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Breakpoint

Breakpoint

Learn about breakpoints and how they are useful for debugging by executing a code line by line.

We'll cover the following...

The print command in GDB prints out the value of a variable that’s defined in the current stack.

We’ll continue to use the “go.c” file from the previous lesson, as a reference for this lesson. We’ll try to print the s variable (which is supposed to be a character buffer). Let’s first however define a breakpoint.

Inserting a breakpoint

A breakpoint is a flag that will stop the program at a specific line of code. Let’s insert a breakpoint on line 9 of our code. This means the program will stop before executing that line of code and leave us in the GDB debugger. We use the GDB command break to insert a breakpoint, and then the run command to run the program:

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/usercode$ gdb go
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
Copyright (C) 2020 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-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 go...
(gdb) break 9
Breakpoint 1 at 0x11ee: file go.c, line 9.
(gdb) run
Starting program: /usercode/go 
Breakpoint 1, getWord (maxsize=256) at go.c:9
9         scanf("%s", s);
(gdb)

Now (gdb) prompt is waiting for input, and we are stopped just short of ...