Question

void StaticControl::addText(LPWSTR text)
{
    lpCurrentText = (LPWSTR)malloc((GetWindowTextLength(hStatic) + 1) * sizeof(LPWSTR));
    GetWindowText(hStatic, lpCurrentText, GetWindowTextLength(hStatic) + 1);

    lpInput = text;
    chInput = (char*)malloc(sizeof (lpInput) *sizeof(LPWSTR)/sizeof(char*));
    chCurrent = (char*)malloc(sizeof (lpCurrentText) *sizeof(LPWSTR)/sizeof(char*));
    wcstombs(chCurrent, lpCurrentText, wcslen(lpCurrentText) + 1);
    wcstombs(chInput, lpInput, wcslen(lpInput) + 1);
    strcat(chCurrent, chInput);
    lpNewText = (LPWSTR)malloc(strlen(chCurrent) * sizeof(char*)); //this is where it crashes
    mbstowcs(lpNewText, chCurrent, strlen(chCurrent) + 1);
    SetWindowText(hStatic, lpNewText);
    return;
}
//where
HWND hStatic;
LPWSTR lpCurrentText;
LPWSTR lpInput;
LPWSTR lpNewText;
char*  chInput;
char*  chCurrent;

This code works fine adding text to the control until the string becomes around 20 characters long in which the program crashes. Stepping through the program, it crashes where I allocate memory for the lpNewText buffer. I don’t know what’s wrong. Visual Studio takes me to the malloc.h header when it crashes.

Answer 1

For a start I’d recommend that you abandon malloc and use C++ memory allocation techniques. Like new, new[], std::vector<>, std::string, std::wstring etc.

That said, I can see the following errors in your code:

lpCurrentText = (LPWSTR)malloc((GetWindowTextLength(hStatic) + 1) * sizeof(LPWSTR));
// should be sizeof(*lpCurrentText), i.e. size of a wide char 
// and not size of a pointer as you have it

chInput = (char*)malloc(sizeof (lpInput) *sizeof(LPWSTR)/sizeof(char*));
// sizeof(lpInput), sizeof(LPWSTR) and sizeof(char*) are all equal to the
// same thing, the size of a pointer

chCurrent = (char*)malloc(sizeof (lpCurrentText) *sizeof(LPWSTR)/sizeof(char*));
// as above

lpNewText = (LPWSTR)malloc(strlen(chCurrent) * sizeof(char*)); 
// sizeof(char*) is the size of a pointer, not what you want

return;
// rather pointless when the return type is void

I don’t really know what your code is trying to do so I’m not going to try to re-write it and correct everything. The fundamental problem you have is that you systematically write sizeof(…) and calculate the size of a pointer when you actually want the size of a character element.

Perhaps what you really need to do is throw away all this horrid code and use std::wstring to do your concatenation.