INTERVIEW QUESTIONS C VARIABLES IN C DETAILS

Question: Constant volatile variable declaration is possible or not? if give any one example and reason.

Answer: Constants can be defined by placing the keyword const in front of any variable declaration. If the keyword volatile is placed after const, then this allows external routines to modify the variable (such as hardware devices). This also forces the compiler to retrieve the value of the variable each time it is referenced rather than possibly optimizing it in a register.

Example: An input-only buffer for an external device could be declared as const volatile (or volatile const, order is not important) to make sure the compiler knows that the variable should not be changed (because it is input-only) and that its value may be altered by processes other than the current program.

Constants can be defined by placing the keyword const in front of any variable declaration. If the keyword volatile is placed after const, then this allows external routines to modify the variable (such as hardware devices). This also forces the compiler to retrieve the value of the variable each time it is referenced rather than possibly optimizing it in a register.

Example: An input-only buffer for an external device could be declared as const volatile (or volatile const, order is not important) to make sure the compiler knows that the variable should not be changed (because it is input-only) and that its value may be altered by processes other than the current program.
Source: CoolInterview.com

The compilation system tries to reduce code size and execution time on all machines, by optimizing code. It is programmer responsibility to inform compilation system that certain code (or variable) should not be optimized.

Volatile Variable
A volatile variable is the one whose values may be changed at any time by some external sources.

Example:

volatile int num;

The value of data may be altered by some external factor, even if it does not appear on the left hand side of the assignment statement. When we declare a variable as volatile the compiler will examine the value of the variable each time it is encountered to see if an external factor has changed the value.

One example of a volatile object might be a buffer used to exchange data with an external device:
int
check_iobuf(void)
{
volatile int iobuf;
int val;

while (iobuf == 0) {
}
val = iobuf;
iobuf = 0;
return(val);
}

if iobuf had not been declared volatile, the compiler would notice that nothing happens inside the loop and thus eliminate the loop
const and volatile can be used together
An input-only buffer for an external device could be declared as const volatile (or volatile const, order is not important) to make sure the compiler knows that the variable should not be changed (because it is input-only) and that its value may be altered by processes other than the current program
Source: CoolInterview.com

If a variable is both const and volatile, the two modifiers can appear in either order.

Yes it can be declared!!! Source: CoolInterview.com

Volatile keyword tells the compiler that the value of the variable can change (may be frequently) so that it can avoid the optimization.

Constant means variable can not be assigned a new value the catch here is using the pointer we can change the value. So when we say a variable is both a constant and volatile that means we are instructing the compiler that the variable can not be assigned a new value and its value can be changed by the use of pointer so don't do any optimization. Source: CoolInterview.com

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