What's the difference between using the variable specifiers static volatile combined? Or using one alone; like static or volatile in microcontroller programming?
static - in this case makes the variable visible only inside the current file
volatile - it is information for the compiler that the object can be changed by something outside the normal execution path (for example, the interrupt routine) and guarantees that the variable will be read before any use and written after every change. volatile (which is a very common misunderstanding) does not guarantee anything else - no atomicity, no cache coherency, etc., etc.
static:
A static variable refers to a class variable that's shared among all instances.
volatile:
Volatile variables are those which are read and written to main memory. They aren't stored in local cache and are always fetched from main memory.
For example, two threads use, say, private volatile int x;. If thread A write(x) and thread B read(x) then, both the times it will write and read from main memory, without using the threads' local cache.
static volatile:
Even if the static variables are shared variables, but in different thread there can be different values for a static variable in the local cache of a thread. To make it consistent for all threads, just declare it as static volatile. So each time it will fetch from main memory.
Many good answers were provided here, but no mention of scope.
Static variables once initialized and later changed within a scope, they retain the changes and never be destroyed or initialized again especially when leaving the scope. Not unless dictated in code. You can say, static variables resemble global variables in terms of their lifecycle but can only be accessed throughout their own scope.
The volatile part has the tendency to force execution to fetch a variable from RAM and not the cached copy in registers or flash. Suppose for example a certain code was submitted to the compiler under certain level of optimization setting. The compiler does not assume any further conditions are attached to variables other than to clear them when they are not used or outside their scope. There are inherently dual uses for volatile, either to disregard optimization offered by the compiler for that variable, or to refrain from using the prefetched copy of that variable except for the one in RAM.
The static volatile is the combination of both behaviors, persistence of that variable in RAM beyond any optimization.
Potential areas of application:
Flash programming
Cyclical buffers
Ring buffers
Concurrency and multiprocessing/multithreading
For the keywords static and volatile there is written enough...
See for example:
Why is volatile needed in C?
What does "static" mean in C?
In the concern of the TWI interface, volatile is needed, because functions which modify these variables could be called from different interrupt service handlers. If volatile would be removed, the compiler will optimize code, not knowing that code can be interrupted. That may lead to failures.
Related
General question in c langage:
Is it safe to initialize data in the declaration?
example:
static unsigned char myVar =5u;
Is there any risk that this value will be overwritten by the startup code?
Generally, embedded systems microcontroller projects come in two flavours and the IDE often lets you pick one:
Standard C compliant (sometimes referred to as "ANSI" by confused tool vendors).
Minimized start-up.
The former, standard C compliant projects require that all variables with static storage duration, such as those declared at file scope and/or with the keyword static are initialized before main() is called. This initialization happens inside the start-up code ("C run-time"/"CRT"). On such a system, the myVar = 5u; is guaranteed to be written (not overwritten) by the start-up code. It copies down the value 5 from flash to RAM.
The latter, "mimizined"/"fast" start-up version is not strictly C standard compliant. In such projects all the initialization code of static storage duration variables is simply removed. This to reduce the time from reset to when main() is called. On such systems, nothing will execute the static unsigned char myVar =5u; code - your variable remains uninitialized and indeterminate even though you explicitly initialized it. You have to set it manually at "run-time", which is usually done from some init "constructor" code.
If you have static uint8_t foo_count; belonging to foo.c, then the foo module will have to provide a function foo_init() from where the code foo_count = 5; is executed.
Since the "minimized start-up" version is very common in embedded systems, it is usually considered dangerous to rely on default initialization of static storage duration variables, in case the code gets ported to such a system.
I hope your start up code runs before main. with that said, when you declare the variable static, the variables scope is bound to the scope of that translation unit (somefile.c), so yes you could overwrite it but that would be hard to do from other units (direct memory assignment) The golden rule for embedded systems is to avoid global variables but if you most, declare global variables as externs in a header file where it makes the most sense to put it.
Alternatively you could replace this with a #define.
#define MY_VAR 5U
The start up code is what is responsible for applying the initialisation (where did you imagine that was done?), so yes it is safe and with respect to initialisation best practice.
Without initialisation a static or global should have a value of zero. It is not unheard of for the startup code in some embedded systems to deliberately omit zero initialisation in order to minimise start up time. Normally that would not be the default startup code, and you would have to make a conscious decision to use it. It is unsafe and unfair to later maintainers who might be unaware is such non-standard behaviour. It is also in most cases an unnecessary and premature optimisation. Nonetheless you might initialise all such variables, even if the unit is zero, to defend against such non standard startup (whilst also rendering it pointless, which it generally is).
What is not best practice is the use of a global in the first instance (https://www.embedded.com/a-pox-on-globals/). Though to be fair the myVar in question is not truly global in this case.
This may not be directly related with the original question, but I think it's worth mentioning:
Initialization of a global variable using another global variable from a different translation unit is not safe. For example:
In a.c
unsigned var_from_a = 5u;
In b.c
extern unsigned var_from_a;
unsigned var_from_b = var_from_a;
There is no guarantee that var_from_b becomes 5u, because the initialization order of translation units is not defined. If you're unlucky, b.c is processed before a.c, and var_from_b may become 0 or some other garbage value, while a.c is processed later and var_from_a properly initialized to 5u.
I'm not sure if it would change this behavior if var_from_a was defined as const.
In case I have a C file that uses an extern declared variable
and in the code the variable is modified and then an external function is called.
will the compiler optimization take into account the possibility that the variable can be touched by the function? so it won't change the C code order and make sure the variable is set in memory before the function is called?
You need to be careful of a number of things... but in a basic C application with a single thread, yes... this should be fine.
If, however you are (non-exhaustive):
Using multiple threads
Using shared memory between a number of processes
Running on a low-level system (e.g: AVR / STM32) and handling the variable in the an interrupt handler and under main()
Handling the variable in a signal handler and under main()
Reading memory / registers that are modified by hardware / DMA
Then you'll need to be careful.
The volatile keyword can be useful - it will inform the compiler that "this variable may change while you're not looking".
Even with the volatile keyword though, you may run into the Read-Modify-Write problem...
Knowing about the Read-Modify-Write problem is half the fight... the other half is mitigating it, which can be achieved by a number of optiosn such as using a technique called Mutual Exclusion / Critical Sections, or if appropriate by copying the data into a local variable before you operate on the value.
In gcc you can declare a thread local variable, for example, as shown below.
__thread long thread_local_variable;
Also, in gcc you can specify a variable to use a certain register, for example, as shown below.
long register_variable asm ("r15" );
I want to combine these two concepts, that is, I want to declare a thread local variable that uses a certain register. How can I do that?
You don't need to do anything special. Your example:
long register_variable asm ("r15" );
is already declaring a thread-local variable, merely due to the fact that each thread has its own set of register values.
There is no possible way to make GCC's global register-storage variables shareable between threads. The fact that this is not well documented speaks to how ill-thought-out and hackish the whole idea of register-storage global variables is...
If you think about it, that couldn't work.
For a thread local variable, each thread needs it's own instance of storage. There's only a single r15 register (or one per core. more precisely), so there's simply no place to put the storage for additional threads.
Also, to put it in GCC's documentation's terms:
The __thread specifier may be used alone, with the extern or static specifiers, but with no other storage class specifier.
the register keyword is a storage class specifier, so cannot be used with __thread.
When threads swap out, they typically save the entire register state of the processor onto a stack somewhere. When the thread is restored, the register state is read from memory back into the registers.
So, if the language you used allowed it then you could in theory make this work. Each thread would have its own copy of the r15 register while it was running. However...
GCC Thread storage just wasn't designed that way. It stores the data in RAM somewhere, so you can later pass the pointer to another thread if you want to.
If you think of it, you don't need anything special for it. register variables always have a lifetime that is the current invocation of the function in which they are defined. These always are "thread local" variables.
The gcc extension __thread and the C11 feature _Thread_local are completely different concepts. They specify that a variable of static storage is instantiated on a per thread base. These kind of variables are never register variables. The register keyword in C forbids you to take the address of a variable, and forces the variable otherwise to be similar to auto variables.
Where is a volatile variable stored in the program memory (in which section)?
volatile is a type qualifier not a storage class specifier, so it does not determine storage location at all; it affects the definition of a variable's type, not its storage.
It simply forces the compiler to explicitly read a variable whose type is volatile from the variable's storage location (wherever that may be) rather than assuming that some previously read value in a register for example remains valid.
In C volatile just tells the compiler - "You don't have enough knowledge to assume the value of this variable hasn't changed". There is no "section" eg BSS, CSS for it.
Consider it a flag to the compiler to prevent certain types of optimisations. Its very handy in embedded programming, where memory at a certain address may "change" due to a hardware device input.
Here's a good explanation: http://www.embedded.com/columns/programmingpointers/174300478?_requestid=137658
The volatility of a variable does not change the place in which a variable is stored. What it changes is the semantics around how it is accessed with respect to reads and writes.
I do not believe the C standard says anything about the implementation of volatile. But typically, volatile guarantees release semantics for write operations on a variable and aquire semantics on read operations of a variable. This will not be true for every implementation though and you should read up on what your specific compiler guarantees
volatile has nothing to deal with storage class.
volatile just tells the compiler or force the compiler to "not to do the optimization" for that variable.
so compiler would not optimize the code for that variable and reading the value from the specified location, not through interal register which holds the previous value.
So, by declaring variable as volatile.. it gives garrantee that you will get the latest value, which may be alterred by an external event.
your code may be work fine if haven't declare that variable as volatile, but there may be chance of not getting correct value sometimes..
so to avoid that we should declare variable as volatile.
volatile is generally used when dealing with external events, like interrupts of hardware related pins.
example.
reading adc values.
const voltile means
you can not modify or alter the value of that variable in code. only external event can change the value.
controller pins are generally defines as volatile.
may be by declaring variable as volatile controller will do "read by pin" not "read by latch"... this is my assumtion. may be wrong...
but still there is lots of confusion when to choose variable as volatile..
A variable should be declared volatile whenever its value could change unexpectedly. In practice, only three types of variables could change:
Memory-mapped peripheral registers
Global variables modified by an interrupt service routine
Global variables within a multi-threaded application
Link : http://eetimes.com/discussion/beginner-s-corner/4023801/Introduction-to-the-Volatile-Keyword
So It is proffered to variable as volatile in such cases.
There's no reason for a volatile variable to be stored in any "special" section of memory. It is normally stored together with any other variables, including non-volatile ones. If some compiler decides to store volatile variables in some special section of memory - there's nothing to prevent it from doing so. But at the language level there's absolutely no reason for this.
Why are you asking such a question? What made you think that it should be stored in some special section of memory?
"Volatile" was used in C/C++ specifications to allow use of memory mapped devices. It directs the compiler not to optimize the variable defined with this keyword, just because the variable doesn't seem to change its state in compiler-visible code.
I have used a static global variable and a static volatile variable in file scope,
both are updated by an ISR and a main loop and main loop checks the value of the variable. here during optimization neither the global variable nor the volatile variable are optimized. So instead of using a volatile variable a global variable solves the problem.
So is it good to use global variable instead of volatile?
Any specific reason to use static volatile??
Any example program would be appreciable.
Thanks in advance..
First let me mention that a static global variable, is the same as a global variable, except that you are limiting the variable to the scope of the file. I.e. you can't use this global variable in other files via the extern keyword.
So you can reduce your question to global variables vs volatile variables.
Now onto volatile:
Like const, volatile is a type modifier.
The volatile keyword was created to prevent compiler optimizations that may make code incorrect, specifically when there are asynchronous events.
Objects declared as volatile may not be used in certain optimizations.
The system always reads the current true value of a volatile object at the point it is used, even if a previous instruction asked for a value from the same object. Also, the value of the object is written immediately on assignment. That means there is no caching of a volatile variable into a CPU register.
Dr. Jobb's has a great article on volatile.
Here is an example from the Dr. Jobb's article:
class Gadget
{
public:
void Wait()
{
while (!flag_)
{
Sleep(1000); // sleeps for 1000 milliseconds
}
}
void Wakeup()
{
flag_ = true;
}
...
private:
bool flag_;
};
If the compiler sees that Sleep() is an external call, it will assume that Sleep() cannot possibly change the variable flag_'s value. So the compiler may store the value of flag_ in a register. And in that case, it will never change. But if another thread calls wakeup, the first thread is still reading from the CPU's register. Wait() will never wake-up.
So why not just never cache variables into registers and avoid the problem completely?
It turns out that this optimization can really save you a lot of time overall. So C/C++ allows you to explicitly disable it via the volatile keyword.
The fact above that flag_ was a member variable, and not a global variable (nor static global) does not matter. The explanation after the example gives the correct reasoning even if you're dealing with global variables (and static global variables).
A common misconception is that declaring a variable volatile is sufficient to ensure thread safety. Operations on the variable are still not atomic, even though they are not "cached" in registers
volatile with pointers:
Volatile with pointers, works like const with pointers.
A variable of type volatile int * means that the variable that the pointer points to is volatile.
A variable of type int * volatile means that the pointer itself is volatile.
They are different things. I'm not an expert in volatile semantics. But i think it makes sense what is described here.
Global
Global just means the identifier in question is declared at file-scope. There are different scopes, called function (where goto-labels are defined in), file (where globals reside), block (where normal local variables reside), and function prototype (where function parameters reside). This concept just exist to structure the visibility of identifiers. It doesn't have anything to do with optimizations.
Static
static is a storage duration (we won't look at that here) and a way to give a name declared within file scope internal linkage. This can be done for functions or objects only required within one translation unit. A typical example might be a help function printing out the accepted parameters, and which is only called from the main function defined in the same .c file.
6.2.2/2 in a C99 draft:
If the declaration of a file scope
identifier for an object or a function
contains the storage class specifier
static, the identifier has internal
linkage.
Internal linkage means that the identifier is not visible outside the current translation unit (like the help function of above).
Volatile
Volatile is a different thing: (6.7.3/6)
An object that has volatile-qualified
type may be modified in ways unknown to
the implementation or have other
unknown side effects. Therefore any
expression referring to such an object
shall be evaluated strictly according
to the rules of the abstract machine,
as described in 5.1.2.3. Furthermore,
at every sequence point the value last
stored in the object shall agree with
that prescribed by the abstract
machine, except as modified by the
unknown factors mentioned
previously.
The Standard provides an excellent example for an example where volatile would be redundant (5.1.2.3/8):
An implementation might define a
one-to-one correspondence between
abstract and actual semantics: at
every sequence point, the values of
the actual objects would agree with
those specified by the abstract
semantics. The keyword volatile
would then be redundant.
Sequence points are points where the effect of side effects concerning the abstract machine are completed (i.e external conditions like memory cell values are not included). Between the right and the left of && and ||, after ; and returning from a function call are sequence points for example.
The abstract semantics is what the compiler can deduce from seeing only the sequence of code within a particular program. Effects of optimizations are irrelevant here. actual semantics include the effect of side effects done by writing to objects (for example, changing of memory cells). Qualifying an object as volatile means one always gets the value of an object straight from memory ("as modified by the unknown factors"). The Standard doesn't mention threads anywhere, and if you must rely on the order of changes, or on atomicity of operations, you should use platform dependent ways to ensure that.
For an easy to understand overview, intel has a great article about it here.
What should i do now?
Keep declaring your file-scope (global) data as volatile. Global data in itself does not mean the variables' value will equal to the value stored in memory. And static does only make your objects local to the current translation unit (the current .c files and all other files #include'ed by it).
The "volatile" keyword suggests the compiler not to do certain optimizations on code involving that variable; if you just use a global variable, nothing prevents the compiler to wrongly optimize your code.
Example:
#define MYPORT 0xDEADB33F
volatile char *portptr = (char*)MYPORT;
*portptr = 'A';
*portptr = 'B';
Without "volatile", the first write may be optimized out.
The volatile keyword tells the compiler to make sure that variable will never be cached. All accesses to it must be made in a consistent way as to have a consistent value between all threads. If the value of the variable is to be changed by another thread while you have a loop checking for change, you want the variable to be volatile as there is no guarantee that a regular variable value won't be cached at some point and the loop will just assume it stays the same.
Volatile variable on Wikipedia
They may not be in different in your current environment, but subtle changes could affect the behavior.
Different hardware (more processors, different memory architecture)
A new version of the compiler with better optimization.
Random variation in timing between threads. A problem may only occur one time in 10 million.
Different compiler optimization settings.
It is much safer in the long run to use proper multithreading constructs from the beginning, even if things seem to work for now without them.
Of course, if your program is not multi-threaded then it doesn't matter.
I +1 friol's answer. I would like to add some precisions as there seem to be a lot of confusions in different answers: C's volatile is not Java's volatile.
So first, compilers can do a lot of optimizations on based on the data flow of your program, volatile in C prevents that, it makes sure you really load/store to the location every time (instead of using registers of wiping it out e.g.). It is useful when you have a memory mapped IO port, as friol's pointed out.
Volatile in C has NOTHING to do with hardware caches or multithreading. It does not insert memory fences, and you have absolutely no garanty on the order of operations if two threads do accesses to it. Java's volatile keyword does exactly that though: inserting memory fences where needed.
volatile variable means that the value assinged to it is not constant, i.e if a function containing a volatile variable "a=10" and the function is adding 1 in each call of that function then it will always return updated value.
{
volatile int a=10;
a++;
}
when the above function is called again and again then the variable a will not be re-initialised to 10, it will always show the updated value till the program runs.
1st output= 10
then 11
then 12
and so on.