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Table of Contents
excludeAt a Glance

Prerequisites

Debugging

More important than knowing how to make something work is knowing what to do when it doesn't. Debugging is probably one of the most important skills you can learn, since things rarely work perfectly the first time.

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Code Block
languagecpp
titlemain.c
linenumberstrue
#include <stdint.h>
#include "soft_timer.h"
#include "gpio.h"
#include "interrupt.h"
#include "log.h"
#include "wait.h"

#define DEBUG_TEST_TIMEOUT_MS 50

typedef struct DebugTestStorage {
  GpioAddress *leds;
  size_t num_leds;
  size_t current_led;
} DebugTestStorage;

static DebugTestStorage s_storage;

static void prv_init_leds(DebugTestStorage *storage) {
  GpioAddress leds[] = {
    { .port = GPIO_PORT_C, .pin = 8 },  //
    { .port = GPIO_PORT_C, .pin = 9 },  //
    { .port = GPIO_PORT_C, .pin = 6 },  //
    { .port = GPIO_PORT_C, .pin = 7 },  //
  };

  GPIOSettingsGpioSettings led_settings = {
    .direction = GPIO_DIR_OUT,
    .state = GPIO_STATE_HIGH
  };

  storage->leds = leds;
  storage->num_leds = SIZEOF_ARRAY(leds);
  storage->current_led = 0;

  for (size_t i = 0; i < SIZEOF_ARRAY(leds); i++) {
    gpio_init_pin(&leds[i], &led_settings);
  }
}

static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
  DebugTestStorage *storage = context;

  // Toggle the current LED
  GpioAddress *led = &storage->leds[storage->current_led];
  LOG_DEBUG("Toggling LED P%d.%x\n", led->port, led->pin);
  gpio_toggle_state(led);

  // Next LED
  storage->current_led = (storage->current_led + 1) % storage->num_leds;

  soft_timer_start_millis(DEBUG_TEST_TIMEOUT_MS, prv_timeout_cb, &storage, NULL);
}

int main(void) {
  interrupt_init();
  soft_timer_init();
  gpio_init();

  prv_init_leds(&s_storage);
  uint32_t buffer[20];
  LOG_DEBUG("%ld LEDs set up\n", s_storage.num_leds);
  soft_timer_start_millis(DEBUG_TEST_TIMEOUT_MS, prv_timeout_cb, &s_storage, NULL);

  size_t i = 0;
  while (true) {
    wait();
    buffer[i % SIZEOF_ARRAY(buffer)] += i;
    i++;
  }

  return 0;
}

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Code Block
languagebash
titleGDB
(gdb) r
Starting program: /home/vagrant/shared/firmware/build/bin/x86/debug_test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[0] projects/debug_test/src/main.c:58: 4 LEDs set up
[0] projects/debug_test/src/main.c:43: Toggling LED P2.8

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400c01 in printf (__fmt=0x401623 "[%u] %s:%u: Toggling LED P%d.%x\n")
    at /usr/include/x86_64-linux-gnu/bits/stdio2.h:104
104       return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
(gdb)

Woah, what happened? It seems like we hit a segfault, but GDB was able to catch it. Let's take a closer look at what caused that.

...


 Info
If instead you get an error like

Program received signal SIG37, Real-time event 37.
0x00007ffff75fd369 in __sigqueue (pid=5662, sig=37, val=..., val@entry=...) at ../sysdeps/unix/sysv/linux/sigqueue.c:40
40 ../sysdeps/unix/sysv/linux/sigqueue.c: No such file or directory.
Try exiting gdb and running the following command, then trying again (see https://stackoverflow.com/a/16742034):
echo "set auto-load safe-path /" >> ~/.gdbinit
Woah, what happened? It seems like we hit a segfault, but GDB was able to catch it. Let's take a closer look at what caused that.
 Info
It's also possible that you won't get a segfault, but it should produce the wrong output!

First, we use backtrace to print the state of the stack. We can see that the segfault is occuring in prv_timeout_cb, which is the timer callback we registered. We can use up and down to move between function scopes in the stack. In our case, we move up to prv_timeout_cb. Since it's a segfault, we're probably attempting to access invalid memory. Looking at led and storage, we see that they're completely invalid. Since led is derived from storage, we should look at where storage is defined.
...
 Code Block
languagebash
titleGDB
(gdb) bt
#0  0x0000000000400c11 in printf (__fmt=0x401623 "[%u] %s:%u: Toggling LED P%d.%x\n")
    at /usr/include/x86_64-linux-gnu/bits/stdio2.h:104
#1  prv_timeout_cb (timer_id=<optimized out>, context=0x7fffffffd9c0)
    at projects/debug_test/src/main.c:45
#2  0x0000000000400e7e in prv_soft_timer_interrupt () at libraries/ms-common/src/x86/soft_timer.c:38
#3  prv_soft_timer_handler (interrupt_id=<optimized out>)
    at libraries/ms-common/src/x86/soft_timer.c:49
#4  <signal handler called>
#5  0x0000000000400ad0 in main () at projects/debug_test/src/main.c:64
(gdb) up
#1  prv_timeout_cb (timer_id=<optimized out>, context=0x7fffffffd9c0)
    at projects/debug_test/src/main.c:45
45        LOG_DEBUG("Toggling LED P%d.%x\n", led->port, led->pin);
(gdb) p led
$1 = (GPIOAddress *) 0x800000c029c0
(gdb) p *led
Cannot access memory at address 0x800000c029c0
(gdb) l
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
41        DebugTestStorage *storage = context;
42
43        // Toggle the current LED
44        GPIOAddress *led = &storage->leds[storage->current_led];
45        LOG_DEBUG("Toggling LED P%d.%x\n", led->port, led->pin);
46        gpio_toggle_state(led);
47
48        // Next LED
49        storage->current_led = (storage->current_led + 1) % storage->num_leds;
(gdb) p storage
$2 = (DebugTestStorage *) 0x7fffffffd9c0
(gdb) p *storage
$3 = {leds = 0x7fffffffd9c0, num_leds = 3257981071963740416, current_led = 6301696}
(gdb)

...
 Code Block
languagebash
titleGDB
(gdb) b prv_timeout_cb
Breakpoint 1 at 0x400bd6: file projects/debug_test/src/main.c, line 40.
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/vagrant/shared/firmware/build/bin/x86/debug_test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[0] projects/debug_test/src/main.c:60: 4 LEDs set up
Breakpoint 1, prv_timeout_cb (timer_id=0, context=0x602130 <s_storage>)
    at projects/debug_test/src/main.c:40
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
(gdb) n
45        LOG_DEBUG("Toggling LED P%d.%x\n", led->port, led->pin);
(gdb)
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
(gdb)
44        GPIOAddress *led = &storage->leds[storage->current_led];
(gdb)
41        DebugTestStorage *storage = context;
(gdb)
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
(gdb) p storage
$1 = (DebugTestStorage *) 0x602130 <s_storage>
(gdb) p &s_storage
$2 = (DebugTestStorage *) 0x602130 <s_storage>
(gdb)
...
 Code Block
languagebash
titleGDB
(gdb) c
Continuing.
[0] projects/debug_test/src/main.c:45: Toggling LED P2.8

Breakpoint 1, prv_timeout_cb (timer_id=1, context=0x7fffffffd9c0)
    at projects/debug_test/src/main.c:40
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
(gdb) n
45        LOG_DEBUG("Toggling LED P%d.%x\n", led->port, led->pin);
(gdb)
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
(gdb)
44        GPIOAddress *led = &storage->leds[storage->current_led];
(gdb)
41        DebugTestStorage *storage = context;
(gdb)
40      static void prv_timeout_cb(SoftTimerIDSoftTimerId timer_id, void *context) {
(gdb) p storage
$3 = (DebugTestStorage *) 0x7fffffffd9c0
(gdb) p *storage
$4 = {leds = 0x7fffffffd9c0, num_leds = 5124990741377889536, current_led = 6301696}
(gdb)
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Hopefully, you were able to see how GDB allows you to observe the state and data of your program without recompiling or relying on debug output. If you are more of a visual learner, CS246 has an amazing GDB tutorial video on their Youtube page that goes over these commands and some other helpful ones. Although they use C++, many of the same principles still apply to C. 
 Info
There's actually another issue in this code. The expected behavior is for all LEDs to toggle in sequence, but the values are all wrong. Can you figure out why?
Hint: Be careful with addresses to stack-allocated variables!
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