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NASA JPL C Coding Standard [pdf]

• ninetax 14y ago
  Oh if only the project that I had been working on followed any of these rules. Most of the code was generated from Matlab, but some had to be translated by hand. I'm not sure any of us knew this even existed...
• losethos 14y ago
  It is perfectly acceptible to use global variables in a 1000 line user game. It would take you bureaucrats a week to verify that. I am CEO of my company with a vision, not some nigger Indian. Some nigger Indian would sleepwalk while remaking Unix complete with all it's flaws. What use is another Unix? No use. If it's not different it's worthless.
• DiabloD3 14y ago
  I agree with almost all of it except for one thing: gotos have pretty much one legitimate use, as C's equiv to finally {} as part of a try {} block, ie, that specific form of cleanup after error management. NASA implies they have no legitimate use. longjmp banning is also slightly questionable (although I can see why because it is very easy to do wrong). I use it inside of my code as part of an STM implementation (so begin_tx() setjmps[1], abort_tx() longjmps; its faster than manually unwinding with if(tx error) { return; } spam in deep call stacks.) Using longjmp for this makes writing code much easier (no needing to error check every single tx function call), so less chance for bugs to slip in. 1: The only ugly part of that is begin_tx() is a function macro, which I prefer never to use in code that is executed; I tolerate it in "fancy template-like generator" setups, though.
  • mvanveen 14y ago
    Anything that makes it "very easy to do wrong" is not a good idea in flight software. A missing semi-colon in FORTRAN once completely killed a space mission.
    • andrewcooke 14y ago
      comma.
      • mvanveen 14y ago
        I remember reading semi-colon in the report ;-) . I can't find the PDF that I believe I read this in, unfortunately, and I'm not sure if it's publicly available. However, this wikipedia article points to a hypen and suggests that there's some folklore involved, and that revisions have been made in the past: http://en.wikipedia.org/wiki/Mariner_1 http://en.wikipedia.org/wiki/Mariner_1 . Edit: Andrew, thanks for pointing out my ignorance, I've never coded FORTRAN, and have been thinking it's a semicolon this entire time. What fitting irony. Sort of makes for an even better story now. >_<
        • andrewcooke 14y ago
          fortran syntax doesn't use semicolons. see http://www.lncc.br/sta/manuais/Fortran77_Lang_Ref.pdf http://www.lncc.br/sta/manuais/Fortran77_Lang_Ref.pdf page 11.
          • andrewcooke 14y ago
            this seems like it may be definitive (well, it's a quote that gives a reference; it also fits with what i remember from the news at the time, which is that it was a do statement and related to the parsing of spaces, but i have no idea how reliable that is): http://catless.ncl.ac.uk/Risks/5.66.html#subj2 http://catless.ncl.ac.uk/Risks/5.66.html#subj2 if it's correct than it was actually a decimal point instead of a comma.
  • mcpherrinm 14y ago
    You mention longjmp as useful in an STM implementation. But you'd never see something like that in flight control software. Simplicity begets correctness. Correctness begets safety. When the software is flying a rocket ship, I'm okay with it being 10% longer but 10% safer.
    • cantankerous 14y ago
      I think many people would be okay with it being 100% longer if you could quantify 10% increased safety.
    • DiabloD3 14y ago
      I wouldn't use threads in mission critical software such as that, so that solves the problem.
• mangamadaiyan 14y ago
  One thing that stood out for me was the vehement "no" to goto. That's a little too harsh, one thinks?
  • jlgreco 14y ago
    That is anything but an uncommon stance.
    • andrewcooke 14y ago
      for C? it's very commonly used to jump to implement "catch" (jump to clean-up on error).
      • _kst_ 14y ago
        Yes, but there are other ways to implement error handling -- and a ban on gotos is a lot easier to check and enforce than a ban on just "bad" gotos.
      • jlgreco 14y ago
        goto is commonly used that way in C... and it is just about as commonly straight up banned in C. I am not defending the bans, but find it strange that people would be surprised by them.
        • andrewcooke 14y ago
          > I am not defending the bans, but find it strange that people would be surprised by them. yes, i think a lot of replies here are speaking past each other, making valid and not actually contradictory points, but not really replying to each other. if you read the entire doc it is clear that they are pushing c in a very safe, but somewhat unusual direction. there's no dynamic memory for example, so the main reason that most of my c code uses goto - to free memory on failure in a "catch" - is irrelevant. taken as a whole, it's not what i would call normal c use, and i don't think it's very useful for most other people as a guideline, but it is internally consistent and, for the specific use case, reasonable.
• mvanveen 14y ago
  A few summers ago I was an intern at JPL working on a static analysis suite for this exact standard. Writing code checkers for these sorts of rules is a really interesting exercise and it helped me grow a lot as a programmer! I went from having no exposure to formal languages, parsing, and grammars to actively playing around with these concepts to try and help build more reliable software. It was a humbling, challenging, and incredibly rewarding experience. Sometimes, a rule is extremely simple to implement. For example, checking a rule that requires that an assert is raised after every so many lines within a given scope is just a matter of picking the right sed expression. Other times, you really need an AST to be able to do anything at all. A rule like "In compound expressions with multiple sub-expressions the intended order of evaluation shall be made explicit with parentheses" is particularly challenging. I spent a few weeks on this rule! I was banging my head, trying to learn the fundamentals of parsing languages, spending my hours diving into wikipedia articles and learning lex and yacc. The grad students at LaRS were always extremely helpful and were always willing to help tutor me and teach me what I needed to learn (hi mihai and cheng if you're reading!). After consulting them and scratching our heads for a while, we figured we might be able to do it with a shift-reduce parser when a shift or reduce ambiguity is introduced during the course of parsing a source code file. This proved beyond the scope of what I'd be able to do within an internship, but it helped me appreciate the nuance and complexity hidden within even seemingly simple statements about language properties. Automated analysis of these rules gives you a really good appreciation of the Chomsky language hierarchy because the goal is always to create the simplest possible checker you can reliably show is able to accurately cover all the possible cases. Sometimes that is simple as a regular language, but the next rule might require you to have a parser for the language. For what it's worth, this is only one of the ways the guys at LaRS (http://lars-lab.jpl.nasa.gov/ http://lars-lab.jpl.nasa.gov/) help try to improve software reliability on-lab. Most of the members are world-class experts in formal verification analysis and try to integrate their knowledge with missions as effectively as possible. Sometimes, this means riding the dual responsibility of functioning as a researcher and a embedded flight software engineer, working alongside the rest of the team. If anyone's interested in trying out static analysis of C on your own, I highly reccomend checking out Eli Bendersky's awesome C parser for Python (http://code.google.com/p/pycparser/ http://code.google.com/p/pycparser/). I found it leaps and bounds better than the existing closed-source toolsets we had licenses for, like Coverity Extend. At the time, it had the extremely horrible limitation of only parsing ANSI 89, but Eli has since improved the parser to have ANSI 99 compliance. Analyzing C in Python is a dream.
• afhof 14y ago
  Wait.... no malloc or sbrk? That means all space has to be stack allocated? That's a pretty serious limitation and would probably make it hard to do anything really interesting.
  • mcpherrinm 14y ago
    You don't want your space ship flight to be "interesting". Note that it says "after task initalization". What this really implies is that you must use O(1) heap space, and you should get what you need ahead of time. Having to deal with out of memory conditions 4 seconds after main engine turn-on is not a fun party. Neither is blocking on malloc() so you can prepare your struct course_adjustment to send to another task.
  • mvanveen 14y ago
    You don't want to do anything interesting in a space flight mission. Code affecting a space mission only has one chance to get it right in many cases. In general, memory management in these systems is extremely serious business. Think of all the ways that manual memory management in C has damaged the reliability of software you have written in the past. In addition, a lot of the rules are created to make the task of reading source code easier, both for humans and machines. I remember Dr. Gerard Holzmann once half-joked in a meeting that he wanted to disallow any declaration of pointers except at static initialization. I sort of thought he was joking, but then he assured me that it was a serious consideration. He reminded me of the gravity of the situation and explained that $2 billion of public funds were on the line. Disallowing pointer indirection would make the task of certain automated analysis techniques much, much simpler to perform. Adding a pointer indirection can really conflate matters sometimes.
    • afhof 14y ago
      But without pointer indirection and dynamic memory allocation, why even use C? The big idea of C is pointers. Aren't there languages designed for mission critical and embedded environments (Ada for instance) ?
      • 14y ago
• lottoro 14y ago
  Compared to other coding standards I like the brevity: 22 pages, 31 rules.
• SjuulJanssen 14y ago
  "A recommended use of assertions is to follow the following pattern:" if (!c_assert(p >= 0) == true) { return ERROR; } Why not: if (!c_assert(p >= 0)) { return ERROR; }
  • vbtemp 14y ago
    Because the whole point is to be maximally explicit and not implicit.
  • jbester 14y ago
    It's one of the MISRA rules - implicit comparisons aren't allowed.
• alexeiz 14y ago
  This document reinforces my opinion that most coding standard documents suck. I’ve seen a countless number of coding standards from different companies (some of the companies I even worked for) and they all sucked. No exception. Even though coding standards have some common sense advice and guidelines which is generally helpful for producing code of good quality, the amount of arbitrary irrational rules and beliefs that coding standards writers put into the standards and try to enforce through the standards actually end up hurting the quality of the code produced by developers trying to follow those rules. Case in point with examples from the NASA JPL coding standards for C: * no direct or indirect recursion What is it, FORTRAN-77? Some algorithms are way easier to implement recursively whereas the iterative algorithm can be much less straightforward and buggier. Think sorting: it’s easy to prove that the recursion is finite and that the implementation of the algorithm is correct. Do they use sorting in NASA or is it prohibited by this rule? * no dynamic memory after initialization FORTRAN-77 again! While dynamic memory management can be challenging in real-time systems and the generic malloc/free implementation is not acceptable, it doesn’t mean that statically pre-allocated fixed-size memory is better. It inevitably leads to brittle code ripe with excessive memory use, bugs like static buffer overruns, and sometimes even inability to use dynamic data structures like linked lists. To work around this restriction, a developer can construct a linked list structure in a statically allocated memory, but doing so is essentially equivalent to creating your own dynamic memory manager which is more likely to be poorly implemented than a good dynamic memory manager. Instead of denying the use of dynamic memory they should develop memory managers with acceptable performance characteristics. * The return value of non-void functions shall be checked or used by each calling function, or explicitly cast to (void) if irrelevant. Given that there are a lot of library functions in C that return some error code rarely useful, this rule leads to code littered with (void) casts: “(void) printf(…)”, “(void) close(…)”, etc. Along with the littering the rule doesn’t make the code any more robust because it encourages to use (void) casts to ignore error codes and therefore error codes will likely be ignored rather than handled correctly. * All functions of more than 10 lines should have at least one assertion. This leads to littering code with assertions in those functions that don’t necessarily have anything to assert and that are accidentally longer than 10 lines (for example, due to mandatory parameter validation checks. I hope parameter validation checks are not assertions, are they?). * All #else, #elif and #endif preprocessor directives shall reside in the same file as the #if or #ifdef directive to which they are related. This is just a bizarre rule. What developer puts #ifdef in one file and #endif in another? Unless of course he’s drunk or high but I hope that’s not how NASA develops its software. * Conversions shall not be performed between a pointer to a function and any type other than an integral type. Wait, pointers to functions should be converted to which integral type? They are a number of integral types: char, short, unsigned long long. Which one do I choose? Why not void* or intptr_t? * Functions should be no longer than 60 lines of text and define no more than 6 parameters. Finally a good rule. But what does the explanation say? “A function should not be longer than what can be printed on a single sheet of paper in a standard reference format with one line per statement and one line per declaration.” Printed on a sheet of paper? Is this still how code is reviewed in NASA? And before you say "these coding standards are for a special kind of software that runs on space flight control systems," embedded devices these days are more powerful than desktop computers ten years ago. Embedded sortware grew beyond draconian restrictions a long time ago and it's much closer now to non-embedded software. Let's not forget that NASA did use Lisp in their systems and they were able to solve pretty difficult problems remotely with help of Lisp REPL (http://www.flownet.com/gat/jpl-lisp.html http://www.flownet.com/gat/jpl-lisp.html). Lisp code certainly can't be subject to any of the restrictions from these coding standards, which is another indication of how irrelevant these coding standards are for producing robust software.