CPSC 213: gdb Tutorial

Purpose

The purpose of gdb is to allow for easy debugging. Although you may argue that you can debug a program easily through visual checks, this is often not the case when there are multiple calls and dependencies. This is especially true when programs are thousands of lines long.

What is gdb?

gdb is a debugging program for code written in C/C++ that is executed through the command line. It works the same way as many other debugging programs. For example, it has a lot of the functionality available in the Eclipse IDE. However, as there isn't a GUI, it is best for you to get acquainted to using commands within the terminal: it follows a simple command-response model of interaction.

How To Start - Compilation

You should now be familiar with compiling C programs with gcc. A typical command that you may use is

gcc -g -o executable_name source.c

where the -o flag specifies a different executable filename than the default a.out. However, to allow the debugger to output additional information for the end user, you need to add the -g flag to allow the compiler to include additional debugging information into the executable.

When you write any code, it is suggested that you keep the -g flag on to allow for easy debugging. The tradeoff is a small increase in compilation time and size of the executable. For the remainder of the CS213 course, please compile with the flag on.

To learn more about other flags for gcc, please see the official online documentation.

How To Start - Executing

Executing gdb is as simple as

gdb executable_name

You need not supply any other arguments. It's easy! You should see a (gdb) prompt once it has loaded your program, indicating that it is ready for use.

gdb's Interface

The interface of gdb works like a usual command-line interface, i.e. you type commands at the prompt and press Enter to execute them. Some useful features to know are:

• Command completion - gdb can complete your command if it has enough characters to determine which command you want to enter. The automated completion occurs when you hit the tab key.
• History - You can see a history of the commands you have entered. This can achieved with the arrow keys or Ctrl+P and Ctrl+N.
• Help - Just type help if you need general help or help command to learn more about a specific gdb command.
• Info - This command allows you get relevant information about various things. For example info locals will tell you the local variables that you have access to. info break will tell you the breakpoints you have set.

Running the Debuggee

To run the program, type

run [arguments]

or just

run

if you want to keep the same arguments as the previous run.

If the program executed successfully, it will tell you the program exited normally, else it will give you an error message.

About Error Messages

Error messages tell you the error that occurred and the line where it occurred. They are very useful for narrowing down where exactly the execution faulted. A common error message you will see is SIGSEGV, Segmentation fault.. It simply means that you have tried to access an invalid memory address. This can mean you are accessing an array out of bounds! Google is always helpful to learn more about your error message.

Useful Functions

• print expression - Prints almost all C expressions. Try these:
  • print x - Prints the value of x
  • print *x - Prints the value stored in the memory location pointed to by x. (x must be of pointer type.)
  • print 3 + 4 * 5 - Prints the calculation of 3 + 4 * 5.
• backtrace - Shows the call stack. Let's say main() called helperFunc(), which called printf. Since printf is a library function, the error message will show you that a line within the library caused an error, but through the call stack, you can see that main() called helperFunc() on line x, and helperFunc() called printf on line y.
• break - Sets a breakpoint. This can be done with a line number in the source file using break source_file.c:line_number, or with a function name using break helper_func. To delete a breakpoint, first list all the breakpoints with info break, find the number of the desired breakpoint, and delete it with del breakpoint_number.
• watch - Watches a variable. Halts execution when variable is changed. watch variable.

Control Flow

• continue, cont - Continues execution until the next breakpoint, watch, or an error occurs.
• step - Steps over the next source line (OVER function calls)
• next - Steps into the source line (INTO function calls)

Shortcuts

These are some shortcuts to help you debug faster.

• b is break
• bt is backtrace
• d is delete
• n is next
• p is print
• s is step
• Just hitting Enter at the prompt, with nothing typed, will repeat the previous command.^*

Advanced Commands

• Conditional breakpoints - You can make breakpoints conditional, so that they break when a condition is true, using e.g. breakpoint foo if x==0
• Advanced printing
  • Printing structs - print linkedlist->next
  • Printing the value that a pointer is pointing to - print *r
  • Printing in hex - print/x variable
  • Casting a struct - print *(struct_name) variable