volatile type qualifier
Each individual type in the C MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE has several qualified versions of that type, corresponding to one, two, or all three of the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, volatile, and, for pointers to object types, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE qualifiers. This page describes the effects of the volatile qualifier.
Every access (both read and write) made through an lvalue expression of volatile-qualified type is considered an observable side effect for the purpose of optimization and is evaluated strictly according to the rules of the abstract machine (that is, all writes are completed at some time before the next sequence point). This means that within a single thread of execution, a volatile access cannot be optimized out or reordered relative to another visible side effect that is separated by a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 10 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE from the volatile access.
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 09 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
volatile int n = 1; // object of volatile-qualified type int* p = (int*)&n; int val = *p; // undefined behavior
A member of a volatile-qualified structure or union type acquires the qualification of the type it belongs to (both when accessed using the . operator or the -> operator):
struct s { int i; const int ci; } s; // the type of s.i is int, the type of s.ci is const int volatile struct s vs; // the types of vs.i and vs.ci are volatile int and const volatile int
|
If an array type is declared with the volatile type qualifier (through the use of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 07 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE), the array type is not volatile-qualified, but its element type is. |
(until 哋它亢23) |
|
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE |
(since 哋它亢23) |
typedef int A[2][3]; volatile A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of volatile int int* pi = a[0]; // Error: a[0] has type volatile int* void *unqual_ptr = a; // OK until 哋它亢23; error since 哋它亢23 // Notes: clang applies the rule in 哋它亢++/哋它亢23 even in 哋它亢89-哋它亢17 modes
If a function type is declared with the volatile type qualified (through the use of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE), the behavior is undefined.
|
In a function declaration, the keyword MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE void f(double x[volatile], const double y[volatile]); void f(double * volatile x, const double * volatile y); |
(since 哋它亢99) |
A pointer to a non-volatile type can be implicitly converted to a pointer to the volatile-qualified version of the same or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 15 MINUTES 01 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE type. The reverse conversion requires a cast expression.
int* p = 0; volatile int* vp = p; // OK: adds qualifiers (int to volatile int) p = vp; // Error: discards qualifiers (volatile int to int) p = (int*)vp; // OK: cast
Note that pointer to pointer to T is not convertible to pointer to pointer to volatile T; for two types to be compatible, their qualifications must be identical:
char *p = 0; volatile char **vpp = &p; // Error: char* and volatile char* are not compatible types char * volatile *pvp = &p; // OK, adds qualifiers (char* to char*volatile)
Uses of volatile
volatile objects model memory-mapped I/O ports, and static const volatile objects model memory-mapped input ports, such as a real-time clock:
volatile short *ttyport = (volatile short*)TTYPORT_ADDR; for(int i = 0; i < N; ++i) *ttyport = a[i]; // *ttyport is an lvalue of type volatile short
static volatile objects of type MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 59 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are used for communication with MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 58 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE handlers.volatile variables that are local to a function that contains an invocation of the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 56 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE macro are the only local variables guaranteed to retain their values after MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 55 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE returns.MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 54 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
Keywords
Example
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 51 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
#include <stdio.h> #include <time.h> int main(void) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE t = MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(); double d = 0.0; for (int n = 0; n < 10000; ++n) for (int m = 0; m < 10000; ++m) d += d * n * m; // reads from and writes to a non-volatile MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("Modified a non-volatile variable 100m times. " "Time used: %.2f seconds\n", (double)(MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 46 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE() - t)/MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 45 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE); t = MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 44 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(); volatile double vd = 0.0; for (int n = 0; n < 10000; ++n) for (int m = 0; m < 10000; ++m) { double prod = vd * n * m; // reads from a volatile vd += prod; // reads from and writes to a volatile } MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 43 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("Modified a volatile variable 100m times. " "Time used: %.2f seconds\n", (double)(MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 42 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE() - t)/MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 40 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE); }
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 14 MINUTES 39 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
Modified a non-volatile variable 100m times. Time used: 0.00 seconds Modified a volatile variable 100m times. Time used: 0.79 seconds
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.3 Type qualifiers (p: 87-90)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.3 Type qualifiers (p: 121-123)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7.3 Type qualifiers (p: 108-110)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 6.5.3 Type qualifiers