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 (memory-mapped I/O), 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, which is based on C++, there is a separate keyword shared for the threading usage, but no volatile keyword exists.

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 do they establish a proper happens-before relationship for threading. This is according to the relevant standards (C, C++, POSIX, WIN32),^[1] and this is the matter of fact for the vast majority of current implementations. Thus, the usage of volatile keyword as a portable synchronization mechanism is discouraged by many C/C++ groups.^[2]^[3]^[4]

Example of memory-mapped I/O 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.

Generally, there are memory barrier operations available on platforms (which are exposed in C++11) 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 (before C11) nor the C++ specification (before C++11) specifies a multi-threaded memory model, so volatile may not behave deterministically across OSes/compilers/CPUs).^[5]

Optimization comparison in C

The following C programs, and accompanying assemblies, demonstrate how the volatile 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.

Assembly comparison
Without `volatile` keyword	With `volatile` keyword
# include <stdio.h> 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 <stdio.h> int main() { volatile int a = 10, b = 100, c = 0, d = 0; printf("%d", a + b); a = b; c = b; d = b; printf("%d", c + d); return 0; }
gcc -S -O3 -masm=intel noVolatileVar.c -o without.s	gcc -S -O3 -masm=intel VolatileVar.c -o with.s
.file "noVolatileVar.c" .intel_syntax noprefix .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "%d" .section .text.startup,"ax",@progbits .p2align 4,,15 .globl main .type main, @function main: .LFB11: .cfi_startproc sub rsp, 8 .cfi_def_cfa_offset 16 mov esi, 110 mov edi, OFFSET FLAT:.LC0 xor eax, eax call printf mov esi, 200 mov edi, OFFSET FLAT:.LC0 xor eax, eax call printf xor eax, eax add rsp, 8 .cfi_def_cfa_offset 8 ret .cfi_endproc .LFE11: .size main, .-main .ident "GCC: (GNU) 4.8.2" .section .note.GNU-stack,"",@progbits	.file "VolatileVar.c" .intel_syntax noprefix .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "%d" .section .text.startup,"ax",@progbits .p2align 4,,15 .globl main .type main, @function main: .LFB11: .cfi_startproc sub rsp, 24 .cfi_def_cfa_offset 32 mov edi, OFFSET FLAT:.LC0 mov DWORD PTR [rsp], 10 mov DWORD PTR [rsp+4], 100 mov DWORD PTR [rsp+8], 0 mov DWORD PTR [rsp+12], 0 mov esi, DWORD PTR [rsp] mov eax, DWORD PTR [rsp+4] add esi, eax xor eax, eax call printf mov eax, DWORD PTR [rsp+4] mov edi, OFFSET FLAT:.LC0 mov DWORD PTR [rsp], eax mov eax, DWORD PTR [rsp+4] mov DWORD PTR [rsp+8], eax mov eax, DWORD PTR [rsp+4] mov DWORD PTR [rsp+12], eax mov esi, DWORD PTR [rsp+8] mov eax, DWORD PTR [rsp+12] add esi, eax xor eax, eax call printf xor eax, eax add rsp, 24 .cfi_def_cfa_offset 8 ret .cfi_endproc .LFE11: .size main, .-main .ident "GCC: (GNU) 4.8.2" .section .note.GNU-stack,"",@progbits

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 provides std::atomic<T> templates.^[6]

Compile-time detection of race conditions

Although in C/C++ using volatile keyword on a shared resource does not guarantee that otherwise unsynchronized threads will not attempt to access it simultaneously, it is possible to create more advanced constructs that will help to detect potential race conditions at compile time, in a way similar to how const keyword prevents unintended modification of data. A possible implementation of such a construct has been proposed by Andrei Alexandrescu.^[7]

According to C/C++ specification, for a volatile-qualified object shared, a compiler will generate an error on attempting to accessing its value directly from a non-volatile function. By using a simple helper object that acquires a corresponding mutex and removes the volatile qualifier, it is possible to access the value of shared in a synchronized critical section. In this case, the usage of volatile has little in common with the original intention of its designers and acts based on a fact that it is just a qualifier: similar to const in a way that compiler validates type correctness, yet distinguishable.

Despite preventing from optimizing some low-level code, a proper usage of the above technique can even improve program performance when providing volatile (synchronized and slower) and non-volatile (unsynchronized and faster) overloads of a function and making compiler choose between them when dealing with volatile (shared) and non-volatile (not shared) objects.

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 qualifier volatile provides the following guarantees:

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.)
In Java 5 or later, volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.^[8]

Using volatile may be faster than a lock, 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 now works correctly.^[9]

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. 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.^[10]

Basically volatile is a shorthand for calling Thread.VolatileRead and Thread.VolatileWrite. These methods are special. In effect, these methods disable some optimizations usually performed by the C# compiler, the JIT compiler, and the CPU itself . The methods work as follows:^[11]

The Thread.VolatileWrite method forces the value in address to be written to at the point of the call. In addition, any earlier program-order loads and stores must occur before the call to VolatileWrite.
The Thread.VolatileRead method forces the value in address to be read from at the point of the call. In addition, any later program-order loads and stores must occur after the call to VolatileRead.
The Thread.MemoryBarrier method does not access memory but it forces any earlier program order loads and stores to be completed before the call to MemoryBarrier. It also forces any later program-order loads and stores to be completed after the call to MemoryBarrier. MemoryBarrier is much less useful than the other two methods.

In Fortran

The volatile is part of the Fortran 2003 standard,^[12] 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

References

1 2 "Publication on C++ standards committee".
↑ "Volatile Keyword In Visual C++". Microsoft MSDN.
↑ "Linux Kernel Documentation – Why the "volatile" type class should not be used". kernel.org.
↑ "C++ and the Perils of Double-Checked Locking" (PDF). DDJ.
↑ "Linux: Volatile Superstition". kerneltrap.org. Retrieved Jan 9, 2011.
↑ "volatile (C++)". Microsoft MSDN.
↑ "volatile: The Multithreaded Programmer's Best Friend". Dr Dobbs.
↑ Section 17.4.4: Synchronization Order "The Java® Language Specification, Java SE 7 Edition". Oracle Corporation. 2013. Retrieved 2013-05-12.
↑ Neil Coffey. "Double-checked Locking (DCL) and how to fix it". Javamex. Retrieved 2009-09-19.
↑ Richter, Jeffrey (February 11, 2010). "Chapter 7: Constants and Fields". CLR Via C#. Microsoft Press. p. 183. ISBN 0-7356-2704-5.
↑ Richter, Jeffrey (February 11, 2010). "Chapter 28: Primitive Thread Synchronization Constructs". CLR Via C#. Microsoft Press. pp. 797–803. ISBN 0-7356-2704-5.
↑ "VOLATILE Attribute and Statement". Cray.

External links

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