Synopsis

Expressions that depend on order of evaluation were found.

Enabled by default

Yes

Severity/Certainty

Medium/High

Full description

One and the same variable is changed in different parts of an expression with an unspecified evaluation order, between two consecutive sequence points. Standard C does not specify an evaluation order for different parts of an expression. For this reason different compilers are free to perform their own optimizations regarding the evaluation order. Projects containing statements that violate this check are not easily ported to another architecture or compiler, and if they are they might be difficult to debug. Only four operators have a guaranteed order of evaluation: logical AND (a && b) evaluates the left operand, then the right operand only if the left is found to be true; logical OR (a || b) evaluates the left operand, then the right operand only if the left is found to be false; a ternary conditional (a ? b : c) evaluates the first operand, then either the second or the third, depending on whether the first is found to be true or false; and a comma (a , b) evaluates its left operand before its right. This check is identical to MISRAC++2008-5-0-1_a, MISRAC2004-12.2_a, MISRAC2012-Rule-1.3_i, MISRAC2012-Rule-13.2_a, CERT-EXP30-C_a.

Coding standards

Code examples

The following code example fails the check and will give a warning:

int main(void) {
  int i = 0;
  i = i * i++;  //unspecified order of operations
  return 0;
}

The following code example passes the check and will not give a warning about this issue:

int main(void) {
  int i = 0;
  int x = i;  
  i++;
  x = x * i;  //OK - statement is broken up  
  return 0;
}