Embedded development: five C features that may no longer be prohibited

　　 Best practices and industry standards have evolved over time , But they represent snapshots of guiding wisdom . Although technological advances have invalidated previously known best practices , But best practices can develop slowly , And often become entrenched . Used in the field of embedded development C Linguistic features have suffered the same fate . Many best practices originate from 80 Age and 90 years , The compiler was still weird , And microcontrollers do have limited resources . From then on , Compilers and microcontrollers have come a long way , many “ Forbidden ” Features and design techniques may no longer be the case .

　　 features 1 – Float

　　 Used in embedded systems Float It has been strongly opposed for a long time .Float Traditionally, there are many potential sources of problems . First , The microcontroller does not have a floating point unit . in the majority of cases , For the lowest end microcontroller , This is still true today , But it contains floating-point units (FPU) The cost has dropped significantly to mid-range and general purpose microcontrollers have begun to include FPU The degree of . Advances in microcontroller technology now include hardware acceleration of mathematical functions , Even without FPU It can also help speed up the calculation .

　　 secondly , In the absence of FPU In case of use float The compiler is required to introduce a large and slow software library . since 20 Since the late 20th century , Compiler technology has been greatly improved , Even in software 8 Floating point calculations performed on bit microcontrollers are also optimized to a negligible degree . Technology is changing , Embedded developers need to change .

　　 features 2–malloc

　　Malloc Allows developers to dynamically allocate memory during program execution , If used improperly in embedded systems , It can be a very dangerous tool . Traditionally , It is forbidden to use in the system with limited resources malloc, There's a reason . What happens when the heap becomes fragmented ? How developers handle memory allocation failures ? What about memory leaks ? These are difficult procedures , It takes code space and horsepower to handle , Traditional microcontrollers cannot handle .

　　 Microcontrollers no longer “ Tradition ”.2015 year , The clock speed of a low-cost microcontroller may exceed 200 MHz, Flash memory space exceeded 1 MB,RAM the height is 64 KB( the height is 256 MB). Even in a less powerful system , It also has correct implementation and use malloc Tools for . The application may well guarantee this as well , Therefore, outdated best practices should not be excluded from consideration from the beginning just because of them .　　

 

　　 features 3 – printf

　　 Generally speaking , Embedded developers use in embedded systems C Library functions are considered bad practice . majority C Libraries are not reentrant , Usually bulky and slow to execute ( Or is it ?), Or it can be implemented by blocking execution until completion .printf The use of belongs to many of these categories , And has avoided using in embedded systems .

　　 As mentioned earlier , Many changes have taken place between compiler optimization and hardware improvements , Use printf It's the guilt of the past . Considering the typical 32 kB Flash space , This function is considered to be “ Big ”, But it takes up very little code space . A typical implementation would printf Used as blocking function , This affects real-time response and consumes potential shared resources . By way of printf Link to circular buffer and interrupt driver transport driver , Response problems can be easily overcome , This allows program execution to continue , At the same time, the driver performs the necessary work .

　　 features 4 – memset

　　 Most involve memory operations or dynamic memory C Features are ultimately prohibited . The reason is obvious , Many embedded developers and teams have suffered in the past when using such features . When things like malloc or memset When functions like this make the most sense , Although they should not be avoided . contrary , Developers should ensure that they fully understand how to use this feature , What precautions are required for proper use , How to recover when the worst happens , Of course, there are appropriate tests to ensure that everything goes according to plan .

　　 features 5 – C Bit fields

　　 A concept is used in C Fuzzy features defined in the standard , The use of this concept is not rooted in the progress of compiler technology or hardware . A good example of this feature is the bit field . The biggest problem with using bit fields is that there are usually portability problems . A simple example is , Bit ordering is not standard , Compilers can usually rearrange these bits , Make it the most effective implementation . Adding padding bytes to the overall structure is also problematic .

　　 The general rule of thumb is to avoid using bit fields , But there are many cases where they make sense , Even portability issues don't matter . A good example of using a bit field is to create a structure , This structure is used to create the configuration table of the initialization driver or application . Read the value of the bit by writing initialization , Reordering or even filling bytes is not a problem .

　　 Conclusion

　　 Standards and best practices are designed to help developers avoid throwing themselves in the foot , But they are just best practices . A standard might say that the use of C features , But the truth is , It is up to the developer to decide, in his own unique circumstances, whether traditional wisdom is applicable . Challenge our preconceptions , And make sure we understand why those best practices are appropriate , This is as important as following them . Don't forget about their applications 、 Give up risk and reward . Embedded developers are considering these “ prohibit ” The characteristics of , Need to know their time 、 Performance and size limitations .