Re: [PATCH] Support for unknown size and not seekable files

[Vladimir 'φ-coder/phcoder' Serbinenko]

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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