fgetpos fsetpos problem/confusion

This is a discussion on fgetpos fsetpos problem/confusion within the C Programming forums, part of the General Programming Boards category; I'm having a problem with fgetpos and fsetpos when i call fsetpos twice. Code: while(0 != fgets(buffer,100,input) && strtod(buffer,NULL) < ...

  1. #1
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    fgetpos fsetpos problem/confusion

    I'm having a problem with fgetpos and fsetpos when i call fsetpos twice.

    Code:
    while(0 != fgets(buffer,100,input) && strtod(buffer,NULL) < LOWER)
    	fgetpos(input,&start);
    	
    fsetpos(input,&start);
    	
    while(0 != fgets(buffer,100,input) && strtod(buffer,NULL) <= upper)
    	lines_needed++;
    	
    rewind(input);
    fsetpos(input,&start_copy);
    the code above works correctly, however when the rewind(input); line is removed, the second fsetpos fails to do anything at all.

    At first i thoght that maybe fsetpos can only work forwards in the file, but the first fsetpos in fact works backwards, so the function must work both ways

    Any ideas as to why this is happening?

  2. #2
    cas
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    Check the return value of fsetpos(). If it returns error, check errno.

  3. #3
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    fsetpos returns 0 for both calls, with or without the rewind line
    Last edited by Skeksis; 12-06-2010 at 01:52 PM.

  4. #4
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    BTW, i've also tried making a copy of start and using it in the second call just to check whether fsetpos was changing anything there, no difference.

  5. #5
    and the Hat of Guessing tabstop's Avatar
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    I guess you should make sure that the relationship between "start" and "start_copy" is what we would expect from the names, as the "easy" diagnosis from what we see is that you set start_copy to the end of where you were just reading instead of the same as start.

    EDIT: This appears to have crossed your other message. Can you give us something compilable? My tests here are working fine.
    Last edited by tabstop; 12-06-2010 at 02:03 PM.

  6. #6
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    sorry to have bothered you guys, the problem appears to have disappeared. Must have had something minor wrong that got fixed while messing around with it debugging.

    There was also the secondary problem that if LOWER == 0 then fgetpos would never run, now fixed by using a do while loop instead of while.

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