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In C, function arguments are passed by value rather than by reference. Although a function may change the values passed in, these changed values are discarded once the function returns. For this reason, many programmers assume a function will not change its arguments , and that declaring the function's parameters as const is unnecessary.

void foo(int x) {
  x = 3; /* Visible only in the function. */
  /* ... */
}

Pointers behave in a similar fashion. A function may change a pointer to reference a different object, or NULL, yet that change is discarded once the function exits. Consequently, declaring a pointer as const is unnecessary.

void foo(int *x) {
  x = NULL; /* Visible only in the function. */
  /* ... */
}

Noncompliant Code Example

Unlike passed-by-value arguments and pointers, pointed-to values are a concern. A function may modify a value referenced by a pointer argument, leading to a side effect that persists even after the function exits. Modification of the pointed-to value is not diagnosed by the compiler, which assumes this behavior was the intended behavior.

void foo(int *x) {
  if (x != NULL) {
    *x = 3; /* Visible outside function. */
  }
  /* ... */
}

If the function parameter is const-qualified, any attempt to modify the pointed-to value should cause the compiler to issue a diagnostic message.

void foo(const int *x) {
  if (x != NULL) {
    *x = 3; /* Compiler should generate diagnostic message. */
  }
  /* ... */
}

As a result, the const violation must be resolved before the code can be compiled without a diagnostic message being issued.

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In the first strcat_nc() call, the compiler generates a warning about attempting to cast away const on c_str2 because strcat_nc() does not modify its second argument yet fails to declare it const.

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In the final strcat_nc() call, the compiler generates a warning about attempting to cast away const on c_str4, which is a valid warning.

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The const-qualification of the second argument, s2, eliminates the spurious warning in the initial invocation but maintains the valid warning on the final invocation in which a const-qualified object is passed as the first argument (which can change). Finally, the middle strcat() invocation is now valid , as because c_str3 is a valid destination string and may be safely modified.

Risk Assessment

Not declaring Failing to declare an unchanging value const prohibits the function from working with values already cast as const. This problem can be sidestepped by type casting away the const, but doing so violates EXP05-C. Do not cast away a const qualification.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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