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Minimalist Assert Function for Embedded C


Assert is a great tool to find errors, bugs, or unintended behaviors on runtime. If implemented correctly, asserts give developer context information about when and where in the code an issue showed up.

In this post, I will discuss a minimalist assert implementation in C suitable for bare-metal implementations.

Disclaimer

  • You should apply this information only to non-safety-critical embedded systems.
  • This post is based on a post by Memfault.
  • I run the examples on my laptop because when I wrote this post I was on vacation, but the ideas are still valid for any architecture.

Requirements

You will need a data structure to save all the Program Counter (PC) and Linker Register (LR).

  • The PC is the current program address. This register can be written to control the program flow.
  • The LR register that stores the return address, such as when making a function call.

For example, in the following code,

    int y = 0;
    y = 2; 
    y = funcA();
    printf("y = %i\r\n", y);

in the function call y = funcA(), once the code is inside funcA(). The value of LR will be the address of y = funcA(). You can verify it by looking at dissambly with, for example, objdump -D. If you want to know how? Read this post.

In order to save the LR and PC, I created a struct called sAssertInfo and defined a function my_assert(). This function will save the values of LR and PC in variable g_assert_info.

typedef struct sAssertInfo {
    uint64_t pc;
    uint64_t lr;
} sAssertInfo;

sAssertInfo g_assert_info; 

void my_assert(const uint64_t *pc, const uint64_t *lr) {
    g_assert_info.pc = pc;
    g_assert_info.lr = lr;
    while(1){};
}

Note that there is a while(1){} at the end of my function. For debugging purposes it is useful, because the code will stop there and that allows you to solve one problem at a time. That means, for production, you may use a recovery and logging mechanism.

Base Implementation

Let’s start with the C file. Note that my_assert() needs uint64_t pointers. You should change them based on your architecture. For example for ARM 32-bit chips, you should use uint32_t for the PC and LR pointers.

*.c File

// my_asserts.c
#include "my_asserts.h"

static sAssertInfo g_assert_info;

void my_assert(const uint64_t *pc, const uint64_t *lr) {
    g_assert_info.pc = pc;
    g_assert_info.lr = lr; 
    while(1){};
}

Now let’s write the header file, here is where the magic happens.

*.h File

I used macros for readability purposes, because the code for reading the PC and LR involves assembly.

#pragma once

#ifndef _MY_ASSERTS_H_ 
#define _MY_ASSERTS_H_ 

#include <stdint.h>

typedef struct sAssertInfo {
    uint64_t pc;    
    uint64_t lr;   
} sAssertInfo;

void my_assert(const uint64_t *pc, const uint64_t *lr);

#define GET_LR() __builtin_return_address(0)
// #define GET_PC(_a) __asm volatile ("mov %0, pc" : "=r" (_a)) // Comment out this for ARM
#define GET_PC(_a) __asm volatile("1: lea 1b(%%rip), %0;": "=a"(pc));  // Comment this for x86

#define MY_ASSERT_RECORD()       \
    do {                         \
      void *pc = NULL;           \
      GET_PC(pc);            \
      const void *lr = GET_LR(); \
      my_assert(pc, lr);         \
    } while (0)

#define ASSERT_EXP(exp)                          \
    do {                                        \
        if (!(exp)) { MY_ASSERT_RECORD(); }     \
    } while (0)

#endif /* _MY_ASSERTS_H_ */

Things to notice:

  • #pragma once. From Wikipedia: “is a non-standard but widely supported preprocessor directive designed to cause the current source file to be included only once in a single compilation. Thus, #pragma once serves the same purpose as #include guards, but with several advantages, including: less code, avoiding name clashes, and improved compile speed.”

  • void *pc. It’s to take advantage that a void pointer can hold address of any type and it can be really useful if the you are not sure about the data type.

  • while(0) at the end of each #define. The reason is good explained in stackoverflow. Basically, it helps to group multiple statements into a single one so that a function-like macro can actually be used as a function.

Simple Example

There are two functions funcA() and funcB(). funcB() calls funcA(), and this one contains a failing assert. ASSERT_EXP(0) means that the code will crash there, because 0 is interpreted as false .

#include <stdio.h>
#include <inttypes.h>
#include "my_asserts.h"

uint64_t x = 0;
uint64_t funcA(void)
{
    x = 8;
    ASSERT_EXP(0);
    return x;
}

uint64_t funB(void)
{
    uint64_t y = 0;
    y = funcA();
    return y;
}


void main ()
{
    funB();
}

Let’s compile the code:

$ gcc -g -c -m64 main.c
$ gcc -g -c -m64 my_asserts.c
$ gcc my_asserts.o main.o -o example_assert

I use gdb to get the values of PC/LR held by g_assert_info.

$ gdb example_assert 
# Inside GDB execute run and then ctrl-D
GNU gdb (Ubuntu 8.1.1-0ubuntu1) 8.1.1
Copyright (C) 2018 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 example_assert...done.


(gdb) run
Starting program: /home/me/Documents/git/t/example_assert 
^C
Program received signal SIGINT, Interrupt.


my_assert (pc=0x555555554639 <funcA+27>, lr=0x55555555467d <funB+21>) at my_asserts.c:8
8           while(1){};

(gdb) quit

As you can see, the function my_assert() is called and gdb prints the values of PC/LR.

my_assert (pc=0x555555554639 <funcA+27>, lr=0x55555555467d <funcB+21>) at main.c:35

That means that: PC was 0x639 and LR was 0x67d, when the program crashed due to ASSERT_EXP(0).

Retrieving Code Info

With the tool addr2line you can convert addresses into file names and line numbers.

Let’s start with the PC:

$  addr2line -e example_assert 0x639
/home/me/main.c:9

$  sed -n '9,1p' main.c
    ASSERT_EXP(0);

And now the LR:

$  addr2line -e example_assert 0x67d
/home/me/main.c:16

$  sed -n '16,1p' main.c
    y = funcA();

As expected, PC is the address where the code stopped ASSERT_EXP(0), and LR is where funcA() was called.

As a note, I encourage you to check the dissambly of example_assert to actually see which instructions the addresses correspond to. Try it with objdump -D example_assert.

Conclusion

Asserts are great tools to find errors, bugs or unintended behaviors on runtime. If implemented correctly, asserts give developer context information about when and where in the code an issue showed up.

I discussed a minimalist assert implementation in C suitable for bare-metal implementations. It’s not as straightforward as a simple printf, but it’s way more lightweight and most likely useful.

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