Run-Time Check Failure #2 - Stack around the variable 'x' was corrupted

Question

I receive this Run-Time Check Failure upon the return in the following code. I believe similar code is running fine elsewhere in the program. Any ideas?

String GetVariableName(CString symbol, CString filepath)
{
    char acLine[512];
    char acPreviousLine[512];
    CString csFile;
    FILE *fp;   

    csFile.Format("%svariables.txt", filepath);

    fp = fopen(csFile, "r");

    if (! fp)
        return("");

    for (;;)
    {
        strcpy(acPreviousLine, acLine);

        // NULL means we are out of lines in the file.
        if (myfgets(acLine, 511, fp) == NULL)
            break;

        // "END" indicates end of file
        if (! strcmp(acLine, "END"))
            break;

        if (! strcmp(acLine, csVarSymbol))
        {
            // Previous line should be variable name
            fclose(fp);

            // Following line results in Check Failure while in Debug mode
            return(acPreviousLine);
        }
    }   
    fclose(fp);
    return("");
}

Answer 1

There is no variable 'x' in the above example, but I'll presume you edited the error message!

acLine isn't initialised, so the first time you copy it to acPreviousLine, you are copying whatever happens to be on the stack. This can give you a buffer overflow and therefore stack corruption in some situations - not all, because you might be lucky and find a null in acLine before you get to 512 bytes.

The stack gets checked for corruption on return, because there are guard words (on this platform and build configuration - which I presume is on Windows, compiling on VS in debug mode) inserted around all stack variables to check for just that problem.

Initialise acLine[0] to 0.