Dangerous Embedded C Coding Standard Rules (2011) | Hacker News Reader

Dangerous Embedded C Coding Standard Rules (2011)

embeddedgurus.com

34 pointsmomo-reina48 comments

Well, let's see.

#1: Avoiding shifting versus doing division: Nope. Crappy compilers and bad architectures will yield bad results. Know what you're doing.

#2: Use a typedef instead of a bare C type, then. Also, on many systems space is king (the last embedded system I wrote had 6 bytes left over in code space, and 16-bit operations were incredibly expensive and nearly unaffordable).

#3: Just make the code clear. This rule is lunacy.

#4: Well, maybe. Except that variable initialization can often be made quite compact by the runtime (e.g., an expansion of a compressed data segment into RAM), and so save you code space.

#5: Again, the compiler can screw you here. Also, it's possible to take the address of a constant and have it be wiped out (for naïve implementations, anyway).

My basic rules:

#0: Don't make assumptions. Know what the heck is going on, fractally. Look at the generated code.

#1: Be a responsible engineer. Design and code for production and testing, and for the next guy. This often means not being clever, but if you are being clever, be forthright about it and don't leave your brilliance as a puzzle. (Don't be clever just to be clever. That will get you fired, no foolin').

#2: Avoid platitudes. Be wary of people who tell you not to use something, always, or to use a particular contruct, always. Be suspicious of code that follows someone else's dogma. [I once worked with a code base where the engineer had used the ternary -- "?:" -- operator everywhere because he thought it was more efficient than using 'if'. Of course it was just guesswork on his part and not only was his code a train wreck, but he had never measured his results. We fired that guy).

Embedded systems are just software, they're not magical, they're not (usually) that hard, and for some reason people are afraid of them. Some of my best moments in the industry have been writing these little things, they're fun as hell.

Why are agreeing with OP on rule #3 and then saying it is lunatic? Did I miss something?

Quote:Better Rule: Use whatever comparison operator is easiest to read in a given situation.

One of the very best things any embedded programmer can do is to make their code as readable as possible to as broad an audience as possible.

> #1: Avoiding shifting versus doing division: Nope. Crappy compilers and bad architectures will yield bad results. Know what you're doing.

Which compiler, among those still used in 2014, does not convert a division by 2 into a shift?

Memory escapes me...is a shift guaranteed by the standard (and if so, which) to sign-extend or not sign-extend?

A right shift of a signed integer is implementation defined.

No, it isn't.

A right shift of a signed, negative value is implementation-defined (which means, something in particular happens, and the compiler or its documentation is supposed to tell you what).

A left shift of a signed value is undefined (which means, the compiler is allowed to launch the missiles, or fail to launch the missiles, whichever is worse for you) if a nonzero bit could conceivable get into the sign.

It's not just division by 2. Most compilers will convert any division by a constant into either a shift (powers of 2) or a multiply (all other).

But there are lots of oddball compilers for CPUs, DSPs, and other embedded quasi processors you've never heard of. Many don't really optimise at all.

This is one of those blog posts where the comments/discussion are really good - maybe even better than the article itself. I recommend reading the discussion on-page if you haven't.

Another reason to pay attention to #2: Most CPUs perform simple arithmetic operations fastest on their native word size. On a 32-bit machine, addition and subtraction on 8-bit variables is probably slower than on 32-bit variables. (Often the hardware will transparently cast to 32 bits, perform the operation, then cast back to 8 bits.)

I'm not an authority on this topic, so I don't want you to take away the message "always use the machine word size". What I do want you to take away is that there are many cases where an "obvious" optimization is counterproductive in practice. Always start with simple, working code, and never optimize without benchmarking.

For this case, the "fast" int types are provided, eg. uint_fast8_t is the fastest unsigned integer which can hold at least 8 bits; uint8_t is required to have exactly 8 bits.

http://pubs.opengroup.org/onlinepubs/7999959899/basedefs/std...

With regards to Bad Rule #3, "Avoid >= and use <," if I saw

    if (speed < 100) ... else if (speed > 99)

in code, my only response would be to question the author's sanity.

But then, I question the sanity of C standards committee members all the time, specifically regarding his actual complaint related to Bad Rule #2 and unspecified-width integer types[1].

[1] http://www.netrino.com/Embedded-Systems/How-To/C-Fixed-Width...

The phrase “C standards committee” is eminently ambiguous, as the standards that define the C language (e.g. ISO/IEC 9899) are the work of a committee. These committee members have constraints coming at them from all sides and do a very good job or maintaining compatibility with the past and future-proofing the C language at the same time.

This has nothing to do with coding standards, which any fool can define (and some have), and for which the only instance that really deserves the name of “standard” is MISRA-C. The other such “standards” have a scope typically limited to a company and its providers, and reflect the style and idiosyncrasies of the person who was in charge of defining it (they are rarely the work of a committee).

I'm not completely on board with #4 (initialization of variables). I agree that 'declaring and then assigning a value' _within the same block_ vs 'initialized declaration' has no speed gains, only downsides.

However, if the assignment can happen in a _different block_ (maybe inside an 'if' block) you could save 1 memory write, depending on how many times the if condition is satisfied.

This obviously optimizes for speed at the expense of maintainability, and it's for the programmer to make intelligent trade offs, but the fact is that one method is faster than the other.

Silly example that decrements 'a' repeatedly if condition is non-zero:

   //temp = 0 inside if block
   int dec(int a, int condition) {
       int temp;

       if (condition) {
           temp = 0;  //<-- memory write depends on condition

           //compute something here in a loop
           while (temp < a) {
               a -= (temp++);
           }
       }

       return a;
   }

   //temp = 0 at declaration
   int dec2(int a, int condition) {
       int temp = 0; // <-- memory write always executed

       if (condition) {
           //compute something here in a loop
           while (temp < a) {
               a -= (temp++);
           }
       }

       return a;
   }

We can disassemble the output to verify that dec() will only write to memory if the condition was satisfied (see 400458), while dec2() will always write into temp (see 400482):

    # <dec>
    400448:  push   %rbp
    400449:  mov    %rsp,%rbp
    40044c:  mov    %edi,0xffec(%rbp)
    40044f:  mov    %esi,0xffe8(%rbp)
    400452:  cmpl   $0x0,0xffe8(%rbp)  # if (condition)
    400456:  je     400473 <dec+0x2b>
    400458:  movl   $0x0,0xfffc(%rbp)  # temp = 0 <-- depends on condition
    40045f:  jmp    40046b <dec+0x23>  # while
    400461:  mov    0xfffc(%rbp),%eax
    400464:  sub    %eax,0xffec(%rbp)
    400467:  addl   $0x1,0xfffc(%rbp)
    40046b:  mov    0xfffc(%rbp),%eax
    40046e:  cmp    0xffec(%rbp),%eax
    400471:  jl     400461 <dec+0x19>  # loop
    400473:  mov    0xffec(%rbp),%eax  # return a
    400476:  leaveq
    400477:  retq

    # <dec2>
    400478:  push   %rbp
    400479:  mov    %rsp,%rbp
    40047c:  mov    %edi,0xffec(%rbp)   
    40047f:  mov    %esi,0xffe8(%rbp)
    400482:  movl   $0x0,0xfffc(%rbp)   # temp = 0 <-- always executed
    400489:  cmpl   $0x0,0xffe8(%rbp)   # if (condition)
    40048d:  je     4004a3 <dec2+0x2b>
    40048f:  jmp    40049b <dec2+0x23>  # while
    400491:  mov    0xfffc(%rbp),%eax
    400494:  sub    %eax,0xffec(%rbp)
    400497:  addl   $0x1,0xfffc(%rbp)
    40049b:  mov    0xfffc(%rbp),%eax
    40049e:  cmp    0xffec(%rbp),%eax
    4004a1:  jl     400491 <dec2+0x19>  # loop
    4004a3:  mov    0xffec(%rbp),%eax   # return a
    4004a6:  leaveq 
    4004a7:  retq

The above code was compiled with gcc 4.1.2 on amd64/linux. gcc -O2 and gcc -O3 completely do away with the 'temp' variable and generate fewer instructions.

I'd suggest that, if a variable with an undefined value is a valid state for the program, the declaration is in the wrong place. How about this alternative?

   int dec(int a, int condition) {
       if (condition) {
           int temp = 0;

           //compute something here in a loop
           while (temp < a) {
               a -= (temp++);
           }
       }

       return a;
   }

That's the obvious next step, but it's not valid C89 and that may or may not be relevant for people doing embedded programming. Personally, I'm all for using newer standards and declaring variables locally when it makes sense. Sometimes I like to see all variables at the top of the function though.

I'm not particularly familiar with x86-64 calling conventions. Where is the code that sets up the stack frame? Would this code look different if temp were created inside the if() block, not just initialized there?

In C, variables have function-level storage. I don't recall the actual term from the C standard, but the storage for _all_ variables declared in any blocks inside a function is allocated at the beginning of the function, when the stack frame is set up. So there is 'no way' to create 'temp' only inside the if block.

Okay. I knew variables had block-level scope. I've read some of the ISO C standard, but hadn't caught the part where variables have function-level storage. I was wondering because, years ago, I read some disassembled code that did lots of stack allocations in the middle of functions, and not just for the purpose of calling another function.

Can you point to a reference showing that variables have function-level storage? The closest I can find in the C standard draft I'm looking at is 6.2.4.6, which suggests block-level lifetimes for variables: "For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way."

Completely agree with no. 5. After a year with C I realized that macros are only to be used as control structures for headers and ifdefs to enable compatibility.

If you need macros to hack C to enable some functionality not inherent to the language, you should change the approach or switch to a different language.

Also macros are usually used to "speed up" the code. You should never optimize before finding the real bottleneck in your code. And thus always use functions instead of macros.

Macros enable you to replace things at compile time.

The C preprocessor is a powerful and awesome thing by itself and I miss it when it's not available in other languages. It enables you to do things like pass line and file numbers automatically. For instance -

    #define log_message(format...) _log_message(__FILE__, __LINE__, format)
    void _log_message(char* filename, int line, char* format, ...);

And there's nothing inherently wrong with #define for frequently used magic numbers. They have the advantage of being pre-processed out rather than being separate variables at compile time.

Macros are also useful to simplify little repeated chunks of code that don't really need their own function.

I've not often seen macros used to try to speed up code.

--edit-- I don't know how long you've kept up with your C, but what you've realised after a year with the language doesn't seem as obvious to me after 15.

> If you need macros to hack C to enable some functionality not inherent to the language, you should change the approach or switch to a different language.

I think if you continue to do serious C hacking, you'll find a lot of places where macros are legitimately a good choice.

For example, I've found that macros are very, very useful when it comes to implementing error handling in a robust way. In C, the only real way to detect whether a function has failed is to return an error code. The caller then has to test for the error code and take the appropriate action if there was an error. I'm working on a side project now that was doing a LOT of this because it does a lot of IO and memory allocation (and basically any operation you do on a file or any call to `malloc` or a function that calls `malloc` can spontaneously fail in C). So my source code, after a while, ended up looking more or less like this:

    if ((f = open_file()) == NULL)
        return ERROR_CODE;
    if ((s = allocate_string()) == NULL)
        return ERROR_CODE;
    if (write_string_to_file(s, f) < 0)
        return ERROR_CODE;

... and so on, and so forth. It got to the point where basically all of the function calls I did needed to be manually checked for an error, which made all of my code really messy and hard to read. The only real way to refactor these checks in pure C without sacrificing robustness is to use a macro:

    #define MY_ASSERT(c) do { if (!(c)) return ERROR_CODE; } while (0)
    MY_ASSERT((f = open_file()) != NULL);
    MY_ASSERT((s = allocate_string()) != NULL);
    MY_ASSERT(write_string_to_file(s, f) >= 0);

This has the same effect as the code above, without sacrificing comprehensibility or readability.

> Also macros are usually used to "speed up" the code. You should never optimize before finding the real bottleneck in your code. And thus always use functions instead of macros.

I think a lot of people on HN and /r/programming fail to understand why premature optimization is a bad thing. A lot of programmers hide behind the "premature optimization is the root of all evil" thing to justify their pre-existing biases or their inefficient code. The point is that you shouldn't spend a great deal of time optimizing functions or algorithms unless optimizing those functions or algorithms will be productive. It's wasteful to spend a bunch of time optimizing your IO operations, for example, if 95% of the time is being spent in the database. The macro-vs-function debate has nothing to do with that. Writing a macro definition takes no more effort than writing a function definition, so you shouldn't be opposed to writing

    #define SOME_OPERATION(x) (((x) * 100) / 3 + 500)

rather than

    int some_operation(int x) { return (x * 100) / 3 + 500; }

That isn't premature optimization, you're just guaranteeing that the compiler won't introduce the overhead of a function call.

I have to strongly disagree with your example. In fact I feel it may just be one of the worst usages of macros I have seen in that you are hiding the control flow of the program and thus obfuscating your code for other programmers. It might be easier for you to read because you wrote it but for every one that comes after you it will be a massive pain that adds unneeded additional cognitive overhead.

I say this from the point of view of someone having to debug your code: "OK so step, step, step, run to this line... wait, what? How did this function return when there is no return statement? Let me run that again from the beginning... ... ... What? This ASSERT macro doesn't actually assert it just returns?"

Keyboard out window etc.

Please stick to language conventions. Macros are bad because they hide what the code is actually doing.

To be fair, I don't see why you would have to throw your keyboard out the window. If I were debugging a source file and saw the same ASSERT macro on every other line, I would break out `grep` and track down the (extremely simple) definition so I could understand what it's doing. Moreover, returning with an error code is IMO entirely sane behavior for a macro with ASSERT in the name, especially considering halting execution of the program altogether generally isn't a good thing if you're writing a C library.

The point of assert is to check your code has values within your required parameters, so you the code is easier to debug while coding. Asserts should never be left in your code compiled for release. Even c library assert is aware of that, as it has the NDEBUG macro, which removes every assert in the code. That is also the reason why you should Never put code that might mutate your variables in them.( function calls or assigning, for example)

http://www.cplusplus.com/reference/cassert/assert/

Quote from the link: Therefore, this macro is designed to capture programming errors, not user or run-time errors, since it is generally disabled after a program exits its debugging phase.

Of course you can name your macro whatever you want, even assert, but it will confuse other developers used to c library functions.

Fair enough -- assert was just the first name that came to mind. Perhaps something more descriptive like RETURN_ERROR_IF_FALSE would have been a better name.

The first block of code seems more readable to me than the second. I'm used to seeing the first; it's idiomatic C. I don't understand why it says "do ... while (0)" in the macro, for instance. Wouldn't you want to handle different errors in different ways anyway? In your example, you're just going to leave the file open and the string allocated? Isn't trying to influence how the compiler compiles it by using a macro instead of a function a clear example of (premature) optimization?

> The first block of code seems more readable to me than the second. I'm used to seeing the first; it's idiomatic C.

The benefits of the macro (IMO) aren't obvious from the example I gave. Like I said, I'm currently working on a personal project where macros have been the saving grace. Attaching a conditional return to every individual statement was making my code extremely hard to maintain (even for myself, who wrote it) -- wrapping common error cases in a macro made everything a whole lot simpler, and allowed me to work on my algorithms at a high level without having to run my eyes over the same error-checking `if` statements a million times.

> I don't understand why it says "do ... while (0)" in the macro, for instance.

It's normal for many macros, so that you can use them like normal statements.

> In your example, you're just going to leave the file open and the string allocated?

I sort of just made that example up off the top of my head -- you're right about the memory leak.

Regarding do ... while(0): http://stackoverflow.com/questions/1067226/c-multi-line-macr...

That is almost exactly like some code I have written in a project and at that time it did it's job. After revisiting the project months later after I have completely forgotten the details, I got frustrated over the macro and what did it actually do( even though I have written it ). It took me some time to track down the macro definition and convert the code in my head to understand it again. But if the code would be simple C as in your first example I wouldn't waste time on it.

You're right, the example is far too simple to warrant the introduction of a macro like this.

"It took me some time to track down the macro definition "

Right-click -> Go to definition?

Your IDE must suck.

In Vim, Ctrl-W followed by i will try to open the definition of the identifier under the cursor in the files included by the current file (sometimes it only finds references or useless mentions inside of comments, though).

What a great comment from a longtime member of HN. Not only did you make a wonderful example of name-calling and an ad-hominem argument in a single sentence, you also failed to dispute the central argument which was that time is lost while translating the macro.

Apart from your already ( apparently )wast knowledge of various development tools, please also consider improving your conversational skills.

The code I used is more like this:

  if ((f = open_file()) == NULL)
  {
    RETURN_ERROR(OPEN_FILE_ERROR);
  }
  if ((s == allocate_string()) == NULL)
  {
    RETURN_ERROR(ALLOCATE_STRING_ERROR);
  }
  if ...

And RETURN_ERROR is defined as:

  #define RETURN_ERROR(my_error) { \
    if (f != NULL) { fclose(f); f = NULL; } \
    if (s != NULL) { free(s); s = NULL; } \
    ...
    return (my_error); \
  }

The approach I use, and that used many places in the Linux kernel, is to use goto for error handling:

  if(failure_1) {
      goto error;
  }
  if(failure_2) {
      goto error;
  }
  
  // Do stuff
  return 0;
  
  error:
  if(f != NULL) {
      fclose(f);
  }
  if(s != NULL) {
      free(s);
  }
  return -1;

So my source code, after a while, ended up looking more or less like this:
if ((f = open_file()) == NULL) return ERROR_CODE; if ((s = allocate_string()) == NULL) return ERROR_CODE; if (write_string_to_file(s, f) < 0) return ERROR_CODE;*
I hope for you that's more 'less' than 'more'. That code leaks resources whenever an error occurs. Can't allocate the string? Oops, kept the file open. Write failed? Oops, kept the file open, and leaked the result of allocate_string.
In C, this kind of error handling screams for the use of goto:
int result = ERROR_CODE; FILE * f = NULL; char * s = NULL; if((f = open_file()) == NULL) goto ERR; if((s = allocate_string() == NULL) goto ERR; ... result = NOERROR; ERR: if( s != NULL) free_string( s); if( f != NULL) close_file( f); return result;
Given that you need that to write robust code, I don't think the macros make things much clearer.

An advantage of using goto to handle errors is that you don't need that condition checking in the teardown:
int result = ERROR_CODE; FILE * f = NULL; char * s = NULL; if((f = open_file()) == NULL) goto end; if((s = allocate_string() == NULL) goto closefile; if((some_other_thing() == NULL) goto freestring; ... result = NOERROR; freestring: free_string( s); closefile: close_file( f); return result;

That's fine until you have multiple lines that goto closefile or freestring, and you mix them up at some point, or you try to refactor the code and forget to update one of your gotos. The resulting bug will wait beneath the waters until one day you get a mysterious "out of file descriptors" or "out of memory" error at some random place in your program after it's been running for awhile.
And really, the extra condition check likely isn't going to affect the critical path, but it will make it a lot easier to reason about the code, especially when someone else has to modify it.

It's really a matter of both taste and situation. I've seen some very compelling examples from the Linux kernel where this is definitively easier to follow than the alternatives (Linus has made some good defences of goto for this purpose), but I've never felt it was the clearest option in any code I've written myself.

"That isn't premature optimization, you're just guaranteeing that the compiler won't introduce the overhead of a function call."
That's precisely what premature optimization is: Reducing assumed overhead before you've even established where the bottleneck is, if any. By using a macro, you've traded away all of the benefits of a function. If you had demonstrable need to avoid the function call overhead, you could have declared the function inline static.
Most performance issues can be solved with algorithmic changes. This level of optimization should only be done as a last resort, and profiled extensively to ensure that you really are making a difference for the better. Many "low level" tricks that on the surface look like they'd speed things up can in fact have the opposite effect on modern processors. For example, it's possible that this macro optimization could blow up your cache if some_operation gets called a lot and you tried unrolling loops.

Even better rule: Know the limitations of your target processor and compiler and code accordingly.
Example 1: Some (bad) compilers just ignore the const keyword (and therefore treat consts like normal variables). Use #defines with these compilers (or get a better compiler).
Example 2: Some (older, smaller) processors don't have an integer division instruction. Avoid division at all costs on these processors unless you are not worried about space or execution speed.

> Example 2: Some (older, smaller) processors don't have an integer division instruction. Avoid division at all costs on these processors unless you are not worried about space or execution speed.
Ok, so steer clear of division on ARM CPUs.

Depends on the ARM CPU. Some variants of ARMv7 architecture have an integer divide function.
https://en.wikipedia.org/wiki/ARM_architecture#Arithmetic_in...

> Ok, so steer clear of division on ARM CPUs.
Don't ignore the last part of the sentence:
> …unless you are not worried about space or execution speed.
I'd argue that on ARMs you're generally not worried about execution speed (as most run pretty fast nowadays), nor space (RAMs and flashes generally have lots of space).
Now, do you have a really slow ARM, or are you running in a really constrained space (like a tiny metal-mask ROM)? Then yes, avoid it.

Some (older, smaller) processors don't have an integer division instruction. Avoid division at all costs on these processors unless you are not worried about space or execution speed.
...unless you know your compiler is capable of converting the division into cheaper operations via strength reduction, per your well-considered rule.

… which will (normally) only happen if the dividend is unsigned.
Another reason to prefer unsigned ints is that the C standards make signed overflow undefined. This is becoming increasingly important as code for embedded systems is built using ‘big’ optimizing compilers, rather than the simple ‘portable assemblers’ assumed by the old old rules of thumb like manually converting divides to shifts.
“If you lie to the compiler, it will get its revenge.” — Henry Spencer

Another option is reciprocal multiplication, possibly with a larger integer type. Reciprocal multiplication is an important element of my software's ability to run a Kinect at ~30fps on an ARM cpu with no FPU and no division instruction.

Regarding division and shifts, one thing worth noting is that you can now (with C11) bust out static assertions to make sure no one changes the count and the shift incompatibly. It's still probably not worth doing unless there's a good reason to make sure that particular operation is a shift rather than a multiply.

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