EXP10-A. Function arguments that are pointers to values not changed by the function should be declared const

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

In C, function arguments are passed by value rather than by reference. Thus, while a function may change the values passed in, these changed values are forgotten once the function exits. For this reason, most programmers assume a function will not change its arguments, and declaring them const is unnecessary.

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

Pointers themselves are similar. A function may change a pointer to point to a different object, or NULL, yet that change will be forgotten once the function exits. Thus, declaring a pointer as const is unnecessary.

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

But pointed-to values are another matter. A function may modify a value referenced by a pointer argument, with the modification being retained after the function exits.

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

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

Risk Assessment

Not declaring an unchanging value const prohibits the function from working with values already cast as const. One could sidestep this problem by typecasting away the const, but that 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.

EXP09-A. Use sizeof to determine the size of a type or variable      03. Expressions (EXP)       EXP30-C. Do not depend on order of evaluation between sequence points