fseek - need some info please

This is a discussion on fseek - need some info please within the C Programming forums, part of the General Programming Boards category; I have read the boards, and It seems I am doing this right, but I am receiving garbage in my ...

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    fseek - need some info please

    I have read the boards, and It seems I am doing this right, but I am receiving garbage in my file:

    I want to open a file, move to the 98th byte and write an email address:
    Code:
    define EMAILADDRLEN 90 // this is set above in the program
     
    int e_loc = 98; // this is set above in the program
     
     
    if ( (out = fopen("gorseve_10774.csv","w")) != NULL ) 
    { 
    while ( (returnCode = fgets(lineOfText, LINESIZE, in )) != NULL) 
    { 
      e_loc = parseGorseveRecord(lineOfText); 
      printf("location found %d\n", e_loc); 
      if ( (ret = fseek(out, e_loc, SEEK_CUR)) == 0 ) 
      { 
        fwrite("test@aol.com",sizeof("test@aol.com"),EMAILADDRLEN,out); 
      } 
      else 
      { 
        printf("Error locating write position\n"); 
        exit(1); 
      } 
      fclose(out); 
      exit(0); 
      } 
    } 
    else 
    { 
    printf("Error Opening File: filename\n"); 
    exit(1); 
    }
    Last edited by Salem; 08-03-2006 at 12:34 PM. Reason: Nice shiny code tags added, please use them in future

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    should I be using fgetpos and fsetpos instead?

    Thanks in advance !

  3. #3
    ZuK
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    Quote Originally Posted by mhenderson
    I want to open a file, move to the 98th byte and write an email address:
    Your code does something else. First of all the position value is overwritten by
    Code:
    e_loc = parseGorseveRecord(lineOfText);
    and then you use
    Code:
    fseek(out, e_loc, SEEK_CUR)
    This moves e_loc bytes forward from the current position.
    Kurt
    EDIT sorry overlooked that read from a different file. But still the position is overwritten with another value.
    Last edited by ZuK; 08-03-2006 at 12:24 PM.

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    sorry for that confusion,

    that's what I meant by e_loc being set to 98 above, i just didn't want to copy in my parseGorseveRecord function. But I test the value of e_loc here:

    printf("location found %d\n", e_loc);
    This prints out 98.

    Then the program exits.
    When I check the file it has written over everything that was in it and just puts a 2 -3 lines of garbage?

  5. #5
    ZuK
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    The problem is the open mode "w" creates a new file. guess you want "a" ( but then you should use use SEEK_SET )
    Kurt
    Last edited by ZuK; 08-03-2006 at 12:29 PM.

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    Wow, I had that all wrong....

    thank you so much!!

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    One more question

    Once I've written that email address, is there an easy way to skip to the beginning of the next line ?

    thanks in advance )))

  8. #8
    ZuK
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    Quote Originally Posted by mhenderson
    One more question

    Once I've written that email address, is there an easy way to skip to the beginning of the next line ?

    thanks in advance )))
    To find the next line you could open the file in read/write mode ("a+") and just read until you find a '\n'
    On the othe hand combining textmode with fseek() is not a very good idea. try "rb+"
    Kurt

  9. #9
    and the hat of int overfl Salem's Avatar
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    > fwrite("test@aol.com",sizeof("test@aol.com"),EMAIL ADDRLEN,out);
    This writes a hell of a lot more than you bargained for to the file.
    Sizeof on a string constant is the number of chars in the string, and the \0 at the end.
    And you write 90 copies of it!


    In-place edits on text files isn't really the thing to do.

    Read a line from the input file
    Modify the line
    Write the line to the output file.

    When you're done, delete the old file and rename the new file.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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    yes, this sounds much better, if I use write ,will it just write an entire line over current line?

    How do I go to the next line, and the next, etc.

    Thanks again...I've been looking at this too long

  11. #11
    Frequently Quite Prolix dwks's Avatar
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    You can only write to the end of a file opened in append mode. You'd be best off creating a new file as Salem suggested.

    [edit] The thread is continued here: Question regarding File I/O [/edit]
    dwk

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