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An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Referencing a volatile object by using a nonvolatile value non-volatile lvalue results in undefined behavior. The C Standard, subclause 6.7.3 [ISO/IEC 9899:2011], states:

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In this noncompliant code example, a volatile object is accessed through a non-volatile-qualified reference, resulting in undefined behavior:

#include <stdio.h>
 
void func(void) {
  static volatile int **ipp;
  static int *ip;
  static volatile int i = 0;

  printf("i = %d.\n", i);

  ipp = &ip; /* ProducesMay warningsproduce ina modernwarning compilersdiagnostic */
  ipp = (int**) &ip; /* Constraint violation; also produces warningsmay produce a warning diagnostic */
  *ipp = &i; /* Valid */
  if (*ip != 0) { /* Valid */
    /* ... */
  }
}

The assignment ipp = &ip is not safe because it allows the valid code that follows to reference the value of the volatile object i through the non-volatile-qualified reference ip. In this example, the compiler may optimize out the entire if block because i *ip != 0 must be false if i the object to which ip points is not volatile.

Implementation Details

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In this compliant solution, ip is declared volatile:

#include <stdio.h>

void func(void) {
  static volatile int **ipp;
  static volatile int *ip;
  static volatile int i = 0;

  printf("i = %d.\n", i);

  ipp = &ip;
  *ipp = &i;
  if (*ip != 0) {
    /* ... */
  }

}

Risk Assessment

Casting away volatile allows access to Accessing an object with a volatile-qualified type through a nonvolatile reference and can result in undefined and perhaps unintended program reference with a non-volatile-qualified type is undefined behavior.

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Bibliography

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