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CWE Rule 481

Assigning instead of Comparing

Since R2023a

Description

Rule Description

The code uses an operator for assignment when the intention was to perform a comparison.

Polyspace Implementation

The rule checker checks for Invalid use of = (assignment) operator.

Examples

expand all

Issue

This issue occurs when an assignment is made inside the predicate of a conditional, such as if or while.

In C and C++, a single equal sign is an assignment not a comparison. Using a single equal sign in a conditional statement can indicate a typo or a mistake.

Risk
  • Conditional statement tests the wrong values— The single equal sign operation assigns the value of the right operand to the left operand. Then, because this assignment is inside the predicate of a conditional, the program checks whether the new value of the left operand is nonzero or not NULL.

  • Maintenance and readability issues — Even if the assignment is intended, someone reading or updating the code can misinterpret the assignment as an equality comparison instead of an assignment.

Fix
Example — Single Equal Sign Inside an if Condition
#include <stdio.h>

void bad_equals_ex(int alpha, int beta)
{
    if(alpha = beta)  //Noncompliant
    {
        printf("Equal\n");
    }
}

The equal sign is flagged as a defect because the assignment operator is used within the predicate of the if-statement. The predicate assigns the value beta to alpha, then implicitly tests whether alpha is true or false.

Correction — Change Expression to Comparison

One possible correction is adding an additional equal sign. This correction changes the assignment to a comparison. The if condition compares whether alpha and beta are equal.

#include <stdio.h>

void equality_test(int alpha, int beta)
{
    if(alpha == beta)
    {
        printf("Equal\n");
    }
}
Correction — Assignment and Comparison Inside the if Condition

If an assignment must be made inside the predicate, a possible correction is adding an explicit comparison. This correction assigns the value of beta to alpha, then explicitly checks whether alpha is nonzero. The code is clearer.

#include <stdio.h>

int assignment_not_zero(int alpha, int beta)
{
    if((alpha = beta) != 0)
    {
        return alpha;
    }
    else
    {
        return 0;
    }
}
Correction — Move Assignment Outside the if Statement

If the assignment can be made outside the control statement, one possible correction is to separate the assignment and comparison. This correction assigns the value of beta to alpha before the if. Inside the if-condition, only alpha is given to test if alpha is nonzero or not NULL.

#include <stdio.h>

void assign_and_print(int alpha, int beta)
{
    alpha = beta;
    if(alpha)
    {
        printf("%d", alpha);
    }
}

Check Information

Category: Others

Version History

Introduced in R2023a