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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 6461 in Appendix J of the C Standard.

As an illustration, the C Standard [ISO/IEC 9899:2011] provides a footnote (Section subclause 6.7.3, para. paragraph 4):

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.

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

Exceptions

EXP05-C-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 elsewhereelsewhere—cannot - 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-C-EX2: A number of C 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 EXP40-C. Do not modify constant valuesobjects and would lead to undefined behavior. These functions are the following:

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');
  /* modifyingModifying *s is undefined */

Similarly, in the following example, 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);
  /* modifyingModifying **end is undefined */

EXP05-C-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.

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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

Related Vulnerabilities

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

Related Guidelines

Bibliography

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