Thread: system calls in linux

well, the thing is, lets say I am using primary indexing method. That means, my index file contains two fields.
1) the primary key field value.
2) The pointer to the block containing the record corresponding to the value.
Both my index file and my data file are sorted with respect to the primary key field value.
My data file is stored over many disc blocks.
So, if I need to retrieve a record whose promary key value is known, I need to do a binary search on the index file to find out which disc block can hold the record that I am interested in. For this,I need the pointer to that block. Now that I have the pointer, I can load that block to the memory and retrieve the tuple.

So, disk is of relevance here as I cant store my entire data file in the memory.I can only load parts of it as and when required.

What exactly do you think mmap actually does for you?

If your file is really that big that it would not fit in memory, then mmap would fail.

It's really no different to allocating a big array yourself, then reading the whole file into memory. Once there, you can treat it as any other array in memory, and if it is sorted for example, then you can search it with bsearch().

Maybe practice doing this with regular files before thinking about trying to use mmap.

Ok, for the time being forget about the mmap() question that I had initially asked. Is there a system call that allows me to detect the block size on my disc?

Probably (or maybe), but again, what does the physical (or logical) block size have to do with anything?

Your data has a record size of 's'
you get to the 'n'th record by doing fseek( fp, s * n, SEEK_SET );

Then you read the record by doing
fread( &rec, 1, sizeof rec, fp );

Voila.

Just let the OS worry about which disk block(s) your record maps to.

well,the requirement is that the node size in my B+ tree is of the size of the block . So, I need to get the block size. also, I am compelled to work on windows, so, I have to find out if there is one such call for windows.

Unless the size is already pretty close to say 512 or 4096, it sounds like an awful waste of space.

> also, I am compelled to work on windows,
So why is this posted in the Linux forum, and why are you using mmap(), which isn't available on windows AFAIK.

>Unless the size is already pretty close to say 512 or 4096,
Thats what even I was speculating. But my prof told me so. He said I have to use a system call to ascertain the actual block size. Do you think MS releases the list of such available system calls that are avaialble for developers? Are you aware of any such links?

>it sounds like an awful waste of space.
Really? I can have many data pointers in one node right? So, instead of having a large number of small nodes, I have few nodes that are as large as my disc block

>So why is this posted in the Linux forum, and why are you using mmap(), which isn't available on windows AFAIK.
well, I had started off on linux, but I had to switch as my teammates arent that familiar with linux.

Random example which has something to do with disk sizes
http://msdn.microsoft.com/library/de...ogicaldisk.asp

well, thanks for that ..
IS it true that I need to have the cygwin library installed to execute system calls from within my C program in windows?

It does have a data type of "uint64" for the blocksize.
Can you tell me how do I actually use this. I have not scripted in windows before.