[mpich-discuss] question in MPI_Info_get

[Thakur, Rajeev]

I guess it may have been defined that way because in Fortran strings are not null-terminated. The definition in the standard allows the same value to be returned/passed in valuelen in both C and Fortran.

Rajeev

On Feb 3, 2015, at 5:28 PM, Junchao Zhang <jczhang at mcs.anl.gov>
 wrote:

> OK, I guess MPI standard wants to have this kind of code.
> 
> int valuelen;
> MPI_Info_get_valuelen(..., &valuelen,...);
> int buflen = valuelen + 1;
> char* value = (char *)malloc(buflen);
> MPI_Info_get(..., valuelen, value, ...);
> 
> Though, from a user's point of view, I fell better if it was MPI_Info_get(..., buflen, value, ...), that is, using buflen to specify the length of buffer I pass to MPI.
> 
> 
> --Junchao Zhang
> 
> On Tue, Feb 3, 2015 at 4:26 PM, William Gropp <wgropp at illinois.edu> wrote:
> Thanks.  Yes, I believe this is a bug.  The implementation of MPI_Info_get in MPICH passes valuelen to the internal copy routine, yet it is clear that the definition in the standard for MPI_Info_get, it should pass valuelen+1 (it should also check for the error return code from the internal routine and return a value MPI error code if the buffer is too small.
> 
> Bill
> 
> On 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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> 
> 
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