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Full Discussion: Understanding read/write and kernel interaction
Top Forums Programming Understanding read/write and kernel interaction Post 302354035 by Krothos on Thursday 17th of September 2009 12:13:08 AM
Old 09-17-2009
Quote:
Originally Posted by jlliagre
If by main memory you mean kernel memory, no. A process has no access to the kernel memory.
So once the bytes are transferred stored over in the main memory through the DMA, how is what was read returned to the caller? So if I wanted to read 3 bytes from a file containing "Hello", how is "Hel" returned to the library function? What sees this and how/where is it returned from?

Quote:
Yes (bytes)Same as read.I don't get what you mean here.
[/quote]

What I'm talking about what is needed for the write.

What I meant was this:

Your library function is passing a reference to a buffer storing the bytes to replace a certain block in a file (for example). So, the goal is just to replace those bytes in the file with what is in the buffer (the second argument passed in).

What I am confused about is HOW exactly the modification takes place. How those bytes within the user area buffer replace the target file block of bytes.

Are the bytes within the user area buffer somehow transferred over to the buffer which resides on the device controller for let's say, a disk (the file to modify is on the disk)? And then from there, what's in the buffer of the controller somehow replaces the targeted block within the file you are trying to modify?

So you're passing in bytes to modify some block within the target file. I don't get how this takes place for a write.

That was what I was saying. I don't get the process of how write takes place in terms of a file let's say.
 

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LEARN ABOUT DEBIAN

cmdk

cmdk(7D)							      Devices								  cmdk(7D)

NAME

       cmdk - common disk driver

SYNOPSIS

       cmdk@target, lun : [ partition | slice ]

DESCRIPTION

       The  cmdk  device  driver  is  a  common interface to various disk devices. The driver supports magnetic fixed disks and magnetic removable
       disks.

       The block-files access the disk using the system's normal buffering mechanism and are read and written  without	regard	to  physical  disk
       records. There is also a "raw" interface that provides for direct transmission between the disk and the user's read or write buffer. A sin-
       gle read or write call usually results in one I/O operation; raw I/O is therefore considerably more efficient when many bytes are transmit-
       ted. The names of the block files are found in /dev/dsk; the names of the raw files are found in /dev/rdsk.

       I/O  requests  to the magnetic disk must have an offset and transfer length that is a multiple of 512 bytes or the driver returns an EINVAL
       error.

       Slice 0 is normally used for the root file system on a disk, slice 1 as a paging area (for example, swap), and slice 2 for backing  up  the
       entire fdisk partition for Solaris software. Other slices may be used for usr file systems or system reserved area.

       Fdisk partition 0 is to access the entire disk and is generally used by the fdisk(1M) program.

FILES

       /dev/dsk/cndn[s|p]n     block device (IDE)

       /dev/rdsk/cndn[s|p]n    raw device (IDE)

			       where:

			       cn	controller n

			       dn	lun n (0-7)

			       sn	UNIX system slice n (0-15)

			       pn	fdisk partition(0)

       /kernel/drv/cmdk        32-bit kernel module.

       /kernel/drv/amd64/cmdk  64-bit kernel module.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Architecture		     |x86			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       fdisk(1M), mount(1M), lseek(2), read(2), write(2), readdir(3C), scsi(4), vfstab(4), attributes(5), dkio(7I)

SunOS 5.10							    9 Oct 2004								  cmdk(7D)
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