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@suse.de wrote:

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At Thu, 13 Mar 2008 16:21:51 +0000, Alan Horstmann wrote:

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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.

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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