Re: Bug#287824: libc6: fseek() fails and returns -1 without setting errno

[GOTO Masanori <gotom@debian.or.jp>]

At Thu, 13 Jan 2005 05:01:50 -0800,
Johan Walles wrote:
> According to the man page:
> 
> "
> RETURN VALUE
>         The  rewind  function  returns  no  value.  Upon successful 
> completion,
>         fgetpos, fseek, fsetpos return 0, and ftell returns the current 
> offset.
>         Otherwise, -1 is returned and the global variable errno is set 
> to indi-
>        cate the error.
> "
> 
> I.e. fseek() should return 0 on success or -1 on failure.
> 
> You're saying:
> 
> "
> file->f_pos always
> becomes minus values.  Return value from lseek() and fseek() functions
> are negative, but from POSIX or ANSI point of view, those are
> interpreted as error condition.
> "
> 
> Why would the actual file position affect the return value from 
> fseek()?  Especially without
> being a bug (possibly in the man page, but still)?

This is special case.  Your program passed offset as negative, and
kernel should return it's EINVAL case.  However mem_lseek is special,
and it accepts even negative value.  So glibc is worked as undefined
behavior.  The current glibc _IO_new_file_seekoff() checks return
value of _IO_file_seek (thus lseek64) as follows:

  result = _IO_SYSSEEK (fp, new_offset, 0);
  if (result < 0)
    return EOF;

In this case result < 0 && result != EOF.  But it should not be
interpreted because result is also off_t type and the offset pointer
becomes negative position.

I forgot to write in previous mail that you should use lseek system
call directory instead of fseek/fseeko to handle such special offset
condition.

Regards,
-- gotom