Thread: FILES in WinAPI

  1. #1
    the Corvetter
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    FILES in WinAPI

    Okay, here is what I have to do. Work with files. But the thing is, should I use API functions or just standard c functions for this?

    And also, a question about it (which ever way I decide to go): I need to either read a certain file (for example: file.txt) or if it is not existing, I need to create the file (file.txt). How would I approach this? What functions?

    Thanks!
    1978 Silver Anniversary Corvette

  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    The windows API file stuff don't *seem* to be as easy to use in the beginning. I'm still learning them all but they seem to be an overall better way of handling files the standard C

  3. #3
    pronounced 'fib' FillYourBrain's Avatar
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    API functions are far superior. That is if speed is important to you.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  4. #4
    pronounced 'fib' FillYourBrain's Avatar
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    And also, a question about it (which ever way I decide to go): I need to either read a certain file (for example: file.txt) or if it is not existing, I need to create the file (file.txt). How would I approach this? What functions?
    CreateFile()
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  5. #5
    jasondoucette.com JasonD's Avatar
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    Originally posted by FillYourBrain
    API functions are far superior. That is if speed is important to you.
    What do you base this statement on? Have you compared the speed differences between the two? I am curious about this, because I would believe that they would both be the same, as they both make use of the same low level interrupts.

  6. #6
    pronounced 'fib' FillYourBrain's Avatar
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    yes I have run speed trials on the C runtime vs system API. Window API on first access of the file takes about half the time to read. On following reads of the same file however, Windows API takes about 1/10th of the time to read. This is due to caching. Sorry to the "standards" purists. My heart goes out to you.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  7. #7
    the Corvetter
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    Originally posted by FillYourBrain
    CreateFile()
    But how would I first check if the file already exists?
    1978 Silver Anniversary Corvette

  8. #8
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    You need to read the MSDN on that function.

    CreateFile(), despite it's misleading name, can open old files, check for existance, check for write access, create files, and even play with devices.

  9. #9
    jasondoucette.com JasonD's Avatar
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    Originally posted by FillYourBrain
    ...Window API on first access of the file takes about half the time to read. On following reads of the same file however, Windows API takes about 1/10th of the time to read. This is due to caching...
    I'm sorry, but this boggles my mind... What form of caching does the Win32 API do that regular C functions do not do? Also, as evident by your wording, you are not speaking about reading in a single file in its entirety at once. Have you tested this? What are the conditions of your test when reading in the file segments at a time?

  10. #10
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    I don't know about FillYourBrain, but in the compilers I've tested (GNU, Microsoft VC++, Borland C++) the C functions have simply been wrappers for the Windows API functions, and so using the API functions is marginally quicker than use the standard C library.

    By using sensible buffers (64Kb reads for example), you can hardly tell the difference between them. It's only when you are reading such small quantities like a single byte at a time does the extra overhead show through.

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    There is also another advantage if you use advanced WinAPI functions. For example, you can use CreateFile(), CreateFileMapping(), and CreateViewOfFile() to memory-map a file. This allows the OS a lot of flexibility on when it actually performs disk access.

    The latency of a disk is high, because it takes time to physically position the head and wait for the platter to spin into the right position. However, once the head is positioned, you can read a lot of sequential data very fast, because as the platter spins, the head simply moves over the data sequentially. For example, if you were to read 64K worth of data, one byte at a time, it would be vastly slower than reading the whole 64K chunk at once.

    Reading byte-by-byte would look like:
    -Position- -Wait- -Read 1 byte- -Position- -Wait- -Read 1 byte- -Position- -Wait- -Read 1 byte-, etc. A lot of wasted time.

    Reading a block of data would look like:
    -Position- -Wait- -Read 64 kbytes-

    In general, by using the API functions, and by doing so in an intelligent way, you can read/write data in a much faster and more intelligent fashion. You can let the OS decide when to commit data to the disk, or when data needs to be read from the disk, instead of forcing reads or writes on every data access/change.

    (As an aside, memory-mapped files are a great way to share data between processes).
    Last edited by Cat; 09-22-2003 at 10:10 PM.
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  12. #12
    pronounced 'fib' FillYourBrain's Avatar
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    I don't know about FillYourBrain, but in the compilers I've tested (GNU, Microsoft VC++, Borland C++) the C functions have simply been wrappers for the Windows API functions, and so using the API functions is marginally quicker than use the standard C library.
    Is the C runtime not its own DLL? Not sure about that. Regardless. My time trials were very extensive.

    large and small files, repetative opening and closing, repetative seeking, repetative small reads, repetative large reads. In all cases, the Windows function won out by substantial margins.

    edit: The C runtime is in fact its own dll. crtdll.dll to be precise. Therefore unless you disassembled it and found it to be calling the API function then I question the above statement.
    Last edited by FillYourBrain; 09-23-2003 at 07:04 AM.
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    There's no need to disassemble anything, the full source code to the crtdll.dll is installed on my harddisk and in this code I can clearly see it call the Windows API CreateFile, WriteFile, etc functions.

  14. #14
    pronounced 'fib' FillYourBrain's Avatar
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    regardless, do the time test yourself. You'll see what I'm talking about. Major difference.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  15. #15
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    I have, and I've not just used Microsoft VC++ either, I've also tested GNU C++ & Borland C++ (which don't use crtdll.dll). I only see a small improvement in speed between the C library and the Windows API. I wouldn't say that this small improvement in speed was worth risking the portability of the code.

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