seeking binary files

This is a discussion on seeking binary files within the C Programming forums, part of the General Programming Boards category; Hi, Can I seek a file using 2 lseek() at the same time, each one seeks for different thing?...

  1. #1
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    seeking binary files

    Hi,
    Can I seek a file using 2 lseek() at the same time, each one seeks for different thing?

  2. #2
    Gawking at stupidity
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    Nothing happens in a single process "at the same time". If they're separate processes then it makes no difference because each has its own internal offset pointer. You'll have to be more specific about what you're actually doing.
    If you understand what you're doing, you're not learning anything.

  3. #3
    Frequently Quite Prolix dwks's Avatar
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    No, you can't. The second lseek() will start where the first left off.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  4. #4
    Gawking at stupidity
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    You could always do something like this:
    Code:
    #include <stdio.h>
    
    void showline(FILE *fp, long *offset)
    {
      char buf[BUFSIZ];
    
      fseek(fp, *offset, SEEK_SET);
    
      fgets(buf, sizeof(buf), fp);
      fputs(buf, stdout);
    
      *offset = ftell(fp);
    }
    
    int main(void)
    {
      FILE *fp;
      long offsets[2];
      int i;
    
      if(!(fp = fopen("flipflop.txt", "r")))
      {
        puts("Unable to open 'flipflop.txt' for reading!");
        return 1;
      }
    
      offsets[0] = 0;
      offsets[1] = 50;
    
      for(i = 0;i < 5;++i)
      {
        showline(fp, &offsets[0]);
        showline(fp, &offsets[1]);
      }
    
      fclose(fp);
    
      return 0;
    }
    Code:
    itsme@itsme:~/C$ cat flipflop.txt
    000000000
    111111111
    222222222
    333333333
    444444444
    555555555
    666666666
    777777777
    888888888
    999999999
    itsme@itsme:~/C$
    Code:
    itsme@itsme:~/C$ ./flipflop
    000000000
    555555555
    111111111
    666666666
    222222222
    777777777
    333333333
    888888888
    444444444
    999999999
    itsme@itsme:~/C$
    That's using standard streams, but the concept is the same for low-level I/O.
    If you understand what you're doing, you're not learning anything.

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