blacs_get

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 4/28/2026

Version

Public

blacs_get

Gets values that BLACS use for internal defaults.

Syntax

call blacs_get ( icontxt , what , nprow , npcol )

Input Parameters

icontxt

On values of what that are tied to a particular context, this parameter is the integer handle indicating the context. Otherwise, ignored.

what

Indicates what BLACS internal(s) should be returned in val . Present options are:

what = 0 : Handle indicating default system context.
what = 1 : The BLACS message ID range.
what = 2 : The BLACS debug level the library was compiled with.
what = 10 : Handle indicating the system context used to define the BLACS context whose handle is icontxt.
what = 11 : Number of rings multiring broadcast topology is presently using.
what = 12 : Number of branches general tree broadcast topology is presently using.
what = 13 : Number of rings multiring combine topology is presently using.
what = 14 : Number of branches general tree combine topology is presently using.
what = 15 : Whether topologies are forced to be repeatable or not. A non-zero return value indicates that topologies are being forced to be repeatable. See Repeatability and Coherence for more information about repeatability.
what = 16 : Whether topologies are forced to be heterogenous coherent or not. A non-zero return value indicates that topologies are being forced to be heterogenous coherent. See Repeatability and Coherence for more information about coherence.

Output Parameters

val: The value of the BLACS internal.

Description

This routine gets the values that the BLACS are using for internal defaults. Some values are tied to a BLACS context, and some are more general. The most common use is in retrieving a default system context for input into blacs_gridinit (Assigns available processes into BLACS process grid.) or blacs_gridmap (Maps available processes into BLACS process grid.) .

Some systems, such as MPI*, supply their own version of context. For those users who mix system code with BLACS code, a BLACS context should be formed in reference to a system context. Thus, the grid creation routines take a system context as input. If you wish to have strictly portable code, you may use blacs_get to retrieve a default system context that will include all available processes. This value is not tied to a BLACS context, so the parameter icontxt is unused.

blacs_get returns information on three quantities that are tied to an individual BLACS context, which is passed in as icontxt . The information that may be retrieved is:

The handle of the system context upon which this BLACS context was defined
The number of rings for TOP = 'M' (multiring broadcast/combine)
The number of branches for TOP = 'T' (general tree broadcast/general tree gather).
Whether topologies are being forced to be repeatable or heterogenous coherent.

NOTE:

Examples of BLACS Routines Usage

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Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

blacs_get

Syntax

Input Parameters

Output Parameters

Description