[gmx-developers] Getting the last frame in an xtc file

[Ran Friedman]

Mark Abraham wrote:
> Ran Friedman wrote:
>> Mark Abraham wrote:
>>> Ran Friedman wrote:
>>>> One more piece of information:
>>>> I replaced xtc_get_current_frame_time of GMX 3.3.3 with that of GMX
>>>> 3.3.1 and recompiled.
>>>>
>>>> Now xtc_get_last_frame_time does work.
>>> Well that's something.
>>>
>>>> Ran Friedman wrote:
>>>>> Dear Mark, GMX developers,
>>>>>
>>>>> Unfortunately this doesn't work. In any case, this part of the code
>>>>> returns -1 in any case:
>>>>>  }else if(ret == -1){
>>>>>     if(gmx_fseek(xdrfiles[fp+1],off,SEEK_SET)) {
>>>>>       return -1;
>>>>>     }
>>>>>     return -1;
>>>>>  }
>>>>>
>>>>> and even if I try to move backwards 2*sizeoff(int) at a time  it
>>>>> doesn't
>>>>> get me anywhere.
>>> What value does ret have?
>> -1
>> Ran.
>
> Well ret==-1 means that there was an I/O failure in
> xtc_at_header_start.   ret==0 means failure to recognise a valid frame
> header, and should trigger the backward scan.
>
> Closer inspection of xtc_at_header_start reveals that it tries to read
> three ints before testing the first for being the magic number. So the
> "-4" in line 1666 is wrong logically (since the second scan for an int
> will give I/O failure), as well as for hard-coding the number. I think
> line 1666 should read
>
>     if(gmx_fseek(xdrfiles[fp+1],-3*sizeof(int),SEEK_END)){
>
> Line 1634 should have the same modification too.
>
> xtc_get_current_frame_time and xtc_get_current_frame_number should
> have the same algorithm with different return variables, but they
> don't. The latter doesn't test for the case ret==0, and to remedy
> this, needs the four-line fragment at line 1294 added to it, with
> 2*sizeof(int) replacing the 8. My earlier suggestion for line 1328 was
> wrong, so leave it unchanged.
Both seem right.
Actually, xtc_get_current_frame_number didn't work but since
xtc_get_current_frame_time did, I didn't make a fuss about it.
I'll submit it to bugzilla.

Mark - thanks a lot for your help!
Ran