NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/17632

Internet Message Format | 1996-08-05 | 2.1 KB

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From: Lally Singh <[e-mail]>
Newsgroups: comp.lang.c++
Subject: Re: HELP Borland C++ 5.0 pointer problem
Date: Tue, 16 Apr 1996 16:58:04 -0400
Organization: Capital Area Internet Service [e-mail] 703-448-4470
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Adam Gensler wrote:
> That won't work because when you leave the function the memeory reserved for
> buffer[20] is marked as free.                            ^^^^- almost, actually, it's created in the stack.
> >Could you please tell me why the function below compiles and operates
> >correctly under win16 but not win32 ?

> >  char* test_function(char *in_data) {
> >
> >       char buffer[20];
> >       strcpy(buffer, in_data);   // copy in_data to buffer
> >       in_data = buffer;   // set pointer to buffer
> >
> >       return(in_data);
> >
> >  }  // end test_function function

Mr. Gensler is correct in that the buffer is not available after the function
ends, but it is not marked free, it was never in the heap, rather in the stack.
When the function returns, the stack is cleaned up and then the next time something
is pushed or popped, then that data is overwritten.
If under Win32 you get garbage, then that is what's happening.  If you get something
like a general protection fault, then the fault is from an attempt to access a
memory address beyond the range of the stack (meaning that the stack pointer has been
moved back before the place where it was when the buffer was allocated) and thus
you're trying to get an invalid address, which Win32 I know doesn't like too much.

To fix it, you have to do something like this:

char * testfunction( char * inData)
{
    char * buffer = new[] char[20]; // allocates on the heap, remember to delete[] it!
    strcpy( buffer, inData );       // copy the string
    return buffer;                  // return the new buffer
};