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http://stackoverflow.com/questions/14332503/what-is-extern-volatile-pointer

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What is extern volatile pointer

What is extern volatile pointer.

extern volatile uint32 *ptr;

Here, what will be the behavior of *ptr? What does this actually mean?

And, when it should be used?

I have tried to google about it, but didn't get any satisfactory answer, there is not much info present about this

combination.

c volatile

edited Jan 25 '13 at 8:26 asked Jan 15 '13 at 6:52

Arti

108 2 8

–You didn't look really hard see stackoverflow.com/questions/9935190/… Mikhail Jan 15 '13 at 6:54

–

Extern and volatile are not related. You have a volatile pointer, and it happens to be defined somewhere else.

The extern qualifier doesn't change the semantics of the volatile pointer. Mat Jan 15 '13 at 6:56

–

I have seen extern volatile *ptr usage in the project I am working in, but due to high data security reasons, I

can not provide the code, for refernce. Also, since product is very vast, so I am unable to understand its

usage. Arti Jan 15 '13 at 6:58

–

tells the compiler to read from its memory location whenever its value is read in the

program/scope. Usually the compiler reads small variables (int, pointer) into a CPU register and use that

register for the next sequential references to that value (for the sake of efficiency). Using makes the

compiler stop that behavior.

volatile obj obj

volatile

ringø Jan 15 '13 at 7:06

–

Can anyone answer, considering both "exter volatile" please? I am not looking for individual definitions. like

what will be the system behaviour if I omit volatile here, what is the behaviour with 'extern volatile'. any

implementaion examples will be helpful. Arti Jan 15 '13 at 7:42

4 Answers

Both and keywords can be considered independently. The role of each of these

keywords does not interact with the other one, and thus an explanation of each of them can be

detailed independently, as below.

extern volatile

tells the compiler that the actual definition of is in another module (another ).

Basically, there is no much change to how the compiler handles - having just tells the

compiler that it does not have to reserve some space in memory for as it is done elsewhere in

another , and its actual memory location will be given by the later.

extern ptr .c

ptr extern

ptr

.c linker

extern uint32 *ptr;

If you omit the compiler will not complain. However, later, the , when it tries to link all

object modules in order to build the final executable program, will throw an error saying " is

defined twice" (since it is already defined in another ).

extern linker

ptr

.c

uint32 *ptr;

tells the compiler that the memory location where resides may be altered / modified

by some external event, and it (the compiler) should not rely on some efficiency optimizations like

considering that the value of will not change during the scope of a few sequential lines of C.

Such an event may be an asynchronous interruption, that happens when the CPU executes the

volatile ptr

ptr

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above said scope, and modifies the value. ptr

Typically (with virtual assembly code of optimized C in comments), REGx are CPU registers, and

we are not much interested in the variable ...y

int x = 10;

int func() {

int y; // REG4

printf("%d\n", x); // Set REG3 = memory(x) and display x

x += 2; // Add 2 to REG3

y = x * x; // REG4 = REG3 * REG3

printf("%d %d\n", x, y); // Do printf(..., REG3, REG4)

x += 5; // REG3 = REG3 + 5

// memory(x) = REG3 (save register to memory) return y; // return REG4

}

should display . For the sake of efficiency (memory accesses are more costly than

register accesses) say the compiler stores the value of in an internal CPU register (REG3) and

uses it in safely until the end where it stores the new value of (it's a global var) to

memory location. is 17 at the end.

10, 12, 144

x

func x x

x

But imagine the program is more complex than that and has a clock interruption every minute that

subtract 10 to . What happens if an interruption... x

void inter_call_by_timer_every_minute() {

x -= 10;

}

... occurs in say just after the line? has in REG3 (10), add 2 (12)

and finally add 5 (17) and stores the REG3 result to memory location (17). This is wrong as theinterruption effect (-10) has been hidden by the compiler optimization, since it stores the value

from REG3 to memory(x) at the end ignoring the done by the interruption. The correct

result was: is initially 10, interrupt subtract 10 to it (0) after the first in , then 2 is

added, then 5. Result 7.

func printf("%d\n", x); func x

x

subtract

x printf func

Adding volatile

volatile int x = 10;

will have the compiler avoid the optimizations in x func

int func() {

int y; // REG4

printf("%d\n", x); // display memory(x)

x += 2; // memory(x) += 2

y = x * x; // REG4 = memory(x) * memory(x)

printf("%d %d\n", x, y); // Do printf(..., memory(x), REG4)

x += 5; // memory(x) += 5

return y; // return REG4

}

and read the value of from memory all the time. Result, having an interruption from

after the first printf, is == 7.

x

inter_call_by_timer_every_minute x

edited Apr 4 at 16:02

rphv

2,650 1 10 31

answered Jan 15 '13 at 8:37

ringø

15.1k 4 31 65

I would explain the way I know through the keywords. means the variable is defined for

use somewhere outside the definition scope. For example it could be defined in header file and

get used in .c file.

extern

means that modification of that variable should be consistent to the external world.

This means that all update should be committed to the main memory so that it can be seen by

other threads which share the same execution space.

volatile


Hassan TM

1,560 6 15

Extern means it is defined elsewhere - probably in the header file. Volatile is info for the compiler

hat it shouldn't try to optimize this.


http://stackoverflow.com/users/1952879/hassan-tmhttp://stackoverflow.com/users/338904/ring%c3%b8http://stackoverflow.com/users/1612562/rphvhttp://stackoverflow.com/posts/14333841/revisions


3/3



KamikazeCZ

515 5 19

2

–

Probably in a header file, it is uncommon to define variables in header files and most of the time also

bad practice. You will likely find extern in the header filer and the actual definition in a corresponding C file.

not

Lundin Jan 15 '13 at 7:54

keyword is used to declare a global variable which is defined somewhere else (that

means its defined in some other file).

extern

.c

For example consider in a project two files and are there. In that a global variable

is defined in , and that variable can be accessed in all the functions which are defined in that

file. If we want to access that same global variable in our second file , then that variable

should be declared as in

.c a.c b.c

a.c

b.c

extern b.c

file is given belowa.c

int flag = 0;

int main()

{

.......

func1();

printf("\nflag value is %d\n", flag).

.......

}

file is given belowb.c

extern int flag;void func1();

{

.....

flag = 10;

.....

}

keyword is used to inform the compiler to avoid doing any sort of optimization while

generating executable instructions.

volatile

int flag = 0;

int main()

{

while(flag == 0);

printf("\nflag value is %d\n", flag);

return 0;

}

Consider the above program, all compiler will optimize the as .

Because in the code there is no where value gets updated before that loop. So if

hat variable value gets updated by some other hardware then it will never get reflected in the

program execution. So if we declare that variable as like below, compiler will not

perform any optimization for that variable, and the behviour of that program will be as intended.

while(flag == 0); while(1);

flag while

volatile

volatile int flag = 0;

But if there is no way for the value of program's variable is getting updated by other hardware,

hen no need to declare that variable as . Because for valatile variables, CPU needs to

perform I/O operation for each instructions which is accessing that variable. This impact in

performance needs to be consider for a variable which will never get updated by other hardwares.

volatile


raja ashok

3,456 9 32 57
http://stackoverflow.com/users/596370/raja-ashokhttp://stackoverflow.com/users/584518/lundinhttp://stackoverflow.com/users/1978950/kamikazecz

c - what is extern volatile pointer - stack overflow

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