[mpich-discuss] question in MPI_Info_get

[Hajime Fujita]

Hi Junchao,

Thank you for your reply.
Okay, so it sounds like in the current dominant implementation I can treat valuelen as “length of the buffer, including a space for the null terminator”.

If anyone could provide a good explanation of the spec it would be great.


Best,
Hajime

> On Feb 2, 2015, at 2:54 PM, Junchao Zhang <jczhang at mcs.anl.gov> wrote:
> 
> Hi, Hajime,
>  MPI spec also says "valuelen is the number of characters available in value". So it seems if you should pass valuelen = 4 if value is a buffer of 4 bytes.
>  I am also confused by "In C, valuelen should be one less than the amount of allocated space to allow for the null terminator".  So I leave the question for others.
>  Interestingly, for MPI_Info_get_valuelen(), MPI spec says "The length returned in C does not include the end-of-string". 
> 
>  BTW, I found MPICH and OpenMPI have the same behavior. 
> 
> --Junchao Zhang
> 
> On Sun, Feb 1, 2015 at 12:13 AM, Hajime Fujita <hfujita at uchicago.edu> wrote:
> Hello,
> 
> I came across some question about a behavior of MPI_Info_get in MPICH.
> 
> If I knew the size of the value, what should I pass to MPI_Info_get as `valuelen`?
> Suppose I have 3-byte value in the info object, and I allocate 4-byte buffer including a space for null-termination. Then I think I should specify 3 as valuelen to retrieve the whole value, as the MPI-3 specifications says
> "In C, valuelen should be one less than the amount of allocated space to allow for the null terminator.”
> 
> However, when I run the attached program in MPICH 3.1.1, I get:
> $ ./test
> key found: vaule=va  # The last character ‘l’ is missing
> 
> If I specify `sizeof(buff)` as valuelen, it shows “val” correctly.
> 
> So apparently MPICH thinks valuelen includes null terminator.
> Am I missing anything, or any misunderstanding here?
> 
> 
> 
> Thank you in advance,
> Hajime
> 
> --
> Hajime Fujita
> Postdoctoral Scholar, Large-Scale Systems Group
> Department of Computer Science, The University of Chicago
> http://www.cs.uchicago.edu/people/hfujita
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