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To: caml-list@inria.fr
From: cashin@cs.uga.edu
Subject: Re: [Caml-list] Unix.lseek versus Pervasives.pos
Date: Wed, 19 Mar 2003 13:36:08 -0500
Message-ID: <877kavryp3.fsf@cs.uga.edu>
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X-Spam: no; 0.00; caml-list:01 lseek:01 pervasives:01 basile:01 flushes:01 openfile:01 rdonly:01 printf:01 unixlabels:01 stdio:01 stdlib:01 unistd:01 char:01 sizeof:01 ocaml:01 
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Sorry if this shows up as a duplicate.  

Basile STARYNKEVITCH <basile@starynkevitch.net> writes:

...
> You apparently forgot to flush the channel.

Flushes are for writes, but even when using a test program that just
reads, zero is returned when it appears that it shouldn't return zero.
Compare the short ocaml program below to the comparable C version.

The ocaml version has lseek returning position zero after reading 10
bytes from the file.  

  ecashin@meili seek-tell$ ./test 
  after reading 10 chars: "let main =", position is 0
  ecashin@meili seek-tell$ cat main.ml
  let main = 
    let fd = Unix.openfile "main.ml" [Unix.O_RDONLY] 0 
    and buf = String.create 1024 in
    Printf.printf "after reading %d chars: \"%s\", position is %d\n"
      (UnixLabels.read fd ~buf ~pos:0 ~len:10)
      buf
      (UnixLabels.lseek fd 0 ~mode:Unix.SEEK_CUR)
  ;;
  
  main
  

... but in the C version you get the expected position reported.
  
  ecashin@meili seek-tell$ ./test 
  after reading "#include <" lseek returns 10
  ecashin@meili seek-tell$ cat main.c
  #include <stdio.h>
  #include <stdlib.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <unistd.h>
  
  int main(void)
  {
      int fd = open("main.c", O_RDONLY);
      char buf[1024];
  
      if (fd == -1) {
        perror("open");
        exit(EXIT_FAILURE);
      }
      memset(buf, '\0', sizeof(buf));
      read(fd, buf, 10);
      printf("after reading \"%s\" lseek returns %d\n",
             buf, (int) lseek(fd, 0, SEEK_CUR));
  
      return 0;
  }


-- 
--Ed L Cashin            |   PGP public key:
  ecashin@uga.edu        |   http://noserose.net/e/pgp/

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