[MPlayer-dev-eng] [PATCH] Add a function for parse and queue multiple cmds seperated by line-break

[Reimar Döffinger]

Hello,
On Sun, Dec 09, 2007 at 09:37:18PM +0800, Ulion wrote:
> 2007/12/9, Reimar Döffinger <Reimar.Doeffinger at stud.uni-karlsruhe.de>:
[...]
> > > +    while (*str == '\n')
> > > +        ++str;
> > > +    while (*str) {
> > > +        size_t len = strcspn(str, "\n");
> > > +        char cmdbuf[len+1];
> >
> > Hmm... I think creating arbitrary-size arrays on the stack is a really
> > bad idea, what does everyone else think?
> 
> I have no idea how the compiler handle such declaration. Indeed I
> never known an array can have a dynamic size declaration before I
> touch mplayer's code. Anyone can explain that to me?

Well, it's not difficult, the size of the array is subtracted from the
current stack pointer and then you have a new stack pointer that points
to the start of your array.
The problem with this here is, that string in theory could be e.g. 500
MB large. As I learned it, the area reserved for your stack is usually
at most 20 MB large though, so like that you would be writing outside
the stack, possibly into something that is already used otherwise,
e.g. another thread's stack or the heap (malloc'd stuff), with
all the problems that entails...
If your data is at most e.g. 1 MB that is not a problem usually, because
your OS should set up "guard" pages below your stack, so as long as
you only allocate small things and access them as well you can be sure
your program will crash before it reaches beyond the stack and somewhere
it shouldn't be.
With 64 bit systems the issue is smaller, since the guard areas can be
made large enough that you can't break out at least with and "int"
index, at least for single-threaded applications.

Greetings,
Reimar Döffinger