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

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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 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 */
  }
  /* ... */
}

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Noncompliant Code Example

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

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

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

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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-AC. 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, const char const *s2); 

char *str1 = "str1";
const char const *str2 = "str2";
char str3[] = "str3";
const 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-AC. Do not cast away a const qualification.

Related Vulnerabilities

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

References

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

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      02. Declarations and Initialization (DCL)       DCL14-AC. Do not make assumptions about the order of global variable initialization across translation units