Debugging with GDB Manual The GNU Source-Level Debugger (769148-001, March 2014)

1 A Sample GDB Session

This chapter describes the most common GDB commands with the help of an example.

The following topics are discussed:

• Loading the Executable

• Setting the Display Width

• Setting Breakpoints

• Running the Executable under GDB

• Stepping to the next line

• Stepping into a Subroutine

• Examining the Stack

• Printing Variable Values

• Listing the Source Code

• Setting Variable Values During a Debug Session

In this sample session, we emphasize user input like this: input, to make it easier to pick out from

the surrounding output.

One of the preliminary versions of GNU m4 (a generic macro processor) exhibits the following bug:

sometimes, when we change its quote strings from the default, the commands used to capture one

macro definition within another stop working. In the following short m4 session, we define a macro

foo which expands to 0000; we then use the m4 built-in defn to define bar as the same

thing. However, when we change the open quote string to <QUOTE> and the close quote string

to <UNQUOTE>, the same procedure fails to define a new synonym baz:

$ cd gnu/m4 //change your current directory to the location where the m4 executable is stored.

$ ./m4 //run the m4 application

define(foo,0000)

foo

0000

define (bar,defn('foo'))

bar

0000

changequote(<QUOTE>,<UNQUOTE>)

define(baz,defn(<QUOTE>foo<UNQUOTE>))

baz

C-d

m4: End of input: 0: fatal error: EOF in string

Loading the Executable

Let us use GDB to try to see what is going on.

$ (gdb) m4

HP gdb 3.0 for PA-RISC 1.1 or 2.0 (narrow), HP-UX 11.00.

Hewlett-Packard Wildebeest 3.0 (based on GDB ) is covered by the

GNU General Public License. Type "show copying" to see the conditions to

change it and/or distribute copies. Type "show warranty" for warranty/support.

GDB reads only enough symbol data to know where to find the rest when needed; as a result, the

first prompt comes up very quickly.

Loading the Executable 15