[SOLVED] random file access from C

caverdave · 07-09-2009, 01:07 PM

Hi,

I'm trying to find a way to open a file for random read/write access in C. Windows provides the O_RANDOM mode, but I can't seem to find anything on how to do this in Linux. Don't tell me you can't do this in Linux!

Thank you

metrofox · 07-09-2009, 03:46 PM

Quote:

Originally Posted by caverdave

Hi,

I'm trying to find a way to open a file for random read/write access in C. Windows provides the O_RANDOM mode, but I can't seem to find anything on how to do this in Linux. Don't tell me you can't do this in Linux!

Thank you

If you talk about C and programming language you can do anything with Linux!

ntubski · 07-09-2009, 03:48 PM

Are you refering to this?
From http://msdn.microsoft.com/en-us/libr...3z(VS.71).aspx

Quote:

The _open function opens the file specified by filename and prepares the file for reading or writing, as specified by oflag.

...

_O_RANDOM
Specifies that caching is optimized for, but not restricted to, random access from disk.

I don't believe there is an equivalent on Linux. This is just an optimization hint anyway, it doesn't affect the behaviour of your program. If you want actual random access you can use mmap.

bannock · 07-09-2009, 04:11 PM

Pretty much all file access is random access, unless you're reading something like a tape drive. Or maybe some special system files like /dev/random.

raconteur · 07-09-2009, 06:33 PM

Quote:

Originally Posted by bannock

Pretty much all file access is random access, unless you're reading something like a tape drive. Or maybe some special system files like /dev/random.

True enough.

If you want to extract or insert data at an arbitrary point in a file, the fseek and ftell functions are your friends.

Hko · 07-10-2009, 03:29 AM

With mmap() you can access a file just like an array. If you do a lot of random access to a file this can be very convenient en efficient. For huge files you may need to revert to fseek() and ftell(). See this thread for some mmap() links and an example.

caverdave · 07-10-2009, 10:35 AM

Thank you all.

Once I posted my question, I kind of realized that I was being dense. I was getting my signals crossed.