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Binary File Access

Posted on 1998-10-21
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Last Modified: 2013-11-13
I am working on a winsock program to do some data testing.
I currently have the socket part working with some text.
I would like to read in a binary file to send.  I am not sure how to open the binary file (such as a zip file).  I am used to working with binary formated files.
Code examples would help
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Question by:jaalex
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steve06 earned 200 total points
ID: 1440999
jaalex,

To work with a binary file, open it the following way:

Open "Myfile.ZIP" for binary as #1

Then, to read the nth byte, use the command:

Dim bytMyVariable as Byte
Get #1, n, bytMyVariable

To create a binary file, use the Put command:
Put #1, n, bytMyVariable

Last but not least, if you repalce a single byte variable by a fixed-length (important!) string, then you can read or write up to 32,767 bytes at a time.

If you need more info, please let me know.
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Expert Comment

by:mark2150
ID: 1441000
You *definiately* do NOT want to read a file *ONE BYTE* at a time. It will be slow as *MUD*. This is due to the overhead of multiple calls. Generally you want to read in blocks that are the same size as a disk cluster. This will give overall best performance from the heads of the drive up to your app. The disk controller is going to read a cluster at a time no matter what so you might as well keep your disk buffers the same size range. On the *last* cluster your returned string will be *shorter* than your cluster size (this is one way to tell you're at the end).

If you want to create a new file as you go you need to kill the file off explicitly prior to opening with BINARY.

M

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Author Comment

by:jaalex
ID: 1441001
Ok I guess I have two questions.

1)  How do I tell the length of the file.

2) I want to feed the to a winsock program tha I wrote.  I want to be able to read in 2048 byte sections.  How do I do that.     Also if the program doesn't happen to end exactly at the end of the block how does the read know that.


I will up the points since I am being such a newbie on this topic
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Expert Comment

by:mark2150
ID: 1441002
filelen() will give you the length before you open the file.
Set your character buffer to 2048 like this:

IsLastBlock = False

(repeat this loop as required)

Buffer$ = Space$( 2048 )   ' Fill with 2k of empty
GET #1,,Buffer$                'Replace with data from disk
IF LEN( Buffer$ ) <> 2048 THEN IsLastBlock = True
.

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Author Comment

by:jaalex
ID: 1441003
Would you want to use LOF for this???
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Author Comment

by:jaalex
ID: 1441004
Would you want to use LOF for this???
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Author Comment

by:jaalex
ID: 1441005
Ok then so I take everything in a nice blocks off 2048 bytes of information.  What happens to that last little bit when I don't hage 2048 bytes of information.
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Author Comment

by:jaalex
ID: 1441006
Ok then so I take everything in a nice blocks off 2048 bytes of information.  What happens to that last little bit when I don't hage 2048 bytes of information.
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Expert Comment

by:mark2150
ID: 1441007
Then you send a short block to finish up.

LOF is used when the file is OPEN. FILELEN when the file is closed.

The length of Buffer$ will be 2048 if you read a full block or something less if you're at the end of the file. Here is some nice code for binary file copying:

'
' FileSpec$ = Full name/path of original file
' filnam$ = Root file name in local directory
'
' Ok, we want to copy in large blocks for speed, but we
' can't get too big or we'll run out of buffer
'
If Len(filnam$) > 0 Then
    rawsize = FileLen(filespec$)
    numblocks = Int(rawsize / 4096)
    fraction = rawsize Mod 4096
    Open filespec$ For Binary As #1
    Open maildir$ + filnam$ For Binary As #2
'
    If numblocks < 1 Then GoTo copytail
    '
    For cindex = 1 To numblocks
        bite$ = Space$(4096)
        Get #1, , bite$
        Put #2, , bite$
    Next cindex
    '
copytail:
    If fraction > 0 Then
        bite$ = Space$(fraction)
        Get #1, , bite$
        Put #2, , bite$
    End If
'
    Close
End If
'

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