My book Embedded C Coding Standard began as an internal coding standard of a consulting company and was first published in by that company Netrino. A update to the book will be released soon and this will be the first time the substance of the standard has changed in over a decade. But we wanted to do more than just remove those noted conflicts and push even further in our embrace of harmonization. In about as many words, BARR-C is a style guide for the C language that reduces the number of defects introduced during the coding phase by increasing readability and portability. It turns out there is quite a bit of value in combining rules from both standards.
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My book Embedded C Coding Standard began as an internal coding standard of a consulting company and was first published in by that company Netrino. A update to the book will be released soon and this will be the first time the substance of the standard has changed in over a decade. But we wanted to do more than just remove those noted conflicts and push even further in our embrace of harmonization.
In about as many words, BARR-C is a style guide for the C language that reduces the number of defects introduced during the coding phase by increasing readability and portability.
It turns out there is quite a bit of value in combining rules from both standards. The rest of this post is a preview of the specific rule changes and additions we will make in BARR-C Omitted from this list are rules reworded simply for greater clarity. Preprocessor directive define shall not be used to alter or rename any keyword or other aspect of the programming language.
It is a preferred practice to avoid all use of the goto keyword. If goto is used it shall only jump to a label declared later and in the same or an enclosing block. There will no longer be any restriction on the use of the break keyword. Always invoke the isfinite macro to check that prior calculations have resulted in neither infinity nor NaN.
Non-Boolean values shall be converted to Boolean via use of relational operators e. That is, the keyword return shall appear a maximum of once.
It is a preferred practice that all functions shall have just one exit point and it shall be via a return at the bottom of the function. The names of all variables representing non-pointer handles for objects, e.
In the case of a variable name requiring multiple of the above prefixes, the order of their inclusion before the first underscore shall be [g][p pp][b h]. If project- or file-global variables are used, their definitions shall be grouped together and placed at the top of a source code file.
It is a preferred practice that the shortest measured in lines of code of the if and else if clauses should be placed first. Any case designed to fall through to the next shall be commented to clearly explain the absence of the corresponding break. If you have questions about any of these draft changes or suggestions for better or other changes, please comment below.
Despite this coding standard being followed by about 1 in 8 professional embedded systems designers , none of us at Barr Group have heard any complaints that this rule is too strict. Instead we hear lots of praise for our unique focus on reducing intra-team stylistic arguments by favoring above all else C coding rules that prevent bugs. However, there exist other coding standards with a more relaxed take on goto. Rule In addition to overlaps, they have more and stricter coding rules and we add stylistic advice.
There seems to be universal agreement that goto should never be used to branch UP to an earlier point in a function. Likewise, branching INTO a deeper level of nesting is a universal no-no. For example, a single goto statement can be used to escape from two or more levels of nesting.
And this is not a behavior that can be done with a single break or continue. To accomplish the same behavior without using goto, one could, e. Given this, the processing of exceptional conditions detected inside nested blocks is a potentially valuable application of goto that could be implemented in a manner compliant with the three MISRA-C rules quoted above.
Such a jump could even proceed from two or more points in a function to a common block of error recovery code. Because good exception handling is a property of higher reliabilty software and is therefore a potential bug killer, I believe I must consider relaxing Rule 1. And none other than Dijkstra 50 years ago now! But—even if all true—none of these assertions is a winning argument for banning the use of goto altogether vs. The best bug-killing argument I have heard for continued use of the current Rule 1.
That is, if a goto jumps over a line of code on which an object would have been constructed or destructed then that step would never occur. This could, for example, lead to a very subtle and difficult type of bug to detect—such as a memory leak. So what do you think? What relevant experiences can bring to bear on this issue in the comments area below? Should I relax the ban on goto or maintain it?
Are there any better bug-killing arguments for or against goto? In a nutshell, the bug is that a bunch of important C source code lines containing digital signature certificate checks were never being run because an extraneous goto fail; statement in a portion of the code was always forcing a jump. This is a bug that put millions of people around the world at risk for man-in-the-middle attacks on their apparently-secure encrypted connections. Moreover, Apple should be embarrassed that this particular bug also represents a clear failure of software process at Apple.
However, whether it was an innocent mistake or an attack designed to look like an innocent mistake, Apple could have and should have prevented this error by writing the relevant portion of code in a simple manner that would have always been more reliable as well as more secure. And thus, in my opinion, Apple was clearly negligent. Importantly, had Apple followed at least the first of these rules, in particular, this dangerous bug should almost certainly have been prevented from ever getting into even a single device.
Braces shall always surround the blocks of code a. Either way, this bug was preventable by following this rule and performing code review. Certainly if that forced each of the goto lines to be replaced with more than one line of code, it would have forced programmers to use curly braces. Dangerous bugs, like this one from Apple, often result from a combination of accumulated errors in the face of flawed software development processes. Too few programmers recognize that many bugs can be kept entirely out of a system simply by adopting and rigorously enforcing a coding standard that is designed to keep bugs out.
However, it is far more common that memory-mapped peripherals have at least a half dozen registers. In this more complicated scenario, a C struct can be defined to encapsulate these registers as a set and a pointer to said data structure can be declared. In this scenario, there are three possible places for the volatile keyword. Finally, the third option is to place one or more volatile keywords within the struct definition itself.
With this placement, only the specific registers within the struct that are declared volatile will be treated, by the compiler, as subject to asynchronous change. Reads and writes from or to other, non-volatile-declared, registers in the struct may potentially be optimized away. Given that there are multiple choices for the placement of volatile, where is the best place to put the volatile keyword in practice?
My preferred placement is typically in the pointer declaration. That way, all of the registers in the struct will be treated, by the compiler, as volatile and yet it is possible to have other e. RAM-based shadows instances of said struct that are not volatile because they are not actually hardware registers underneath. If so, why? And how should you combine volatile and const properly? The volatile keyword, like const, is a type qualifier.
These keywords can be used by themselves or together in variable declarations. As that article makes plain:. It tells the compiler that the value of the variable may change at any time—without any action being taken by the code the compiler finds nearby. By declaring a variable volatile you are effectively asking the compiler to be as inefficient as possible when it comes to reading or writing that variable.
Specifically, the compiler should generate object code to perform each and every read from a volatile variable and each and every write to a volatile variable—even if you write it twice in a row or read it and ignore the result. No read or write can be skipped. Effectively no optimizations are allowed with respect to volatile variables.
The order of accesses of volatile variables A and B in the object code must be the same as the order of those accesses in the source code. The compiler is not allowed to reorder volatile variable accesses for any reason. The first example declares a global flag that can be shared between an ISR and some other part of the code e.
It is important to use volatile to declare all variables that are shared by asynchronous software entities, which is important in any kind of multithreaded programming. Remember, though, that access to global variables shared by tasks or with an ISR must always also be controlled via a mutex or interrupt disable, respectively. The second example declares a pointer to a hardware register at a known physical memory address h —in this case to manipulate the state of one or more LEDs. Because the pointer to the hardware register is declared volatile, the compiler must always perform each individual write.
Even if you write C code to turn an LED on followed immediately by code to turn the same LED off, you can trust that the hardware really will receive both instructions. Because of the sequence point restrictions, you are also guaranteed that the LED will be off after both lines of the C code have been executed.
The const keyword is can be used to modify parameters as well as in variable declarations. Here we are only interested in the use of const as a type qualifier, as in:. In C, this variable will exist in memory at run-time, but will typically be located, by the linker, in a non-volatile memory area such as ROM or flash. Any reference to the const variable will read from that location.
Any attempt the code makes to write to a const variable directly i. To the extent that the const variable is located in ROM or flash, an indirect write i. Declaring the pointer this way, any attempt to write to that physical memory address via the pointer e.
The following declaration uses both const and volatile in the frequently useful scenario of declaring a constant pointer to a volatile hardware register. The proper way to read a complex declaration like this is from the name of the variable back to the left, as in:. Reading it that way, we can see that the keyword const modifies only the pointer i. Whereas the keyword volatile modifies only the type of integer. Another use for a combination of const and volatile is where you have two processors communicating via a shared memory area and you are coding the side of this communications that will only be reading from a shared memory buffer.
In this case you could declare variables such as:. In the case of pointers, the use of const and volatile may become even more complex, as in the next category. Sometimes you will run across a read-only hardware register. By declaring your variable IS A constant pointer TO A constant and volatile memory location you request all of the appropriate protections, as in:.
Embedded C Coding Standard
Email: info ldra. The C language is widely used by organisations in the development of safety-critical software applications. The Embedded C Coding Standard was developed by the Barr Group to minimise firmware bugs through rules that keep bugs out while also improving the maintainability and portability of embedded software. Netrino Compliance Matrix. The coding standard details software programming principles, specifies naming conventions, and outlines rules for data types, functions, preprocessor macros, variables, and much more. Individual rules that have been demonstrated to reduce or eliminate certain types of bugs are highlighted. For every coding standard we support, we offer a complete compliance matrix so you can see exactly which rules are implemented within our tools.
ESC: Partnership extends LDRA’s support of C coding standards
The Embedded C Coding Standard was developed by Netrino to minimize firmware bugs by creating rules that keep bugs out while also improving the maintainability and portability of embedded software. The coding standard details guiding software programming principles, specifies naming conventions and outlines rules for data types, functions, preprocessor macros, variables and much more. Individual rules that have been demonstrated to reduce or eliminate certain types of bugs are highlighted. You must Register or Login to post a comment. This site uses Akismet to reduce spam.