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
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