[mpich-discuss] MPI_File_open(...) - overwrite existing file

[Thakur, Rajeev]

It will not be truncated. If you need the file to be truncated, you will have to delete the file before opening.

Rajeev

> On Feb 4, 2016, at 10:37 AM, Florian.Willich at dlr.de wrote:
> 
> Dear MPI Group,
> 
> I'm wondering if the files created with the modes  
> 
> MPI_MODE_CREATE | MPI_MODE_WRONLY 
> 
> will be truncated to 0 if the file already exists calling MPI_File_open(...). 
> 
> As far as I tested it and also which is written in the standard:
> 
> MPI_MODE_CREATE — create the file if it does not exist
> MPI_MODE_WRONLY — write only
> 
> The existing file will be overwritten, but leaving the bytes that have not been explicitly overwritten. Thus, an existing file with the size of 8 Bytes and data to be written to of 4 Bytes will still be 8 Bytes after writing: With the 4 Bytes of the new data and 4 Bytes of the old data.
> 
> As I read it, there is nothing defined what happens with the data of the file if it already exists [p 494, 1].
> 
> The POSIX / GNU lib c standard states that the file will be truncated to 0 if opened with 'w' and the file does already exists calling fopen(...) [p. 249, 2]:
> 
> ‘w’ - Open the file for writing only. If the file already exists, it is truncated to zero length. Otherwise a new file is created.
> 
> I really would appreciate some input if I'm wrong or not and what to do if the file needs to be truncated before writing (other then deleting the file before).
> 
> Kind regards
> 
> Florian Willich
> 
> [1]: Message Passing Interface Forum. 2015. MPI: A Message-Passing Interface Standard Version 3.1. http://mpi-forum.org/docs/mpi-3.1/mpi31-report.pdf. [Online. Accessed 2nd December 2015].
> 
> [2]: Loosemore, Sandra, Stallman, Richard M., McGrath, Roland, Oram, Andrew, & Drepper, Ulrich. 2015. The GNU C Library Reference Manual. http://www.gnu.org/software/libc/manual/pdf/libc.pdf. [Online. Accessed 24th January 2016].
> 
> Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)
> German Aerospace Center
> Institute of Planetary Research | Planetary Physics | Rutherfordstraße 2 | 12489 Berlin
>  
> Florian Willich| Candidate for Bachelor of Science
> Software Developer (Parallel Applications)
> florian.willlich at dlr.de
> DLR.de
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