C reading/writing files

This is a discussion on C reading/writing files within the C Programming forums, part of the General Programming Boards category; Code: for(i = 0;i < numfiles;++i) extract_file(fdin, files[i]); close(fdin); The structure is filled including name, file size, and offset into ...

  1. #16
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    Code:
      for(i = 0;i < numfiles;++i)
        extract_file(fdin, files[i]);
    
      close(fdin);
    The structure is filled including name, file size, and offset into archive before that block of code is reached. That tiny for() loop simply extracts each file in turn. If you want to rewrite the extract_file() function to just do a simple lseek() and then read(), that's totally fine. Just make sure your buffer is big enough. Maybe something like:
    Code:
    char *buffer;
    for(i = 0;i < numfiles;++i)
    {
      buffer = malloc(files[i].size);
      read(fdin, buffer, files[i].size);
      fdout = open(...);
      write(fdout, buffer, files[i].size);
      close(fdout);
      free(buffer);
    }
    Last edited by itsme86; 12-01-2010 at 02:14 PM.
    If you understand what you're doing, you're not learning anything.

  2. #17
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    itme86,

    hello again, just encountered a error I am stuck on: -

    --) When adding files to archive ( the first program), when we go back to write() the header data, we reserved 2 bytes(short) at beginning of file, two store value of how many files in archive.

    like: write(fdout,(short*)&numfiles, sizeof(short)); - - at the time obviously variable 'numfiles' would have value ie say 4, representing no of files user added and thus that line above should have written say 4 into the first 2 bytes(short) at start of archive file, mistake me if I misunderstand?

    --) So, in the second program (extracting) we have the line: -

    Code:
    int numfiles;
    read(fdin, (short*)&numfiles, sizeof(short));
    Obviously, the var 'numfiles' in this program would be zero intially and using the read() we want to read the value we stored in first 2 bytes of archive file, to get this value back out and put it back into var 'numfiles' correct me if I misunderstood?

    -- ) The problem I have, I believe is either the numfiles value was not written into file in first place, or there is error with read() statement I using below, in my extract program : -

    Code:
    int numfiles; //would be empty obviously in this program until read value from file in
    lseek(fdin, 0, SEEK_SET); 
    read(fdin, (short*)&numfiles, sizeof(short)); 
    //want to check numfiles has number of files stored inside it
    printf("%d is the number of files inside archive\n", numfiles);
    printf("%x is the number of files inside archive- memory address\n", numfiles);
    However, the memory address returns fine, but say I added one file to archive 'file1' then 'numfiles=1' should should it not, however it returns a number in its thousands like 23456 or something?

    Is there a way I an see the header data to see if its written correctly, or is it always hidden?

    What does (short*)&numfiles do, &numfiles is the memeory address of int numfiles variable, but what is (short*) about ?

    Thanks
    daza166

  3. #18
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    You can open the file in a hex editor to see what's going on in there. I use XVI32

    But yes, you're right. If you add one file then the first 2 bytes of the file should be 0x01 0x00, assuming you're using a little endian CPU such as x86.
    If you understand what you're doing, you're not learning anything.

  4. #19
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    itme86,

    the problems is when I say add one file
    read(fdin, (short*)&numfiles, sizeof(short));

    and then
    printf("%d\n", numfiles); -- returns 28265 , should that not be '1' as only 1 file added?

    Thus the loop below would be invalid as says numfiles is - 28265
    for(i = 0; i < numfiles;++i)

    There is an error somewhere when writting and reading out first 2 bytes(short) from archive?

  5. #20
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    Yes, numfiles should be 1 if only 1 file was added.

    Did you try a hex editor on the archive file? What do the first 2 bytes look like? I tested with the code I've pasted here and the first 2 bytes always come out correct for me.

    My guess is that there's something wrong with your code that writes the header.
    Last edited by itsme86; 12-02-2010 at 11:37 AM.
    If you understand what you're doing, you're not learning anything.

  6. #21
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    looked in ghex but could not work out, what it means, here is bit of first line is like

    00000000 69 6E
    00000011 61 72
    00000022 73 74

    nothing on the clear text on side, just content can not see header ie first char is not a number
    using write() read() as shown in previous posts

    lseek(fdout, 0, SEEK_SET);
    write(fdout,(short*)&numfiles, sizeof(short));
    Last edited by daza166; 12-02-2010 at 11:50 AM.

  7. #22
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    Where do you assign numfiles? Can you post the whole code?
    If you understand what you're doing, you're not learning anything.

  8. #23
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    itme86,

    ok managed to sort program myself, just stuck on little issue

    ) How do I write a variable into a file using write() function, it returns an error

    say I want to write() an integer value at start of file, so want to write 4 bytes

    Code:
    int numberfiles=5; 
    open().......
    write(file, numberfiles, sizeof(numberfiles));
    Do I have to use '&' in order to write that value from that variable using its memory address?

    Thanks,

  9. #24
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    Do I have to use '&' in order to write that value from that variable using its memory address?
    Yup. Just use &numberfiles and it should work correctly.
    If you understand what you're doing, you're not learning anything.

  10. #25
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    itme86,

    Say I want to write() 4 bytes(int) to a file and read() that 4 bytes back out and put the value into inside those 4 bytes into a variable

    Code:
    //open () .........opens approp files
    
    int numfile=5; 
    lseek(file, 0, SEEK_SET);
    write(file, &numfile, sizeof(numfile)); // write 4 bytes (int value) to file
    
    //read 4 bytes from file and write into variable readout, also int
    int readout; //put value read from file in this variable 
    
    lseek(file, 0, SEEK_SET);
    read(file, &readout, sizeof(readout));
    printf("number that was put in file originally was %d",readout);
    I know that you can not print or see int values read() or write()en to file, as formats change

    So how can I read() the 4 bytes I know is at start of file and put the value into that int variable (readout) which is the right type to hold that value and print it out to screen?

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