[alsa-devel] Scope of snd_pcm_hw_params_alloca() ?

[Takashi Iwai]

At Thu, 13 Mar 2008 15:41:02 -0300,
Aldrin Martoq wrote:
> 
> Hello,
> 
> On Thu, Mar 13, 2008 at 1:44 PM, Takashi Iwai <tiwai at suse.de> wrote:
> > At Thu, 13 Mar 2008 16:21:51 +0000,
> > Alan Horstmann wrote:
> >  > I've spent a couple of days pining this down.
> >  > snd_pcm_hw_params_alloca() is part of the user API -right?  But accomplished
> >  > as a #define in pcm.h.
> >  Because alloca can be used only with a macro due to its nature.
> >  > However if it is called inside a function, and the pointer stored for future
> >  > use, after leaving the function the allocation seems to have gone, and
> >  > attempting thereafter to use the pointer to access the hwparams space in
> >  > another API function results in seg-fault.
> 
> You should use snd_pcm_hw_params_malloc() instead. *_alloca() from
> asoundlib are based in alloca(). From my system man:
> 
> $ man alloca
> NAME
>        alloca - memory allocator
> 
> SYNOPSIS
>        #include <alloca.h>
> 
>        void *alloca(size_t size);
> 
> DESCRIPTION
>        The  alloca() function allocates size bytes of space in the stack frame
>        of the caller.  This temporary space is automatically  freed  when  the
>        function that called alloca() returns to its caller.
> 
> [..]
> 
> Yup, this is actually very useful in many many many places!

Especially in C, that isn't easy to have implicit destructors.

BTW, a pitfall once I fell into is that the alloca frees the data at
the function exit, not at the block exit.  So, the following code eats
stack up and shall evetually dies:

{
	...
	for (;;) {
		char *tmp = alloca(4096);
		...
	}
	...
}


Takashi