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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, many programmers assume a function will not change its arguments, and declaring the function's parameters as const is unnecessary.

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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.

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Unlike passed-by-value arguments and pointers, pointed-to values are a concern. A function may modify a value referenced by a pointer argument, leading to a side effect which that persists even after the function exits. Modification of the pointed-to value is not diagnosed by the compiler, which assumes this was the intended behavior.

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

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In the second strcat_nc() call, the compiler compiles the code with no warnings, but the resulting code will attempt to modify the "str1" literal. This violates recommendation STR05-C. Use pointers to const when referring to string literals and rule STR30-C. Do not attempt to modify string literals.

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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 , but the second argument cannot.

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

char *str1 = "str1";
const char *str2 = "str2";
char str3[9] = "str3";
const char str4[9] = "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 str3 is a valid destination string and may be safely modified.

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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 recommendation EXP05-C. Do not cast away a const qualification.

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Related Vulnerabilities

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

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CERT C++ Secure Coding Standard: DCL13-CPP. Declare function parameters that are pointers to values not changed by the function as const

ISO/IEC 9899:19992011

ISO/IEC TR 24772 "CSJ Passing parameters and return values"

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