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Re: [bug-gnulib] valloc()?

From: Bruno Haible
Subject: Re: [bug-gnulib] valloc()?
Date: Wed, 2 Mar 2005 13:00:33 +0100
User-agent: KMail/1.5

Derek Price wrote:
> First, I found man pages stating that valloc() was obsoleted in favor of
> posix_memalign().  Does anyone have a feel for how portable
> posix_memalign() is

It appears to be implemented only by glibc.

> Second, if posix_memalign() is not portable, is there any interest in a
> GNULIB module either to replace it or valloc()?  I've attached the
> valloc.c that CVS is currently using.

The spec says that the returned memory must be freeable through free().
I don't see how you can write a portable replacement for posix_memalign()
without digging in undocumented details of the malloc() implementation.

> void *
> valloc (bytes)
>      size_t bytes;
> {
>   long pagesize;
>   void *ret;
>   pagesize = getpagesize ();
>   ret = malloc (bytes + pagesize - 1);
>   if (ret)
>     ret = (long) (ret + pagesize - 1) &~ (pagesize - 1);
>   return ret;
> }

This is the best you can do in a portable way, but
  1. the return value cannot be passed to free(),
  2. it wastes 1/2 page of memory on average.

Therefore I'd suggest a new interface:
  void* pagealign_alloc(size_t);
  void pagealign_free(void*);

and do the implementation as follows:
  - If mmap() is available, use mmap and some bookkeeping for pagealign_alloc,
    and munmap() for pagealign_free,
  - Otherwise, if posix_memalign() is available, use it and free(),
  - Otherwise, use something similar to the valloc() above.

Is this all worth it? For what purpose do you need the memory to be


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