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Declaring function arguments parameters const indicates that the function promises not to change these values.

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

void foo(int x) {
  x = 3; /* only lasts until end of function */
  /* ... */
}

...

void foo(int *x) {
  x = NULL; /* only lasts until end of function */
  /* ... */
}

Non-Compliant 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, with the modification being retained after the function exits. Modification of the pointed-to value is not diagnosed by the compiler, which assumes this was the intended behavior.

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

Non-Compliant Code Example

If a function modifies a In this non-compliant code example, the function parameter is const-qualified. Any attempt to modify the pointed-to value , declaring this value as const will be caught is diagnosed by the compiler.

void foo(int const * x) {
  if (x != NULL) {
    *x = 3; /* generates compiler error */
  }
  /* ... */
}

Compliant Solution

If a function does not modify the pointed-to value, it should declare this value as const. This improves code readability and consistency.

void foo(int const * x) {
  if (x != NULL) {
    printf("Value is %d\n", *x);
  }
  /* ... */
}

Non-Compliant Code Example

This non-compliant code example, defines a fictional version of the standard strcat() function called strcat_nc(). This function differs from strcat() in that the second argument is not const-qualified.

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In the final strcat_nc() call, the compiler generates a warning about attempting to cast away const on str4. This is a valid warning.

Compliant Solution

This compliant solution uses the prototype for the strcat() from C90. Although the restrict type qualifier did not exist in C90, const did. In general, the arguments function parameters should be declared in a manner consistent with the semantics of the function. In the case of strcat(), the initial argument can be changed by the function while the second argument cannot.

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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 str1 is a valid destination string, as the string exists on the stack and may be safely modified.

Risk Assessment

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

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] 
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "CSJ Passing parameters and return values"

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