Page History

...

The C Standard, 6.5.7 paragraph 4 [ISO/IEC 9899:2011], states

...

In almost every case, an attempt to shift by a negative number of bits or by more bits than exist in the operand indicates a bug (logic error). These issues are covered by INT34-C. Do not shift a negative number of bits or more bits than exist in the operand.

Noncompliant Code Example

This noncompliant code example performs a left shift, after verifying that the number being shifted is not negative, and the number of bits to shift is valid.  The PRECISION() macro and popcount() function are explained in INT35-C. Use correct integer precisions. However, because this code does no overflow check, it can result in an unrepresentable value. 

...

Compliant Solution

This compliant solution eliminates the possibility of overflow resulting from a left-shift operation:

...

This compliant solution tests the operands to guarantee there is no possibility of signed overflow. It loads the value stored in the atomic integer and tests for possible overflow before performing the addition. However, this code contains a race condition where i can be modified after the load, but prior to the atomic store. This solution is only compliant if i is guaranteed to only be access by a single thread. See CON43CON08-C. Do not assume that a group of calls to independently atomic methods is atomic for  for more information.

#include <limits.h>
#include <stdatomic.h>
 
atomic_int i;

void func(int si_a) {
  int si_b = atomic_load(&i);

  if (((si_b > 0) && (si_a > (INT_MAX - si_b))) ||
      ((si_b < 0) && (si_a < (INT_MIN - si_b)))) {
    /* Handle error */
  } else {
    atomic_store(&i, si_a + si_b);
  }

  /* ... */
}

...