Fixed #2930 (new check: redundant assignment of pointer function parameter)

lib/checkautovariables.cpp

@@ -22,6 +22,7 @@
 
 #include "checkautovariables.h"
 #include "symboldatabase.h"
+#include "checknullpointer.h"
 
 #include <list>
 #include <string>
@@ -79,6 +80,16 @@ static bool checkRvalueExpression(const Variable* var, const Token* next)
     return((next->str() != "." || (!var->isPointer() && (!var->isClass() || var->type()))) && next->strAt(2) != ".");
 }
 
+static bool pointerIsDereferencedInScope(const Variable *var, const Scope *scope)
+{
+    bool unknown = false;
+    for (const Token *tok = scope->classStart; tok && tok != scope->classEnd; tok = tok->next()) {
+        if (tok->varId() == var->varId() && CheckNullPointer::isPointerDeRef(tok, unknown))
+            return true;
+    }
+    return false;
+}
+
 void CheckAutoVariables::autoVariables()
 {
     const SymbolDatabase *symbolDatabase = _tokenizer->getSymbolDatabase();
@@ -96,6 +107,11 @@ void CheckAutoVariables::autoVariables()
                 const Variable * var = tok->tokAt(5)->variable();
                 if (checkRvalueExpression(var, tok->tokAt(6)))
                     errorAutoVariableAssignment(tok->next(), false);
+            } else if (Token::Match(tok, "[;{}] %var% =") &&
+                       isPtrArg(tok->next()) &&
+                       Token::Match(tok->next()->variable()->typeStartToken(), "struct| %type% * %var% [,)]") &&
+                       !pointerIsDereferencedInScope(tok->next()->variable(), scope)) {
+                errorUselessAssignmentPtrArg(tok->next());
             } else if (Token::Match(tok, "[;{}] %var% . %var% = & %var%")) {
                 // TODO: check if the parameter is only changed temporarily (#2969)
                 if (_settings->inconclusive) {
@@ -218,6 +234,14 @@ void CheckAutoVariables::errorReturnAddressOfFunctionParameter(const Token *tok,
                 "value is invalid.");
 }
 
+void CheckAutoVariables::errorUselessAssignmentPtrArg(const Token *tok)
+{
+    reportError(tok,
+                Severity::warning,
+                "uselessAssignmentPtrArg",
+                "Assignment of function parameter has no effect outside the function.");
+}
+
 //---------------------------------------------------------------------------
 
 // return temporary?

lib/checkautovariables.h

@@ -82,6 +82,7 @@ private:
     void errorReturnTempReference(const Token *tok);
     void errorInvalidDeallocation(const Token *tok);
     void errorReturnAddressOfFunctionParameter(const Token *tok, const std::string &varname);
+    void errorUselessAssignmentPtrArg(const Token *tok);
 
     void getErrorMessages(ErrorLogger *errorLogger, const Settings *settings) const {
         CheckAutoVariables c(0,settings,errorLogger);
@@ -92,6 +93,7 @@ private:
         c.errorReturnTempReference(0);
         c.errorInvalidDeallocation(0);
         c.errorReturnAddressOfFunctionParameter(0, "parameter");
+        c.errorUselessAssignmentPtrArg(0);
     }
 
     static std::string myName() {
@@ -104,7 +106,8 @@ private:
                "* returning a pointer to auto or temporary variable\n"
                "* assigning address of an variable to an effective parameter of a function\n"
                "* returning reference to local/temporary variable\n"
-               "* returning address of function parameter\n";
+               "* returning address of function parameter\n"
+               "* useless assignment of pointer parameter\n";
     }
 };
 /// @}

test/testautovariables.cpp

@@ -66,6 +66,7 @@ private:
         TEST_CASE(testautovar7); // ticket #3066
         TEST_CASE(testautovar8);
         TEST_CASE(testautovar9);
+        TEST_CASE(testautovar10); // ticket #2930 - void f(char *p) { p = '\0'; }
         TEST_CASE(testautovar_array1);
         TEST_CASE(testautovar_array2);
         TEST_CASE(testautovar_return1);
@@ -239,6 +240,19 @@ private:
         ASSERT_EQUALS("[test.cpp:6]: (error) Address of local auto-variable assigned to a function parameter.\n", errout.str());
     }
 
+    void testautovar10() { // #2930 - assignment of function parameter
+        check("void foo(char* p) {\n"
+              "    p = 0;\n"
+              "}");
+        ASSERT_EQUALS("[test.cpp:2]: (warning) Assignment of function parameter has no effect outside the function.\n", errout.str());
+
+        check("void foo(char* p) {\n"
+              "    if (!p) p = buf;\n"
+              "    *p = 0;\n"
+              "}");
+        ASSERT_EQUALS("", errout.str());
+    }
+
     void testautovar_array1() {
         check("void func1(int* arr[2])\n"
               "{\n"