A simple debug session

Let's assume an x86_64 target using a basic TCP/IP configuration. The following lines (from the buildfile you'd use to create the OS image) show what's needed to host the sample session:

io-sock -m phy -m usb -d axge
if_up -p axge0
ifconfig axge0 192.0.2.172/24
devc-pty &
[+session] pdebug 8000 &

We'll be using the x86_64 compiler. Note the -g option, which makes the compiler include debugging information:

$ qcc -V gcc_ntox86_64 -g -o hello hello.c  

For this simple example, the source can be found in our working directory. First, let's start the session; to reduce document clutter, we'll run the debugger in quiet mode:

ntox86_64-gdb -quiet

The gdb debugger provides its own shell; by default its prompt is (gdb). Specify the target IP address and the port used by pdebug:

(gdb) target qnx 10.0.1.172:8000
Remote debugging using 10.0.1.172:8000
MsgNak received - resending
Remote target is little-endian

Upload the debug executable to the target. Note that the file has to be in the target system's pathname space, so we can get the executable via a network filesystem, sftp, or, if no filesystem is present, via the upload command:

(gdb) upload hello /tmp/hello

Load symbolic debug information from the current working directory (here, hello must reside on the host system):

(gdb) file hello
Reading symbols from hello...done.

Start the program:

(gdb) run hello
Starting program:  /tmp/hello
Trying to find symbol file for ldqnx-64.so.2

Set a breakpoint on main():

(gdb) break main
Breakpoint 1 at 0x80483ae: file hello.c, line 8.

Allow the program to continue to the breakpoint found at main():

(gdb) c
Continuing.
Breakpoint 1, main () at hello.c:8
8       gettimeofday (&when, NULL);

While you're in a debug session, you can use any of the following commands as the next action:

next

Continue to the next source line in the current (innermost) stack frame.

list

List the specified function or line.

break

Set a breakpoint on the specified function or line.

help

Get the help main menu.

help data

Get the help data menu.

help inspect

Get help for the inspect command.

inspect y

Inspect the contents of variable y.

set y=3

Assign a value to variable y.

bt

Get a backtrace.

Let's see how to use some of these basic commands. First, here's the list command:

(gdb) list
3
4   main () {
5       struct timeval when;
6       int x,y,z;
7
8       gettimeofday (&when, NULL);
9       printf ("Hi ya!\n");
10
11      x=3;
12      y=2;

Press Enter to repeat the last command:

(gdb) <enter>
13      z=3*2;
14
15      exit (0);
16  }

Break on line 11:

(gdb) break 11
Breakpoint 2 at 0x80483c7: file hello.c, line 11.

Continue until the first breakpoint:

(gdb) continue
Continuing.
Hi ya!
Breakpoint 2, main () at hello.c:11
11      x=3;

Inspect variable y, using the short form of the inspect command:

(gdb) inspect y
$1 = -1338755812

Get some help on the step and next commands:

(gdb) help s
Step program until it reaches a different source line.
Usage: step [N]
Argument N means step N times (or till program stops for another reason).
(gdb) help n
Step program, proceeding through subroutine calls.
Usage: next [N]
Unlike "step", if the current source line calls a subroutine,
this command does not enter the subroutine, but instead steps over
the call, in effect treating it as a single source line.

Go to the next line of execution:

(gdb) next
12      y=2;
(gdb) next
13      z=3*2;
(gdb) inspect z
$2 = 1
(gdb) next
15      exit (0);
(gdb) inspect z
$3 = 6

Continue program execution:

(gdb) continue
Continuing.

Program exited normally.

Quit the debugger session:

(gdb) quit
The program is running. Exit anyway? (y or n) y