Sizeof with side effects¶

ID: cpp/sizeof-side-effect
Kind: problem
Security severity: 
Severity: recommendation
Precision: high
Tags:
   - reliability
   - correctness
   - external/jsf
Query suites:
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

This rule finds sizeof expressions that have a parameter with side-effects. sizeof only uses the type of the parameter, so the parameter will not be evaluated. In the C99 standard, using sizeof on expressions with dynamic arrays may or may not evaluate the side-effect, so it is better to avoid it completely.

int f(void){
	int i = 0;
	char arr[20];
	int size = sizeof(arr[i++]); //wrong: sizeof expression has side effect
	cout << i; //would output 0 instead of 1
}