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Do not use the assignment operator in the outermost expression of a selection statement (if or switch) or an iteration statement (while, do, or for) because this because it typically indicates programmer error and can result in unexpected behavior.

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if (a = b) {
  /* ... */
}

While Although the intent of the code may be to assign b to a and test the value of the result for equality to zero, it is very frequently a case of the programmer mistakenly using the assignment operator = instead of the equals operator ==. Consequently, many compilers will warn about this condition, making this coding error detectable by adhering to recommendation MSC00-C. Compile cleanly at high warning levels.

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if (a == b) {
  /* ... */
}

When the assignment is , if fact, intended, this the following is an alternative compliant solution:

if ((a = b) != 0) {
  /* ... */
}

It is less desirable in general, depending on what was intended, because it mixes the assignment in the condition, but it is clear that the programmer intended the assignment to occur.

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

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ISO/IEC TR 24772 "KOA Likely Incorrect Expressionsincorrect expressions"

MITRE CWE: CWE-480, "Use of Incorrect Operatorincorrect operator"

Bibliography

[Hatton 1995] Section 2.7.2, "Errors of omission and addition"

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