Volatile (computer programming)
In computer programming, particularly in the C, C++, C#, and Java programming languages, the volatile keyword indicates that a value may change between different accesses, even if it does not appear to be modified. This keyword prevents an optimizing compiler from optimizing away subsequent reads or writes and thus incorrectly reusing a stale value or omitting writes. Volatile values primarily arise in hardware access, where reading from or writing to memory is used to communicate with peripheral devices, and in threading, where a different thread may have modified a value.
Despite being a common keyword, the behavior of
volatile differs significantly between programming languages, and is easily misunderstood. In C and C++, it is a type qualifier, like const, and is a property of the type. Furthermore, in C and C++ it does not work in most threading scenarios, and that use is discouraged. In Java and C#, it is a property of a variable and indicates that the object to which the variable is bound may mutate, and is specifically intended for threading. In the D programming language, there is a separate keyword shared for the threading usage, but no volatile keyword exists.In C and C++
In C, and consequently C++, thevolatile keyword was intended to- allow access to memory-mapped I/O devices
- allow uses of variables between
setjmpandlongjmp - allow uses of
sig_atomic_tvariables in signal handlers.
volatile variables are not atomic, nor do they establish a proper happens-before relationship for threading. This is specified in the relevant standards, and volatile variables are not threadsafe in the vast majority of current implementations. Thus, the usage of volatile keyword as a portable synchronization mechanism is discouraged by many C/C++ groups.Example of memory-mapped I/O in C
In this example, the code sets the value stored infoo to 0. It then starts to poll that value repeatedly until it changes to 255:static int foo;
void bar
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
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.To prevent the compiler from optimizing code as above, the
volatile keyword is used:static volatile int foo;
void bar
With this modification the loop condition will not be optimized away, and the system will detect the change when it occurs.
Generally, there are memory barrier operations available on platforms that should be preferred instead of volatile as they allow the compiler to perform better optimization and more importantly they guarantee correct behaviour in multi-threaded scenarios; neither the C specification nor the C++ specification.
Optimization comparison in C
The following C programs, and accompanying assemblies, demonstrate how thevolatile keyword affects the compiler's output. The compiler in this case was GCC.While observing the assembly code, it is clearly visible that the code generated with
volatile objects is more verbose, making it longer so the nature of volatile objects can be fulfilled. The volatile keyword prevents the compiler from performing optimization on code involving volatile objects, thus ensuring that each volatile variable assignment and read has a corresponding memory access. Without the volatile keyword, the compiler knows a variable does not need to be reread from memory at each use, because there should not be any writes to its memory location from any other thread or process.C++11
According to the C++11 ISO Standard, the volatile keyword is only meant for use for hardware access; do not use it for inter-thread communication. For inter-thread communication, the standard library providesstd::atomic templates.In Java
The Java programming language also has thevolatile keyword, but it is used for a somewhat different purpose. When applied to a field, the Java qualifier volatile provides the following guarantees:- In all versions of Java, there is a global ordering on reads and writes of all volatile variables. This implies that every thread accessing a volatile field will read its current value before continuing, instead of using a cached value.
- In Java 5 or later, volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.
volatile may be faster than a lock, but it will not work in some situations before Java 5. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking now works correctly.In C#
In C#,volatile ensures that code accessing the field is not subject to some thread-unsafe optimizations that may be performed by the compiler, the CLR, or by hardware. When a field is marked volatile, the compiler is instructed to generate a "memory barrier" or "fence" around it, which prevents instruction reordering or caching tied to the field. When reading a volatile field, the compiler generates an acquire-fence, which prevents other reads and writes to the field, including those in other threads, from being moved before the fence. When writing to a volatile field, the compiler generates a release-fence; this fence prevents other reads and writes to the field from being moved after the fence.Only the following types can be marked
volatile: all reference types, Single, Boolean, Byte, SByte, Int16, UInt16, Int32, UInt32, Char, and all enumerated types with an underlying type of Byte, SByte, Int16, UInt16, Int32, or UInt32. Using the
volatile keyword does not support fields that are passed by reference or captured local variables; in these cases, Thread.VolatileRead and Thread.VolatileWrite must be used instead.In effect, these methods disable some optimizations usually performed by the C# compiler, the JIT compiler, or the CPU itself. The guarantees provided by
Thread.VolatileRead and Thread.VolatileWrite are a superset of the guarantees provided by the volatile keyword: instead of generating a "half fence", VolatileRead and VolatileWrite generate a "full fence" which prevent instruction reordering and caching of that field in both directions. These methods work as follows:- The
Thread.VolatileWritemethod forces the value in the field to be written to at the point of the call. In addition, any earlier program-order loads and stores must occur before the call toVolatileWriteand any later program-order loads and stores must occur after the call. - The
Thread.VolatileReadmethod forces the value in the field to be read from at the point of the call. In addition, any earlier program-order loads and stores must occur before the call toVolatileReadand any later program-order loads and stores must occur after the call.
Thread.VolatileRead and Thread.VolatileWrite methods generate a full fence by calling the Thread.MemoryBarrier method, which constructs a memory barrier that works in both directions. In addition to the motivations for using a full fence given above, one potential problem with the volatile keyword that is solved by using a full fence generated by Thread.MemoryBarrier is as follows: due to the asymmetric nature of half fences, a volatile field with a write instruction followed by a read instruction may still have the execution order swapped by the compiler. Because full fences are symmetric, this is not a problem when using Thread.MemoryBarrier.In Fortran
VOLATILE is part of the Fortran 2003 standard, although earlier version supported it as an extension. Making all variables volatile in a function is also useful finding aliasing related bugs.integer, volatile :: i ! When not defined volatile the following two lines of code are identical
write i**2 ! Loads the variable i once from memory and multiplies that value times itself
write i*i ! Loads the variable i twice from memory and multiplies those values
By always "drilling down" to memory of a VOLATILE, the Fortran compiler is precluded from reordering reads or writes to volatiles. This makes visible to other threads actions done in this thread, and vice versa.
Use of VOLATILE reduces and can even prevent optimization.