# blacs_set

Sets values that BLACS use for internal defaults.

## Syntax

call blacs_set( icontxt, what, val )

## Input Parameters

icontxt | INTEGER. For values of what that are tied to a particular context, this parameter is the integer handle indicating the context. Otherwise, ignored.
what | INTEGER. Indicates what BLACS internal(s) should be set. Present values are: 1 = The BLACS message ID range 11 = Number of rings for multiring topology to use 12 = Number of branches for general tree topology to use.
val | INTEGER. Array of dimension (*). Indicates the value(s) the internals should be set to. The specific meanings depend on what values, as discussed in Description below.

## Description

This routine sets the BLACS internal defaults depending on what values:

what = 1

  Setting the BLACS message ID range.

  If you wish to mix the BLACS with other message-passing packages, restrict the BLACS to a certain message ID range not to be used by the non-BLACS routines. The message ID range must be set before the first call to blacs_gridinit or blacs_gridmap. Subsequent calls will have no effect. Because the message ID range is not tied to a particular context, the parameter icontxt is ignored, and val is defined as:

  `VAL (input) INTEGER` array of dimension (2)

  `VAL(1)` : The smallest message ID (also called message type or message tag) the BLACS should use.

  `VAL(2)` : The largest message ID (also called message type or message tag) the BLACS should use.

what = 11

  Set number of rings for `TOP = 'M'` (multiring broadcast).This quantity is tied to a context, so icontxt is used, and val is defined as:

  `VAL (input) INTEGER` array of dimension (1)

  `VAL(1)` : The number of rings for multiring topology to use.

what = 12

  Set number of rings for `TOP = 'T'` (general tree broadcast/general tree gather). This quantity is tied to a context, so icontxt is used, and val is defined as:

  `VAL (input) INTEGER` array of dimension (1)

  `VAL(1)` : The number of branches for general tree topology to use.

Parent topic: Initialization Routines