Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords — and, indeed, the meaning of the notion of keyword — differs widely from language to language....
usually has special properties related to optimization and/or threading. Generally speaking, the volatile keyword is intended to prevent the compiler from applying any optimizations on the code that assume values of variables cannot change "on their own."
The actual definition and applicability of the volatile keyword is often misconstrued in the context of C; and because C++, C#, and Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
mystically "inherit" volatile from C, there is a great deal of difference between the semantics and usefulness of volatile in each of these programming languages.
In C and C++
In C, and consequently C++, the volatile keyword was intended to
Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
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 do they 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.
Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
In C
In this example, the code sets the value stored in foo to 0. It then starts to poll
Polling (computer science)
Polling, or polled operation, in computer science, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output , and is also referred to as polled or software driven .Polling is sometimes used...
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
Infinite loop
An infinite loop is a sequence of instructions in a computer program which loops endlessly, either due to the loop having no terminating condition, having one that can never be met, or one that causes the loop to start over...
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
Hardware register
In digital electronics, especially computing, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously.The hardware registers inside a central processing unit are called processor registers....
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
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
. If we observe the assembly code, we can see that the code generated with volatile objects is bigger than the other, because the volatile keyword stops the compiler from performing optimization on code involving volatile objects.
Disassembly comparison
Without volatile keyword
With volatile keyword
include
int main {
/* These variables will never be created on stack*/
int a = 10, b = 100, c = 0, d = 0;
/* "printf" will be called with arguments "%d" and
110 (the compiler computes the sum of a+b),
hence no overhead of performing addition at
run-time */
printf("%d", a + b);
/* This code will be removed via optimization, but
the impact of 'c' and 'd' becoming 100 can be
seen while calling "printf" */
a = b;
c = b;
d = b;
/* Compiler will generate code where printf is
called with arguments "%d" and 200 */
printf("%d", c + d);
return 0;
}
include
int main {
volatile int a = 10, b = 100, c = 0, d = 0;
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:
(In all versions of Java) There is a global ordering on the reads and writes to a volatile variable. This implies that every thread
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
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.)
(In Java 5 or later) Volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.
In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...
, but it will not work in some situations. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking
Double-checked locking
In software engineering, double-checked locking is a software design pattern used to reduce the overhead of acquiring a lock by first testing the locking criterion without actually acquiring the lock...
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
In computer programming, the term directive is applied in a variety of ways that are similar to the term command. It is also used to describe some programming language constructs ....
rather than a keyword. "For a volatile object all reads and updates of the object as a whole are performed directly to memory."
