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In computer programming, particularly in the C, C++, and C# programming languages, a variable or object declared with the volatile keyword may be modified externally from the declaring object. Variables declared to be volatile will not be optimized by the compiler because the compiler must assume that their values can change at any time. [1]. Note that volatile variables in C and C++ on most systems are generally not a useful threading construct (ex: they are not atomic), according to the relevant standards and actual fact of current compiler implementations.

Contents

Use

In this example, the code sets the value stored in foo to 0. It then starts to poll that value repeatedly until it changes to 255:

static int foo;
 
void bar (void) {
    foo = 0;
 
    while (foo != 255)
         ;
}

An optimizing compiler will notice that no other code can possibly change the value stored in foo, and will assume that it will remain equal to 0 at all times. The compiler will therefore replace the function body with an infinite loop similar to this:

void bar_optimized(void) {
    foo = 0;
 
    while (true)
         ;
}

However, foo might represent a location that can be changed by other elements of the computer system at any time, such as a hardware register of a device connected to the CPU. The above code would never detect such a change; without the volatile keyword, the compiler assumes that the current program is the only part of the system that could change the value (which is by far the most common situation).

To prevent the compiler from optimizing code as above, the volatile keyword is used:

static volatile int foo;
 
void bar (void) {
    foo = 0;
 
    while (foo != 255)
        ;
}

With this modification the loop condition will not be optimized away, and the system will detect the change when it occurs.

The volatile keyword is not a threading or synchronization primitive in portable C or C++. In C, and consequently C++, the keyword was mainly intended to:[citation needed]

  • allow access to memory mapped devices
  • allow uses of variables between setjmp and longjmp
  • allow uses of variables in signal handlers

For an example of the use of volatile in context, see busy waiting.

Optimization comparison in C

The following C programs, and accompanying disassemblies, demonstrate how the volatile keyword affects the compiler's output. The compiler in this case was GCC.

In Java

The Java programming language also has the volatile keyword, but it is used for a somewhat different purpose. When applied to a field, the Java volatile guarantees that:

  1. (In all versions of Java) There is a global ordering on the reads and writes to a volatile variable. This implies that every thread accessing a volatile field will read its current value before continuing, instead of (potentially) using a cached value. (However, there is no guarantee about the relative ordering of volatile reads and writes with regular reads and writes, meaning that it's generally not a useful threading construct.)
  2. (In Java 5 or later) Volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.[2]

Using volatile may be faster than a lock, but it will not work in all situations. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking now works correctly.[3]

References

  1. ^ Sams Teach Yourself C in 24 Hours (2nd Edition); 2nd edition (2000);Authors: Tony Zhang, John Southmayd; Chapter 14: The Volatile Modifier; Sams_Publishing ; ISBN-13:978-0672318610
  2. ^ "A field may be declared volatile[...]" "The Java Language Specification, 2nd Edition, Section 8.3.1.4: volatile Fields". Sun Microsystems. 2000. http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#36930. Retrieved 2009-05-25. 
  3. ^ Neil Coffey. "Double-checked Locking (DCL) and how to fix it". Javamex. http://www.javamex.com/tutorials/double_checked_locking_fixing.shtml. Retrieved 2009-09-19. 

External links

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In computer programming, particularly in the C, C++, and C# programming languages, a variable or object declared with the volatile keyword usually has special properties related to optimization and/or threading. Generally speaking, the volatile keyword is intended to prevent the (pseudo)compiler from applying any optimizations on the code inherent the attributed variable or Object, that would be otherwise altered.

The actual definition and applicability of the volatile keyword is often misconstrued in the context of C; and because C++, C#, and Java spiritually "inherit" volatile from C, there is a great deal of difference between the semantics and usefulness of volatile in each of these programming languages.

Contents

In C and C++

In C, and consequently C++, the volatile keyword was intended to[1]

  • allow access to memory mapped devices
  • allow uses of variables between setjmp and longjmp
  • allow uses of sig_atomic_t variables in signal handlers

Operations on volatile variables are not atomic nor establish a proper happens-before relationship for threading. This is according to the relevant standards (C, C++, POSIX, WIN32), and this is the matter of fact for the vast majority of current implementations. The volatile keyword is basically worthless as a portable threading construct.[2][3][4][5][6]

Example of MMIO In C

In this example, the code sets the value stored in foo to 0. It then starts to poll that value repeatedly until it changes to 255:

static int foo;

void bar(void) {

   foo = 0;
   while (foo != 255)
        ;

}

An optimizing compiler will notice that no other code can possibly change the value stored in foo, and will assume that it will remain equal to 0 at all times. The compiler will therefore replace the function body with an infinite loop similar to this:

void bar_optimized(void) {

   foo = 0;
   while (true)
        ;

}

However, foo might represent a location that can be changed by other elements of the computer system at any time, such as a hardware register of a device connected to the CPU. The above code would never detect such a change; without the volatile keyword, the compiler assumes that the current program is the only part of the system that could change the value (which is by far the most common situation).

To prevent the compiler from optimizing code as above, the volatile keyword is used:

static volatile int foo;

void bar (void) {

   foo = 0;
   while (foo != 255)
       ;

}

With this modification the loop condition will not be optimized away, and the system will detect the change when it occurs.

Optimization comparison in C

The following C programs, and accompanying disassemblies, demonstrate how the volatile keyword affects the compiler's output. The compiler in this case was GCC.

In Java

The Java programming language also has the volatile keyword, but it is used for a somewhat different purpose. When applied to a field, the Java volatile guarantees that:

  1. (In all versions of Java) There is a global ordering on the reads and writes to a volatile variable. This implies that every thread accessing a volatile field will read its current value before continuing, instead of (potentially) using a cached value. (However, there is no guarantee about the relative ordering of volatile reads and writes with regular reads and writes, meaning that it's generally not a useful threading construct.)
  2. (In Java 5 or later) Volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.[7]

Using volatile may be faster than a lock, but it will not work in all situations. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking now works correctly.[8]

In Ada

In Ada, pragma Volatile is a directive rather than a keyword. "For a volatile object all reads and updates of the object as a whole are performed directly to memory."[9]

References

  1. ^ Publication on C++ standards committee website; http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2016.html
  2. ^ Publication on C++ standards committee website; http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2016.html
  3. ^ Volatile Keyword In Visual C++; http://msdn2.microsoft.com/en-us/library/12a04hfd.aspx
  4. ^ Linux Kernel Documentation - Why the "volatile" type class should not be used; http://kernel.org/doc/Documentation/volatile-considered-harmful.txt
  5. ^ Volatile: Almost Useless for Multi-Threaded Programming (Intel Software Network); http://softwareblogs.intel.com/2007/11/30/volatile-almost-useless-for-multi-threaded-programming/
  6. ^ C++ and the Perils of Double-Checked Locking; http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf
  7. ^ "A field may be declared volatile[...]" "The Java Language Specification, 2nd Edition, Section 8.3.1.4: volatile Fields". Sun Microsystems. 2000. http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#36930. Retrieved 2009-05-25. 
  8. ^ Neil Coffey. "Double-checked Locking (DCL) and how to fix it". Javamex. http://www.javamex.com/tutorials/double_checked_locking_fixing.shtml. Retrieved 2009-09-19. 
  9. ^ "C.6 Shared Variable Control" "Ada Reference Manual". ISO. 2005. http://www.adaic.com/standards/05rm/html/RM-C-6.html. Retrieved 2010-05-04. 

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