This is important when dealing with hardware registers. One of the most consistently popular articles on the is about C's volatile keyword. A solution of this problem is to make the global flag volatile, this technique prevents the compiler from applying any optimization on the global flag and tells the compiler that value of this flag can change by external event at any time without any action being taken the by code. . The volatile keyword is very important when using optimizing compilers. To resolve this issue, the Standard says that compilers may assume that objects which are not declared volatile will not have their value observed or changed in ways that are outside the compiler's control, or would be outside a reasonable compiler's understanding.
Does the standard specify whether a read is considered 'observable behavior' if the value is never used? If even one bit can change unexpectedly, the entire register must be declared volatile. In this case, x + hh and x differ exactly by hh. Going back to the volatile keyword, placing volatile before a variable, in effect, does the same thing. Unless you are writing code for a specific compiler and you know what extensions to the C language the compiler implements with respect to volatile, you should not use volatile for threading semantics. Consequently, all the code after the while loop may simply be removed by the optimizer. Volatile variables have the visibility features of synchronized but not the atomicity features. Edit: Some code examples would be great! Think about it : when you substract very close numbers, you lose a lot of significant digits which can ruin the computation of the derivative think 1.
However, I'm not sure what compiler optimization if any this prevents. It also leads to potentially less efficient code. Therefore, we need declare the register as volatile that is, reading the value each time. Examples of volatile variables are: Non-automatic variables referenced within an interrupt service routine This is probably the most obvious use of the volatile keyword. This can lead not only to logically incorrect behavior of the program but also to exploitable security holes in it if it processes untrusted files or files from untrusted locations. The DoWork method may optimize the member access, resulting in reading stale data. For more information about multithreading, see.
A is a qualifier which prevents the objects, from the compiler optimization and tells the compiler that the value of the object can be change at any time without any action being taken by the code. Global variables modified by an interrupt service routine 3. And when you modify the object, that modification is written immediately and exactly as you specify, with no buffering or reordering optimizations. To avoid these kinds of scenarios, we declare the flag as volatile, we are telling to compiler that this value may be changed by an external interface or other module of program, i. The way to get around this is to declare the pointer gadget as volatile. In the case of pointers, the use of const and volatile may become even more complex, as in the next category. Appear in any type specifier, including decl-specifier-seq of , to specify constness or volatility of the object being declared or of the type being named.
This article was published in the July 2001 issue of. Reading it that way, we can see that the keyword const modifies only the pointer i. Also I have marked this question as both C and C++ not because I think they are the same language, but because I am interested to know if there are differences in what volatile does in this case in these two languages. But the register may be modified by other programs. C language interview questions solution for freshers beginners placement tricky good pointers answers explanation operators data types arrays structures functions recursion preprocessors looping file handling strings switch case if else printf advance linux objective mcq faq online written test prime numbers Armstrong Fibonacci series factorial palindrome code programs examples on c++ tutorials and pdf When any variable has qualified with const keyword in declaration statement then it is not possible to assign any value or modify it by the program. Otherwise, the compiler is allowed to assume that functions that don't take a reference to the variable can't modify it, so assuming as CesarB intended that the above loop does not set quit, the compiler can optimise it into a constant loop, assuming that there's no way for quit to be changed between iterations.
But here, I would like to take a step back and would want to put in a case for the consequences of over usage of it. There is only one reason to use it: When you interface with hardware. Unfortunately, most books about the C programming language dismiss volatile in a sentence or two. In some cases we may only desire the visibility and not atomicity. Remember, different threads may be operating against inconsistent caches on different processors; there is no requirement that a compiler implement volatile to take that into account.
This is not surprising, as most C texts dismiss it in a sentence or two. If that data happens to change, your program may become using two or more different versions of the data and get into an inconsistent state. Note that although the pointer itself can be volatile, the object that it points to cannot. Peripheral Register In the embedded system, all peripherals are located at the specific memory address. I'll mention another scenario where volatiles are important. You can skip to the end and leave a response.
Despite being a common keyword, the behavior of volatile differs significantly between programming languages, and is easily misunderstood. It is great for use with microcontroller or other hardware registers, where the value of the register changes outside the control of the compiled application running on the microcontroller. After studying more about volatile I found three portable uses for volatile. In the following code without the volatile the while loop may never break out. I'd say volatile is used to prevent the compiler from making optimizations that would normally be useful and desirable. For instance, a lot of peripherals contain registers that are cleared simply by reading them.