How to debug NuttX using QEMU and GDB

This guide explains the steps needed to use QEMU and GDB to debug an ARM board (lm3s6965-ek), but it could be modified to work with other board or architecture supported by QEMU.

Start configuring and compiling the lm3s6965-ek board with qemu-flat profile.

Compiling

Configure the lm3s6965-ek

There is a sample configuration to use lm3s6965-ek on QEMU.

Just use lm3s6965-ek:qemu-flat board profile for this purpose.
$ cd nuttx $ ./tools/configure.sh lm3s6965-ek:qemu-flat
Compile
$ make -j

Start QEMU

You need to start QEMU using the nuttx ELF file just create above:

$ qemu-system-arm -M lm3s6965evb -device loader,file=nuttx -serial mon:stdio -nographic -s
Timer with period zero, disabling
ABCDF
telnetd [4:100]

NuttShell (NSH) NuttX-12.0.0
nsh>

Start GDB to connect to QEMU

These steps show how to connect GDB to QEMU running NuttX:

$ gdb-multiarch -ix tools/nuttx-gdbinit nuttx
(gdb) target extended-remote localhost:1234
Remote debugging using localhost:1234
0x000012ee in up_mdelay (milliseconds=milliseconds@entry=250)
    at common/arm_mdelay.c:51
51             for (j = 0; j < CONFIG_BOARD_LOOPSPERMSEC; j++)
(gdb)

From (gdb) prompt you can run commands to inpect NuttX:

(gdb) info_nxthreads
target examined
_target_arch.name=armv7
$_target_has_fpu : 1
$_target_has_smp : 0
saved current_tcb (pid=0)
*
0 Thread 0x20001538  (Name: Idle Task, State: Running, Priority: 0, Stack: 464/1000) PC: 0x12fc in up_mdelay()
saved current_tcb (pid=0)

1 Thread 0x20005060  (Name: hpwork, State: Waiting,Semaphore, Priority: 224, Stack: 320/1992) PC: 0x47dd in work_thread()
saved current_tcb (pid=0)

2 Thread 0x20005c30  (Name: nsh_main, State: Waiting,Semaphore, Priority: 100, Stack: 1016/2000) PC: 0x1 in _vectors()
saved current_tcb (pid=0)

3 Thread 0x20006b40  (Name: NTP daemon, State: Waiting,Signal, Priority: 100, Stack: 864/1952) PC: 0x0 in _vectors()
saved current_tcb (pid=0)

4 Thread 0x20008540  (Name: telnetd, State: Waiting,Semaphore, Priority: 100, Stack: 616/2008) PC: 0x20008fd4 in No()
saved current_tcb (pid=0)
saved current_tcb (pid=0)
saved current_tcb (pid=0)
saved current_tcb (pid=0)
(gdb)

As you can see QEMU and GDB are powerful tools to debug NuttX without using external board or expensive debugging hardware.