Clang note: explicitly cast the pointer to silence this warning (note_bad_memaccess_silence)

Text	error: explicitly cast the pointer to silence this warning
Type	Note
Category	Semantic Issue
Internal Id	note_bad_memaccess_silence
Internal Message	explicitly cast the pointer to silence this warning
Regular Expression	note\: explicitly cast the pointer to silence this warning
First Commit	2011-06-03 ac6872655bc6 Clean up the "non-POD memaccess" stuff some. This adds a properly named

Description

The note is issued by the Clang compiler when a warning is generated due to the use of a pointer in a context where the action may overwrite special class members, such as a vtable pointer, when using functions like memset, memcpy, memmove, or memcmp on objects of dynamic class types. This is particularly relevant for classes with virtual functions or virtual inheritance, as these features imply the presence of a vtable, which is crucial for the dynamic dispatch of virtual functions. Overwriting this memory can lead to undefined behavior, as it may disrupt the virtual function mechanism. The compiler suggests that an explicit cast of the pointer to void* should be performed to silence this warning. The explicit cast serves as an acknowledgment by the programmer that overwriting this memory, including the vtable pointer, is intentional and that the associated risks have been considered.

AI Generated

Example

In the following example, a struct named S is defined with an integer member x and a virtual function f. This structure introduces a vtable to manage the virtual function. The main function creates an instance of S and attempts to zero out its memory using memset. This action triggers a warning because memset overwrites the entire memory area of the instance, including the space for the vtable pointer. Overwriting the vtable space can lead to undefined behavior due to the corruption of the mechanism that manages virtual function calls. The compiler issues a note suggesting that the pointer be explicitly cast to void* to silence the warning. This suggestion indicates that the programmer is aware of the risks associated with overwriting the object's memory, including the vtable, and confirms that this overwriting is intentional.

AI Generated

Flags	-xc++	[Try out in Compiler Explorer]
Source	#include <string.h> struct S { int x; virtual void f() {} }; int main() { S s; // Virtual function introduces vtable memset(&s, 0, sizeof(s)); return 0; }
Compiler Output	<source>:7:10: warning: destination for this 'memset' call is a pointer to dynamic class 'S'; vtable pointer will be overwritten [-Wdynamic-class-memaccess] <source>:7:10: note: explicitly cast the pointer to silence this warning

Clang Internals (17.0.6)

Git Commit Message

Clean up the "non-POD memaccess" stuff some. This adds a properly named
diagnostic group to cover the cases where we have definitively bad
behavior: dynamic classes.

It also rips out the existing support for POD-based checking. This
didn't work well, and triggered too many false positives. I'm looking
into a possibly more principled way to warn on the fundamental buggy
construct here. POD-ness isn't the critical aspect anyways, so a clean
slate is better. This also removes some silliness from the code until
the new checks arrive.

llvm-svn: 132534

Used in Clang Sources

This section lists all occurrences of the diagnostic within the Clang's codebase. For each occurrence, an auto-extracted snipped from the source code is listed including key elements like control structures, functions, or classes. It should illustrate the conditions under which the diagnostic is activated.

clang/lib/Sema/SemaChecking.cpp (line 12603)

/// Check for dangerous or invalid arguments to memset().
///
/// This issues warnings on known problematic, dangerous or unspecified
/// arguments to the standard 'memset', 'memcpy', 'memmove', and 'memcmp'
/// function calls.
///
/// \param Call The call expression to diagnose.
void Sema::CheckMemaccessArguments(const CallExpr *Call, unsigned BId, IdentifierInfo *FnName) {
  // ...
  for (unsigned ArgIdx = 0; ArgIdx != LastArg; ++ArgIdx) {
    // ...
    DiagRuntimeBehavior(Dest->getExprLoc(), Dest, PDiag(diag::note_bad_memaccess_silence) << FixItHint::CreateInsertion(ArgRange.getBegin(), "(void*)"));

Triggered in Clang Tests

This section lists all internal Clang test cases that trigger the diagnostic.

clang/test/SemaObjC/warn-nontrivial-struct-memaccess.m

• clang/test/SemaObjC/warn-nontrivial-struct-memaccess.m:34:10: note: explicitly cast the pointer to silence this warning

• clang/test/SemaObjC/warn-nontrivial-struct-memaccess.m:36:9: note: explicitly cast the pointer to silence this warning

• clang/test/SemaObjC/warn-nontrivial-struct-memaccess.m:37:10: note: explicitly cast the pointer to silence this warning

• clang/test/SemaObjC/warn-nontrivial-struct-memaccess.m:38:11: note: explicitly cast the pointer to silence this warning