Why does memcpy fail copying to a local array member of a simple object?

Question

Classic memcpy gotcha with C arrays as function arguments. As pointed out below, I have an error in my code but the erroneous code worked in a local context!

I just encountered this weird behaviour in a porting job, where I'm emulating the Macintosh Picture opcode playback using objects. My DrawString object was drawing garbage on playback because it apparently failed to copy the string argument. The following is a test case I wrote - note how a manual copying loop works but memcpy fails. Tracing in the Visual Studio debugger shows the memcpy ovewrites the destination with garbage.

Memcpy on two local Str255 arrays works fine.

When one of them is a member in an object on the stack, it fails (in other testing it also fails when the object is on the heap).

The following sample code shows the memcpy being invoked in an operator=. I moved it there after it failed in a constructor but there was no difference.

typedef unsigned char Str255[257];

// snippet that works fine with two local vars
Str255 Blah("\004Blah");
Str255 dest;
memcpy(&dest, &Blah, sizeof(Str255));  // THIS WORKS - WHY HERE AND NOT IN THE OBJECT?

/*!
class to help test  CanCopyStr255AsMember
*/
class HasMemberStr255  {
public:
    HasMemberStr255()
    {
        mStr255[0] = 0;
    }

    HasMemberStr255(const Str255 s)
    {
        for (int i = 0; i<257; ++i)
        {
            mStr255[i] = s[i];
            if (s[i]==0)
                return;
        }
    }

    /// fails
    void operator=(const Str255 s)  {
        memcpy(&mStr255, &s, sizeof(Str255));
    };
    operator const Str255&() { return mStr255; }

private:
    Str255 mStr255;
};
-

/*!
Test trivial copying technique to duplicate a string
Added this variant using an object because of an apparent Visual C++ bug.
*/
void TestMacTypes::CanCopyStr255AsMember()
{
    Str255 initBlah("\004Blah");
    HasMemberStr255 blahObj(initBlah);
// using the operator= which does a memcpy fails   blahObj = initBlah;

    const Str255& dest = blahObj;  // invoke cast operator to get private back out
    CPPUNIT_ASSERT( dest[0]=='\004' );
    CPPUNIT_ASSERT( dest[1]=='B' );
    CPPUNIT_ASSERT( dest[2]=='l' );
    CPPUNIT_ASSERT( dest[3]=='a' );
    CPPUNIT_ASSERT( dest[4]=='h' );
    CPPUNIT_ASSERT( dest[5]=='\0' );  //  trailing null
}

Answer 1

This is probably a good example of why (in my opinion) it's a bad idea to typedef array types.

Unlike in other contexts, in function declarations a parameter of array type is always adjusted to an equivalent pointer type. When an array is passed to the function it always decays into a pointer to the first element.

These two snippets are equivalent:

typedef unsigned char Str[257];
Str src = "blah";
Str dst;
memcpy( &dst, &src, sizeof(Str) ); // unconventional

---

unsigned char src[257] = "blah";
unsigned char dst[257];
memcpy(&dst, &src, sizeof(unsigned char[257])); // unconventional

In this latter case &dst and &src are both of type unsigned char (*)[257] but the value of these pointers are the same as the value of pointers to the first element of each array, which is what dst and src would decay into if passed directly into memcpy like this.

memcpy(dst, src, sizeof(unsigned char[257])); // more usual

memcpy takes void* arguments so the types of the original pointers don't matter, only their values.

Because of the rule for parameter declarations (an array type of any or unspecified size is adjusted to the equivalent pointer type), these declarations for fn are all equivalent:

typedef unsigned char Str[257];
void fn( Str dst, Str src );

---

void fn( unsigned char dst[257], unsigned char src[257] );

---

void fn( unsigned char dst[], unsigned char src[] );

---

void fn( unsigned char* dst, unsigned char* src );

Looking at this code, it is more obvious that the values being passed into memcpy in this case are pointers to the passed pointers, and not pointers to the actual unsigned char arrays.

// Incorrect
void fn( unsigned char* dst, unsigned char* src )
{
    memcpy(&dst, &src, sizeof(unsigned char[257]));
}

With a typedef, the error is not so obvious, but still present.

// Still incorrect
typedef unsigned char Str[257];
void fn( Str dst, Str src )
{
    memcpy(&dst, &src, sizeof(Str));
}