10. Resume the execution: The execution stops the time the breakpoint is reached. Execute the three lines of code: This allows you to verify the current value of the sum variable before it is assigned to b : 8.9. Additional Resources For more information about the GNU Debugger and all its features, see the resources listed below. Installed Documentation Installing the devtoolset-12-gdb-doc package provides the following documentation in HTML and PDF formats in the /opt/rh/devtoolset-12/root/usr/share/doc/devtoolset-12-gdb-doc-11.2 directory: The Debugging with GDB book, which is a copy of the upstream material with the same name. The version of this document exactly corresponds to the version of GDB available in Red Hat Developer Toolset. The GDB's Obsolete Annotations document, which lists the obsolete GDB level 2 annotations. Online Documentation Red Hat Enterprise Linux 7 Developer Guide - The Developer Guide for Red Hat Enterprise Linux 7 provides more information on the GNU Debugger and debugging. GDB Documentation - The upstream GDB documentation includes the GDB User Manual and other reference material. See Also Chapter 1, Red Hat Developer Toolset - An overview of Red Hat Developer Toolset and more information on how to install it on your system. Chapter 2, GNU Compiler Collection (GCC) - Further information on how to compile programs written in C, C++, and Fortran. Chapter 9, strace - Instructions on using the strace utility to monitor system calls that a program uses and signals it receives. Chapter 11, memstomp - Instructions on using the memstomp utility to identify calls to library functions with overlapping memory regions that are not allowed by various standards.

Commands:
scl enable devtoolset-12 'gcc -g -o output_file input_file ...'
scl enable devtoolset-12 'g++ -g -o output_file input_file ...'
#include <stdio.h> #include <limits.h> int main (int argc, char *argv[]) { unsigned long int a = 0; unsigned long int b = 1; unsigned long int sum; while (b < LONG_MAX) { printf("%ld ", b); sum = a + b; a = b; b = sum; } return 0; }
scl enable devtoolset-12 'gcc -g -o fibonacci fibonacci.c'
debuginfo-install package_name
debuginfo-install glibc Loaded plugins: product-id, refresh-packagekit, subscription-manager --> Running transaction check ---> Package glibc-debuginfo.x86_64 0:2.17-105.el7 will be installed
scl enable devtoolset-12 'gdb file_name '
(gdb) quit
scl enable devtoolset-12 'bash'
which gdb
gdb -v
scl enable devtoolset-12 'gdb fibonacci' GNU gdb (GDB) Red Hat Enterprise Linux 8.2-2.el7 Copyright (C) 2017 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-redhat-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>. Find the GDB manual and other documentation resources online at: <http://www.gnu.org/software/gdb/documentation/>. For help, type "help". Type "apropos word" to search for commands related to "word" Reading symbols from fibonacci...done. (gdb)
(gdb) list
(gdb) list file_name : line_number
(gdb) list file_name : function_name
(gdb) set listsize number
(gdb) set listsize 20
(gdb) list 1 #include <stdio.h> 2 #include <limits.h> 3 4 int main (int argc, char *argv[]) { 5 unsigned long int a = 0; 6 unsigned long int b = 1; 7 unsigned long int sum; 8 9 while (b < LONG_MAX) { 10 printf("%ld ", b); 11 sum = a + b; 12 a = b; 13 b = sum; 14 } 15 16 return 0; 17 }
(gdb) break file_name : line_number
(gdb) break file_name : function_name
(gdb) break 10 Breakpoint 1 at 0x4004e5: file fibonacci.c, line 10.
(gdb) info breakpoints
(gdb) info breakpoints Num Type Disp Enb Address What 1 breakpoint keep y 0x00000000004004e5 in main at fibonacci.c:10
(gdb) clear line_number
(gdb) clear function_name
(gdb) break 7 Breakpoint 2 at 0x4004e3: file fibonacci.c, line 7.
(gdb) clear 7 Deleted breakpoint 2
(gdb) run
(gdb) run argument ...
(gdb) run Starting program: /home/john/fibonacci Breakpoint 1, main (argc=1, argv=0x7fffffffe4d8) at fibonacci.c:10 10 printf("%ld ", b);
(gdb) print variable_name
(gdb) print a USD1 = 0 (gdb) print b USD2 = 1
(gdb) continue
(gdb) continue number
(gdb) step
(gdb) step number
(gdb) continue Continuing. Breakpoint 1, main (argc=1, argv=0x7fffffffe4d8) at fibonacci.c:10 10 printf("%ld ", b);
(gdb) step 3 13 b = sum;
(gdb) print sum USD3 = 2