User guide

Chapter 5. Debugging

not only the characteristics of each individual elements in the structure, but the morphology of the

structure as well.

GDB requires the following items in order to fully function:

Debug Information

Much of GDB's operations rely on a program's debug information. While this information generally

comes from compilers, much of it is necessary only while debugging a program, i.e. it is not

used during the program's normal execution. For this reason, compilers do not always make that

information available by default — GCC, for instance, must be explicitly instructed to provide this

debugging information with the -g flag.

To make full use of GDB's capabilities, it is highly advisable to make the debug information

available first to GDB. GDB can only be of very limited use when run against code with no

available debug information.

Source Code

One of the most useful features of GDB (or any other debugger) is the ability to associate events

and circumstances in program execution with their corresponding location in source code. This

location normally refers to a specific line or series of lines in a source file. This, of course, would

require that a program's source code be available to GDB at debug time.

5.2.1. Simple GDB

GDB literally contains dozens of commands. This section describes the most fundamental ones.

br (breakpoint)

The breakpoint command instructs GDB to halt execution upon reaching a specified point in the

execution That point can be specified a number of ways, but the most common are just as the line

number in the source file, or the name of a function. Any number of breakpoints can be in effect

simultaneously. This is frequently the first command issued after starting GDB.

r (run)

The run command starts the execution of the program. If run is executed with any arguments,

those arguments are passed on to the executable as if the program has been started normally.

Users normally issue this command after setting breakpoints.

Before an executable is started, or once the executable stops at, for example, a breakpoint, the state

of many aspects of the program can be inspected. The following commands are a few of the more

common ways things can be examined.

p (print)

The print command displays the value of the argument given, and that argument can be almost

anything relevant to the program. Usually, the argument is simply the name of a variable of any

complexity, from a simple single value to a structure. An argument can also be an expression valid

in the current language, including the use of program variables and library functions, or functions

defined in the program beingtested.

bt (backtrace)

The backtrace displays the chain of function calls used up until the exectuion was terminated.

This is useful for investigating serious bugs (such as segmentation faults) with elusive causes.

l (list)

When execution is stopped, the list command shows the line in the source code corresponding

to where the program stopped.