Editing Volatile (computer programming)

{{Short description|Keyword used in some programming languages to tag variables}}
{{Lowercase title}}
In [[computer programming]], a [[Variable (computer science)|variable]] is said to be '''''volatile''''' if its [[Value (computer science)|value]] can be read or modified asynchronously by something other than the current [[thread (computing)|thread of execution]]. 
The value of a <code>volatile</code> variable may spontaneously change for reasons such as:
sharing values with other threads; 
sharing values with asynchronous [[signal handler]]s;
accessing hardware devices via [[memory-mapped I/O]] (where you can send and receive messages from [[peripheral device]]s by reading from and writing to memory).
Support for these use cases varies considerably among the programming languages that have the <code>volatile</code> keyword.
Volatility can have implications regarding function [[calling convention]]s and how variables are stored, accessed and cached.

==In C and C++==

In C and C++, <code>volatile</code> is a [[type qualifier]], like <code>[[const (computer programming)|const]]</code>, and is a part of a [[data type|type]] (e.g. the type of a variable or field). 

The behavior of the <code>volatile</code> keyword in C and C++ is sometimes given in terms of suppressing optimizations of an [[optimizing compiler]]: 1- don't remove existing <code>volatile</code> reads and writes, 2- don't add new <code>volatile</code> reads and writes, and 3- don't reorder <code>volatile</code> reads and writes. However, this definition is only an approximation for the benefit of new learners, and this approximate definition should not be relied upon to write real production code.

In C, and consequently C++, the <code>volatile</code> keyword was intended to:<ref name="auto">{{cite web |title=Publication on C++ standards committee|url= http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2016.html}}</ref>
*Allow access to [[memory-mapped I/O]] devices.
*Allow preserving values across a <code>[[setjmp|longjmp]]</code>.
*Allow sharing values between signal handlers and the rest of the program in <code>volatile</code> <code>sig_atomic_t</code> objects.

The C and C++ standards allow writing portable code that shares values across a <code>[[setjmp|longjmp]]</code> in <code>volatile</code> objects, and the standards allow writing portable code that shares values between signal handlers and the rest of the code in <code>volatile</code> <code>sig_atomic_t</code> objects. Any other use of <code>volatile</code> keyword in C and C++ is inherently non-portable or incorrect. In particular, writing code with the <code>volatile</code> keyword for [[memory-mapped I/O]] devices is inherently non-portable and always requires deep knowledge of the specific target C/C++ implementation and platform.   

=== Multi-threading ===
It is a common misconception that the <code>volatile</code> keyword is useful in portable [[thread (computing)|multi-threading]] code in C and C++. The <code>volatile</code> keyword in C and C++ has ''never'' functioned as a useful, portable tool for ''any'' multi-threading scenario.<ref>{{cite web |date=21 September 2021 |title=Volatile Keyword In Visual C++ |url=http://msdn2.microsoft.com/en-us/library/12a04hfd.aspx |work=Microsoft MSDN}}</ref><ref>{{cite web |title=Linux Kernel Documentation – Why the "volatile" type class should not be used |url=https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html |work=kernel.org}}</ref><ref>{{cite web |author1=Scott Meyers |author2=Andrei Alexandrescu |year=2004 |title=C++ and the Perils of Double-Checked Locking |url=http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf |work=DDJ}}</ref><ref>{{cite web |author1=Jeremy Andrews |year=2007 |title=Linux: Volatile Superstition |url=http://kerneltrap.org/Linux/Volatile_Superstition |archive-url=https://web.archive.org/web/20100620121940/http://kerneltrap.org/Linux/Volatile_Superstition |archive-date=2010-06-20 |access-date=Jan 9, 2011 |publisher=kerneltrap.org}}</ref> Unlike the [[Java (programming language)|Java]] and [[C Sharp (programming language)|C#]] programming languages, operations on <code>volatile</code> variables in C and C++ are not [[atomic operation|atomic]], and operations on <code>volatile</code> variables do not have sufficient memory ordering guarantees (i.e. [[memory barrier|memory barriers]]). Most C and C++ compilers, linkers, and runtimes simply do not provide the necessary memory ordering guarantees to make the <code>volatile</code> keyword useful for ''any'' multi-threading scenario. Before the C11 and C++11 standards, programmers were forced to rely on guarantees from the individual implementations and platforms (e.g. POSIX and WIN32) to write [[thread (computing)|multi-threading]] code. With the modern C11 and C++11 standards, programmers can write portable [[thread (computing)|multi-threading]] code using new portable constructs such as the <code>std::atomic<T></code> templates.<ref>{{cite web |title=volatile (C++) |url=https://msdn.microsoft.com/en-us/library/12a04hfd.aspx |work=Microsoft MSDN|date=21 September 2021 }}</ref>  

===Example of memory-mapped I/O in C===
In this example, the code sets the value stored in <code>foo</code> to <code>0</code>. It then starts to [[polling (computer science)|poll]] that value repeatedly until it changes to <code>255</code>:

<syntaxhighlight lang="c">
static int foo;

void bar(void) {
    foo = 0;

    while (foo != 255)
         ;
}
</syntaxhighlight>

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

<syntaxhighlight lang="c">
void bar_optimized(void) {
    foo = 0;

    while (true)
         ;
}
</syntaxhighlight>

However, the programmer may make <code>foo</code> refer to another element of the computer system such as a [[hardware register]] of a device connected to the [[CPU]] which may change the value of <code>foo</code> while this code is running. (This example does not include the details on how to make <code>foo</code> refer to a hardware register of a device connected to the CPU.) Without the <code>volatile</code> keyword, an [[optimizing compiler]] will likely convert the code from the first sample with the read in the loop to the second sample without the read in the loop as part of the common [[Loop-invariant code motion|loop-invariant code-motion optimization]], and thus the code will likely never notice the change that it is waiting for.

To prevent the compiler from doing this optimization, the <code>volatile</code> keyword can be used:

<syntaxhighlight lang="c">
static volatile int foo;

void bar (void) {
    foo = 0;

    while (foo != 255)
        ;
}
</syntaxhighlight>

The <code>volatile</code> keyword prevents the compiler from moving the read out of the loop, and thus the code will notice the expected change to the variable <code>foo</code>.

===Optimization comparison in C===
The following C programs, and accompanying assembler language excerpts, demonstrate how the <code>volatile</code> keyword affects the compiler's output. The compiler in this case was [[GNU Compiler Collection|GCC]].

While observing the assembly code, it is clearly visible that the code generated with <code>volatile</code> objects is more verbose, making it longer so the nature of <code>volatile</code> objects can be fulfilled. The <code>volatile</code> 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 <code>volatile</code> 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.

{|class="wikitable collapsible collapsed" width="100%"
! style="text-align: center" colspan=2 | Assembly comparison
|-
!Without <code>volatile</code> keyword||With <code>volatile</code> keyword
|-
|
<syntaxhighlight lang="c">
# 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;
}
</syntaxhighlight>
|
<syntaxhighlight lang="c">
# 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;
}
</syntaxhighlight>
|-

! {{Mono|gcc -S -O3 -masm{{=}}intel noVolatileVar.c -o without.s}}
! {{Mono|gcc -S -O3 -masm{{=}}intel VolatileVar.c -o with.s}}
|- valign="top"
|<syntaxhighlight lang="asm">
 .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
</syntaxhighlight>
|<syntaxhighlight lang="asm">
 .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
</syntaxhighlight>
|}

=== Standards defects ===
While intended by both C and C++, the current C standard fails to express that the <code>volatile</code> semantics refer to the lvalue, not the referenced object. The respective defect report ''DR 476'' (to C11) is still under review with [[C17 (C standard revision)|C17]].<ref>[http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2244.htm ''Clarification Request Summary for C11.''] Version 1.13, October 2017.</ref>

=== Compiler defects ===
Unlike other language features of C and C++, the <code>volatile</code> keyword is not well supported by most C/C++ implementations - even for portable uses according to the C and C++ standards. Most C/C++ implementations are buggy regarding the behavior of the <code>volatile</code> keyword.<ref>{{Cite journal |last1=Eide |first1=Eric |last2=Regehr |first2=John |date=October 2008 |title=Volatiles Are Miscompiled, and What to Do about It |url=https://users.cs.utah.edu/~regehr/papers/emsoft08-preprint.pdf |journal=Proceedings of the Eighth ACM and IEEE International Conference on Embedded Software (EMSOFT), Atlanta, Georgia, USA |via=cs.utah.edu}}</ref><ref>{{Cite web |title=Volatile Bugs, Three Years Later – Embedded in Academia |url=https://blog.regehr.org/archives/503 |access-date=2024-08-28 |website=blog.regehr.org}}</ref> Programmers should take great care whenever using the <code>volatile</code> keyword in C and C++.

==In Java==
In all modern versions of the [[Java programming language]], the <code>volatile</code> keyword gives the following guarantees:

* <code>volatile</code> reads and writes are [[atomic operation|atomic]]. In particular, reads and writes to <code>long</code> and <code>double</code> fields will not tear. (The [[atomic operation|atomic]] guarantee applies only to the <code>volatile</code> primitive value or the <code>volatile</code> reference value, and ''not'' to any Object value.)
* There is a single global ordering of all <code>volatile</code> reads and writes. In other words, a <code>volatile</code> read will read the current value (and not a past or future value), and all <code>volatile</code> reads will agree on a single global order of <code>volatile</code> writes.
* <code>volatile</code> reads and writes have "acquire" and "release" [[memory barrier]] semantics (known in the Java standard as [[happened-before|happens-before]]).<ref>Section 17.4.4: Synchronization Order
{{cite web |year=2013 |title=The Java® Language Specification, Java SE 7 Edition |url=http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.4 |access-date=2013-05-12 |publisher=[[Oracle Corporation]]}}</ref><ref>{{cite web |date=2021-03-08 |title=Java Concurrency: Understanding the 'Volatile' Keyword |url=https://dzone.com/articles/java-concurrency-understanding-the-volatile-keyword |archive-url=https://web.archive.org/web/20210509104459/https://dzone.com/articles/java-concurrency-understanding-the-volatile-keyword |archive-date=2021-05-09 |access-date=2021-05-09 |publisher=dzone.com}}</ref> In other words, <code>volatile</code> provides guarantees about the relative order of <code>volatile</code> and non-<code>volatile</code> reads and writes. In other words, <code>volatile</code> basically provides the same memory visibility guarantees as a Java [[lock (computer science)|synchronized block]] (but without the mutual exclusion guarantees of a [[lock (computer science)|synchronized block]]).

Together, these guarantees make <code>volatile</code> into a useful [[thread (computing)|multi-threading]] construct in [[Java programming language|Java]]. In particular, the typical [[double-checked locking]] algorithm with <code>volatile</code> works correctly in [[Java programming language|Java]].<ref>{{cite web |author1=Neil Coffey |title=Double-checked Locking (DCL) and how to fix it |url=http://www.javamex.com/tutorials/double_checked_locking_fixing.shtml |access-date=2009-09-19 |publisher=Javamex}}</ref> 

=== Early versions of Java ===
Before Java version 5, the Java standard did not guarantee the relative ordering of <code>volatile</code> and non-<code>volatile</code> reads and writes. In other words, <code>volatile</code> did not have "acquire" and "release" [[memory barrier]] semantics. This greatly limited its use as a [[thread (computing)|multi-threading]] construct. In particular, the typical [[double-checked locking]] algorithm with <code>volatile</code> did ''not'' work correctly.

==In C#==

In [[C Sharp (programming language)|C#]], <code>volatile</code> 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 <code>volatile</code>, 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 <code>volatile</code> field, the compiler generates an ''acquire-fence'', which prevents other reads and writes to the field from being moved ''before'' the fence. When writing to a <code>volatile</code> field, the compiler generates a ''release-fence''; this fence prevents other reads and writes to the field from being moved ''after'' the fence.<ref name="Albahari">{{cite web |last1=Albahari |first1=Joseph |title=Part 4: Advanced Threading |url=http://www.albahari.com/threading/part4.aspx |website=Threading in C# |publisher=O'Reilly Media |access-date=9 December 2019 |archive-url=https://web.archive.org/web/20191212032535/http://www.albahari.com/threading/part4.aspx#_Nonblocking_Synchronization |archive-date=12 December 2019 |url-status=bot: unknown }}</ref>

Only the following types can be marked <code>volatile</code>: all reference types, <code>Single</code>, <code>Boolean</code>, <code>Byte</code>, <code>SByte</code>, <code>Int16</code>, <code>UInt16</code>, <code>Int32</code>, <code>UInt32</code>, <code>Char</code>, and all enumerated types with an underlying type of <code>Byte</code>, <code>SByte</code>, <code>Int16</code>, <code>UInt16</code>, <code>Int32</code>, or <code>UInt32</code>.<ref>{{cite book |last1=Richter |first1=Jeffrey |title=CLR Via C# |url=https://archive.org/details/clrviac00rich_000 |url-access=limited |publisher=Microsoft Press |date=February 11, 2010 |pages=[https://archive.org/details/clrviac00rich_000/page/n200 183] |chapter=Chapter 7: Constants and Fields |isbn=978-0-7356-2704-8}}</ref> (This excludes value [[struct]]s, as well as the primitive types <code>Double</code>, <code>Int64</code>, <code>UInt64</code> and <code>Decimal</code>.)

Using the <code>volatile</code> keyword does not support fields that are [[Evaluation strategy#Call by reference|passed by reference]] or [[Closure (computer programming)|captured local variables]]; in these cases, <code>Thread.VolatileRead</code> and <code>Thread.VolatileWrite</code> must be used instead.<ref name="Albahari"/>

In effect, these methods disable some optimizations usually performed by the C# compiler, the JIT compiler, or the CPU itself. The guarantees provided by <code>Thread.VolatileRead</code> and <code>Thread.VolatileWrite</code> are a superset of the guarantees provided by the <code>volatile</code> keyword: instead of generating a "half fence" (ie an acquire-fence only prevents instruction reordering and caching that comes before it), <code>VolatileRead</code> and <code>VolatileWrite</code> generate a "full fence" which prevent instruction reordering and caching of that field in both directions.<ref name="Albahari"/> These methods work as follows:<ref>{{cite book |last1=Richter |first1=Jeffrey |title=CLR Via C# |url=https://archive.org/details/clrviac00rich_000 |url-access=limited |publisher=Microsoft Press |date=February 11, 2010 |pages=[https://archive.org/details/clrviac00rich_000/page/n814 797]–803 |chapter=Chapter 28: Primitive Thread Synchronization Constructs |isbn=978-0-7356-2704-8}}</ref>
*The <code>Thread.VolatileWrite</code> method 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 to <code>VolatileWrite</code> and any later program-order loads and stores must occur after the call.
*The <code>Thread.VolatileRead</code> method 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 to <code>VolatileRead</code> and any later program-order loads and stores must occur after the call.

The <code>Thread.VolatileRead</code> and <code>Thread.VolatileWrite</code> methods generate a full fence by calling the <code>Thread.MemoryBarrier</code> 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 <code>volatile</code> keyword that is solved by using a full fence generated by <code>Thread.MemoryBarrier</code> is as follows: due to the asymmetric nature of half fences, a <code>volatile</code> 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 <code>Thread.MemoryBarrier</code>.<ref name="Albahari"/>

==In Fortran==
<code>VOLATILE</code> is part of the [[Fortran#Fortran 2003|Fortran 2003]] standard,<ref>{{cite web|url=http://docs.cray.com/books/S-3692-51/html-S-3692-51/zfixedn3c8sk4c.html|title=VOLATILE Attribute and Statement|publisher=Cray|access-date=2016-04-22|archive-date=2018-01-23|archive-url=https://web.archive.org/web/20180123165050/http://docs.cray.com/books/S-3692-51/html-S-3692-51/zfixedn3c8sk4c.html|url-status=dead}}</ref> although earlier version supported it as an extension. Making all variables <code>volatile</code> in a function is also useful finding [[aliasing (computing)|aliasing]] related bugs.
<syntaxhighlight lang="fortran">
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
</syntaxhighlight>

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.<ref>{{cite web
|url=https://software.intel.com/en-us/forums/intel-moderncode-for-parallel-architectures/topic/279191
|title=Volatile and shared array in Fortran |website=Intel.com}}</ref>

Use of VOLATILE reduces and can even prevent optimization.<ref>{{cite web
|url=https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vnbq/index.html
|title=VOLATILE  |website=Oracle.com}}</ref>

==References==
{{Reflist|30em}}

==External links==
*[http://www.adaic.com/standards/05rm/html/RM-C-6.html Ada Reference Manual C.6: Shared Variable Control]
*[https://web.archive.org/web/20160304053622/https://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt Linux kernel: volatile-considered-harmful]

[[Category:C (programming language)]]
[[Category:Concurrency control]]
[[Category:Variable (computer science)]]