Some operators do not evaluate their operands beyond the type information the operands provide. When using one of these operators, do not pass an operand which that would otherwise yield a side effect , as since the side effect will not be generated.
The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. In most cases, the operand is not evaluated. A possible exception is when the type of the operand is a variable length array type (VLA); the expression is evaluated. When part of the operand of the sizeof operator is a VLA type and when changing the value of the VLA's size expression would not affect the result of the operator, it is unspecified whether or not the size expression is evaluated (see unspecified behavior 22).
...
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <stdio.h>
void func(void) {
int val = 0;
/* ... */
++val;
size_t align = _Alignof(int[val]);
printf("%zu, %d\n, align, val);
/* ... */
} |
Exceptions
EXP44-C-EX1: Reading a volatile-qualified value is a side-effecting operation. However, accessing a value through a volatile-qualified type does not guarantee side effects will happen on the read of the value unless the underlying object is also volatile-qualified. Idiomatic reads of a volatile-qualified object are permissible as an operand to a sizeof(), _Alignof(), or _Generic expression, as in the following example:
...