Question

I’m writing a function that creates a file using the win32 API

void createFile(HWND hwnd, LPTSTR pszFile, PBITMAPINFO pbi, HBITMAP hBMP, HDC hDC)

I’ve also written a function that dynamically generates a file name based on the current time.

char * getFilename() {
   char filename[200] = "";
   char buf[40];
   SYSTEMTIME st;
   GetSystemTime(&st);
   itoa(st.wHour, buf,10)
   strcat(filename,buf);
   .....
   return filename;
}

I call the above function like:

createFile(hwnd, getFilename, pbi, hBMP, hDC);

Passing through the debugger, the filename is correct until after the first statement in createFile() is executed and the string suddenly becomes “ÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ”

It’s clear I have some sort of type error, but I am unsure how to fix it. I am new to the win32 API and have a lot of trouble understanding the various types used, so I would appreciate any help fixing this issue

Answer 1

You are returning a pointer to filename, which has been allocated on the stack inside getFilename and so is freed when the function returns.

EDIT: To avoid confusion – the memory is allocated on the stack, not on the heap (via malloc or new). So “it is freed” does not refer to calling free (or delete). Rather to the fact that it is no longer reserved. Not available for use anymore. Deallocated. Bereft of bytes. This is a non-buffer.

For a solution, consider allocating the memory outside of getFileName. This can be done on the stack by the caller, who passes the buffer as a parameter. Naturally, using a string class would be an option.