DCL13-A. Declare function parameters that are pointers to values not changed by the function as const

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

In C, function arguments are passed by value rather than by reference. While a function may change the values passed in, these changed values are discarded once the function returns. For this reason, many programmers assume a function will not change its arguments, and declaring the functions parameters as const is unnecessary.

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

Pointers behave in a similar fashion. A function may change a pointer to reference a different object, or NULL, yet that change is discarded once the function exits. Consequently, declaring a pointer as const is unnecessary.

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

Non-Compliant Code Example

Unlike passed-by-value arguments and pointers, pointed-to values are a concern. A function may modify a value referenced by a pointer argument, with the modification being retained after the function exits. Modification of the pointed-to value is not diagnosed by the compiler, which assumes this was the intended behavior.

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

Non-Compliant Code Example

In this non-compliant code example, the function parameter is const-qualified. Any attempt to modify the pointed-to value is diagnosed by the compiler.

void foo(int const * x) {
  if (x != NULL) {
    *x = 3; /* generates compiler error */
  }
  /* ... */
}

Compliant Solution

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

void foo(int const * x) {
  if (x != NULL) {
    printf("Value is %d\n", *x);
  }
  /* ... */
}

Non-Compliant Code Example

This non-compliant code example, defines a fictional version of the standard strcat() function called strcat_nc(). This function differs from strcat() in that the second argument is not const-qualified.

char *strcat_nc(char *s1, char *s2);

char *str1 = "str1";
char const *str2 = "str2";
char str3[] = "str3";
char const str4[] = "str4";

strcat_nc(str3, str2);	/* Compiler warns that str2 is const */
strcat_nc(str1, str3);  /* Attempts to overwrite string literal! */
strcat_nc(str4, str3);  /* Compiler warns that str4 is const */

The function would behave the same as strcat(), but the compiler generates warnings in incorrect locations, and fails to generate them in correct locations.

In the first strcat_nc() call, the compiler will generate a warning about attempting to cast away const on str2. This is a good warning, as strcat_nc() does not modify its second argument, yet fails to declare it const.

In the second strcat_nc() call, the compiler will happily compile the code with no warnings, but the resulting code will attempt to modify the "str1" literal, which may be impossible; the literal may not be defined in the heap. This violates STR05-A. Use pointers to const when referring to string literals.

In the final strcat_nc() call, the compiler generates a warning about attempting to cast away const on str4. This is a valid warning.

Compliant Solution

This compliant solution uses the prototype for the strcat() from C90. Although the restrict type qualifier did not exist in C90, const did. In general, function parameters should be declared in a manner consistent with the semantics of the function. In the case of strcat(), the initial argument can be changed by the function while the second argument cannot.

char *strcat(char *s1, char const *s2); 

char *str1 = "str1";
char const *str2 = "str2";
char str3[] = "str3";
char const str4[] = "str4";

strcat(str3, str2); 

/* Args reversed to prevent overwriting string literal */ 
strcat(str3, str1);  
strcat(str4, str3);  /* Compiler warns that str4 is const */

The const-qualification of the second argument s2 eliminates the spurious warning in the initial invocation, but maintains the valid warning on the final invocation in which a const-qualified object is passed as the first argument (which can change). Finally, the middle strcat() invocation is now valid, as str1 is a valid destination string, as the string exists on the stack and may be safely modified.

Risk Assessment

Not declaring an unchanging value const prohibits the function from working with values already cast as const. This problem can be sidestepped by type casting away the const, but doing so 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.

References

[[ISO/IEC 9899:1999]]
[[ISO/IEC PDTR 24772]] "CSJ Passing parameters and return values"

DCL12-A. Implement abstract data types using opaque types      02. Declarations and Initialization (DCL)       DCL14-A. Do not make assumptions about the order of global variable initialization across translation units