EXP05-CPP. Do not cast away a const qualification

Do not cast away a const qualification on a variable type. Casting away the const qualification allows a program to modify a constant value, which results in undefined behavior.

As an illustration, C99 provides a footnote:

The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

Noncompliant Code Example

The remove_spaces() function in this noncompliant code example accepts a pointer to a string str and a string length slen and removes the space character from the string by shifting the remaining characters toward the front of the string. The function remove_spaces() is passed a const char pointer as an argument. The const qualification is cast away and then the contents of the string are modified.

void remove_spaces(const char *str, size_t slen) {
  char *p = (char *)str;
  size_t i;
  for (i = 0; i < slen && str[i]; i++) {
    if (str[i] != ' ') *p++ = str[i];
  }
  *p = '\0';
}

Compliant Solution

In this compliant solution, the function remove_spaces() is passed a non-const char pointer. The calling function must ensure that the null-terminated byte string passed to the function is not const by making a copy of the string or by other means.

void remove_spaces(char *str, size_t slen) {
  char *p = str;
  size_t i;
  for (i = 0; i < slen && str[i]; i++) {
    if (str[i] != ' ') *p++ = str[i];
  }
  *p = '\0';
}

Noncompliant Code Example

In this noncompliant code example, the contents of the const int array vals are cleared by the call to memset().

const int vals[3] = {3, 4, 5};
memset(vals, 0, sizeof(vals));

Because the memset() function takes a (non-const) pointer to void, the compiler must implicitly cast away const.

Implementation Details

The compiler GCC issues a warning when an implicit cast is performed.

Compliant Solution

If the intention is to allow the array values to be modified, do not declare the array as const.

int vals[3] = {3, 4, 5};
memset(vals, 0, sizeof(vals));

Otherwise, do not attempt to modify the contents of the array.

Exceptions

EXP05-EX1: An exception to this rule is allowed when it is necessary to cast away const when invoking a legacy API that does not accept a const argument, provided the function does not attempt to modify the referenced variable. For example, the following code casts away the const qualification of INVFNAME in the call to the audit_log() function.

/* Legacy function defined elsewhere - cannot be modified */
void audit_log(char *errstr) {
  fprintf(stderr, "Error: %s.\n", errstr);
}

/* ... */
const char INVFNAME[]  = "Invalid file name.";
audit_log((char *)INVFNAME); /* EXP05-EX1 */
/* ... */

Risk Assessment

If the object is constant, the compiler may allocate storage in ROM or write-protected memory. Attempting to modify such an object may lead to a program crash or denial-of-service attack.

Automated Detection

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

GCC Compiler can detect violations of this rule when the -Wcast-qual flag is used.

Compass/ROSE can detect violations of this recommendation.

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as EXP05-C. Do not cast away a const qualification.

References

[[ISO/IEC 9899:1999]] Section 6.7.3, "Type qualifiers"
[[ISO/IEC PDTR 24772]] "HFC Pointer casting and pointer type changes" and "IHN Type system"
[[MISRA 04]] Rule 11.5

EXP04-CPP. Do not perform byte-by-byte comparisons between classes or structs      03. Expressions (EXP)       EXP06-CPP. Operands to the sizeof operator should not contain side effects