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The C language provides several different kinds of constants: integer constants, such as 10 and 0x1C; floating constants, such as 1.0 and 6.022e+23; and character constants, such as 'a' and '\x10'. C also provides string literals, such as "hello, world" and "\n". These constants can all be referred to as literals.

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Objects that are const-qualified have scope and can be type-checked by the compiler. Because these they are named objects (unlike macro definitions), some debugging tools can show the name of the object. The object also consumes memory.

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• size of a bit-field member of a structure.
• size of an array (except in the case of variable length arrays).
• value of an enumeration constant.
• value of a case constant.

If any of these are required, then an integer constant (which would be an rvalue) must be used.

const-qualified objects allow the programmer to take the address of the object.:

const int max = 15;
int a[max]; /* invalidInvalid declaration outside of a function */
const int *p;

/* aA const-qualified object can have its address taken */
p = &max;

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enum { max = 15 };
int a[max]; /* OK outside function */
const int *p;

p = &max; /* errorError: '&' on enum constant */

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Noncompliant Code Example

The meaning of the integer literal 18 is not clear in this example.:

/* ... */
if (age >= 18) {
   /* Take action */
}
else {
  /* Take a different action */
}
/* ... */

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This compliant solution replaces the integer literal 18 with the symbolic constant ADULT_AGE to clarify the meaning of the code.:

enum { ADULT_AGE=18 };
/* ... */
if (age >= ADULT_AGE) {
   /* Take action */
}
else {
  /* Take a different action */
}
/* ... */

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Integer literals are frequently used when referring to array dimensions, as shown in this noncompliant code example.:

char buffer[256];
/* ... */
fgets(buffer, 256, stdin);

This use of integer literals can easily result in buffer overflows , if, for example, the buffer size is reduced but the integer literal used in the call to fgets() is not.

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In this noncompliant code example, the string literal "localhost" and integer constant 1234 are embedded directly in program logic and are consequently difficult to change.:

LDAP *ld = ldap_init("localhost", 1234);
if (ld == NULL) {
  perror("ldap_init");
  return(1);
}

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In this compliant solution, the host name and port number are both defined as object-like macros, so they can be passed as compile-time arguments.:

#ifndef PORTNUMBER     /* mightMight be passed on compile line */
#  define PORTNUMBER 1234
#endif

#ifndef HOSTNAME        /* mightMight be passed on compile line */
#  define HOSTNAME "localhost"
#endif

/* ... */

LDAP *ld = ldap_init(HOSTNAME, PORTNUMBER);
if (ld == NULL) {
  perror("ldap_init");
  return(1);
}

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Related Vulnerabilities

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

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