# BLACS Broadcast Routines

This topic describes BLACS broadcast routines.

A broadcast sends data possessed by one process to all processes within a scope. Broadcast, much like point to point communication, has two complementary operations. The process that owns the data to be broadcast issues a broadcast/send. All processes within the same scope must then issue the complementary broadcast/receive.

The BLACS define that both broadcast/send and broadcast/receive are globally-blocking. Broadcasts/receives cannot be locally-blocking since they must post a receive. Note that receives cannot be locally-blocking. When a given process can leave, a broadcast/receive operation is topology dependent, so, to avoid a hang as topology is varied, the broadcast/receive must be treated as if no process can leave until all processes have called the operation.

Broadcast/sends could be defined to be locally-blocking. Since no information is being received, as long as locally-blocking point to point sends are used, the broadcast/send will be locally blocking. However, defining one process within a scope to be locally-blocking while all other processes are globally-blocking adds little to the programmability of the code. On the other hand, leaving the option open to have globally-blocking broadcast/sends may allow for optimization on some platforms.

The fact that broadcasts are defined as globally-blocking has several important implications. The first is that scoped operations (broadcasts or combines) must be strictly ordered, that is, all processes within a scope must agree on the order of calls to separate scoped operations. This constraint falls in line with that already in place for the computation of message IDs, and is present in point to point communication as well.

A less obvious result is that scoped operations with `SCOPE = 'ALL'` must be ordered with respect to any other scoped operation. This means that if there are two broadcasts to be done, one along a column, and one involving the entire process grid, all processes within the process column issuing the column broadcast must agree on which broadcast will be performed first.

The convention used in the communication routine names follows the template ?xxyy2d, where the letter in the ? position indicates the data type being sent, xx is replaced to indicate the shape of the matrix, and the yy positions are used to indicate the type of communication to perform:

i

  integer

s

  single precision real

d

  double precision real

c

  single precision complex

z

  double precision complex

ge

  The data to be communicated is stored in a general rectangular matrix.

tr

  The data to be communicated is stored in a trapezoidal matrix.

bs

  Broadcast/send. A process begins the broadcast of data within a scope.

br

  Broadcast/receive A process receives and participates in the broadcast of data within a scope.

BLACS Broadcast Routines
Routine name | Operation performed
--- | ---
?gebs2d ?trbs2d | Start a broadcast along a scope.
?gebr2d ?trbr2d | Receive and participate in a broadcast along a scope.