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Re: [PATCH] Support for unknown size and not seekable files
From: |
Vladimir 'φ-coder/phcoder' Serbinenko |
Subject: |
Re: [PATCH] Support for unknown size and not seekable files |
Date: |
Sat, 15 Jan 2011 14:02:14 +0100 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.16) Gecko/20101226 Icedove/3.0.11 |
On 01/15/2011 01:15 PM, Szymon Janc wrote:
> On Saturday 15 January 2011 01:37:13 Seth Goldberg wrote:
>
>> Hi,
>>
>> Thanks for this. What's the goal of this patch? It seems that it marks
>> the size and unknown until it gets to the metadata that indicates the
>> file's size, then updates the size to the correct size, right? Where do
>> you see this being useful?
>>
> The biggest problem now is that opening compressed files require seeking to
> the
> end to get uncompressed size so on not seekable fs this causes
> grub_file_open("foo") to read whole file. With this patchset this is no
> longer a
> case. So in that use case
>
> file = grub_file_open ("foo");
> while (grub_file_read (file, buf, sizeof (buf)) > 0) { ... }
>
> foo is read only once.
>
>
>
>> It would seems that even with this patch, one
>> would still need to read a gzipped file twice to be able to get its size,
>> then allocate a contiguous buffer large enough to hold the entire thing
>> (if that is desired (which, in some cases, it is :))).
>>
> Wouldn't sth like that work?
>
>
It's O(N^2).
You need to always at least double the allocation every time you exceed
already allocated buffer in order to get O(N)
> char * ptr = 0;
> char buf[0x10000];
> unsigned size, size_total = 0;
>
> grub_file_t file = grub_file_open ("foo");
> while ( (size = grub_file_read (file, buf, sizeof (buf))) > 0)
> {
> ptr = grub_realloc (ptr, size_total + size);
> grub_memcpy (ptr+size_total, buf, size);
> size_total += size;
> }
>
>
--
Regards
Vladimir 'φ-coder/phcoder' Serbinenko
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