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Do not cast away a const qualification on an object of pointer type. Casting away the const qualification allows a program to modify the object referred to by the pointer, which may result in undefined behavior. See undefined behavior 61 of Appendix J.

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 recommendation 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 */
/* ... */

EXP05-EX2: A number of C99 standard library functions are specified to return non-const pointers that refer to their const-qualified arguments. When the actual arguments to such functions reference const objects, attempting to use the returned non-const pointers to modify the const objects would be a violation of rule EXP40-C. Do not modify constant values and lead to undefined behavior. These functions are the following:

`memchr`	`strchr`	`strpbrk`	`strrchr`
`strstr`	`strtod`	`strtof`	`strtold`
`strtol`	`strtoll`	`strtoul`	`strtoull`
`wmemchr`	`wcschr`	`wcspbrk`	`wcsrchr`
`wcsstr`

For instance, in following example the function strchr returns an unqualified char* that points to the terminating null character of the constant character array s (which could be stored in ROM). Even though the pointer is not const, attempting to modify the character it points to would lead to undefined behavior.

  extern const char s[];
  char* where;
  where = strchr(s, '\0');
  /* modifying *s is undefined */

Similarly, in the example below, the function strtol sets the unqualified char* pointer referenced by end to point just past the last successfully parsed character of the constant character array s (which could be stored in ROM). Even though the pointer is not const, attempting to modify the character it points to would lead to undefined behavior.

  extern const char s[];
  long x;
  char* end;
  x = strtol(s, &end, 0);
  /* modifying **end is undefined */

EXP05-EX3: Because "const" means read-only and not constant, it is sometimes useful to declare struct members as (pointer to) const objects to obtain diagnostics when the user tries to change them in some way other than via the functions that are specifically designed to maintain that data type. Within those functions, however, it may be necessary to strip off the const qualification to update those members.

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.