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Declaring function arguments 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  Although a function may change the values passed in, these changed values are discarded once the function returns. For this reason, most many programmers assume a function will not change its arguments , and declaring them that declaring the function's parameters as const is unnecessary.

void foo(int x) {
  x = 3; /* Visible only lastsin until endthe 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; /* Visible only lasts until end ofin the function */
  /* ... */
}

...

Noncompliant 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 leading to a side effect that persists even after the function exits. Modification of the pointed-to value is not diagnosed by the compiler, which assumes this behavior was intended.

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

Non-Compliant Code Example

If a function modifies a If the function parameter is const-qualified, any attempt to modify the pointed-to value , declaring this value as const will be caught by the compilershould cause the compiler to issue a diagnostic message.

void foo(const int * x) {
  if (x != NULL) {
    *x = 3; /* Compiler should generatesgenerate compilerdiagnostic warningmessage */
  }
  /* ... */
}

As a result, the const violation must be resolved before the code can be compiled without a diagnostic message being issued.

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.This compliant solution addresses the const violation by not modifying the constant argument:

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

...

Noncompliant Code Example

This non-compliant noncompliant 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 *c_str1 = "c_str1";
const char *c_str2 = "c_str2";
char c_str3[9] = "c_str3";
const char c_str4[9] = "c_str4";

strcat_nc(c_str3, c_str2);  /* Compiler warns that c_str2 is const */
strcat_nc(c_str1, c_str3);  /* Attempts to overwrite string literal! */
strcat_nc(c_str4, c_str3);  /* Compiler warns that c_str4 is const */

The function would behave behaves 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 generates a warning about attempting to cast away const on str2. This is a good warning, as c_str2 because 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 compiles the code with no warnings, but the resulting code will attempt to modify the "c_str1" literal, which may be impossible; the literal may not be defined in the heap. This violates STR05-AC. Use pointers to const char when referencing when referring to string literals and STR30-C. Do not attempt to modify string literals.

In the final strcat_nc() call, the compiler generates a warning about ateempting attempting to cast away const on c_str4. This , which 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, the arguments 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 , but the second argument cannot.

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

char *c_str1 = "c_str1";
const char *c_str2 = "c_str2";
char c_str3[9] = "c_str3";
const char c_str4[9] = "c_str4";

strcat(c_str3, c_str2); 

/* Args reversed to prevent overwriting string literal */ 
strcat(str1c_str3, str3c_str1);  /* Note: reversed args */
strcat(c_str4, c_str3);  /* different 'const' qualifiers Compiler warns that c_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 because c_str3 is a valid destination string, as the string exists on the stack and may be safely modified.

Risk Assessment

Not declaring Failing to declare an unchanging value const prohibits the function from working with values already cast as const. One could sidestep this problem by typecasting This problem can be sidestepped by type casting away the const, but that doing so violates EXP05-AC. Do not cast away a const qualification.

Automated Detection

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] 
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "CSJ Passing parameters and return values"

Related Guidelines

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