.TH MPI_Buffer_detach 3 "11/12/2019" " " "MPI"

.SH NAME

MPI_Buffer_detach \-  Removes an existing buffer (for use in MPI_Bsend etc) 

.SH SYNOPSIS

.nf

int MPI_Buffer_detach(void *buffer_addr, int *size)

.fi

.SH OUTPUT PARAMETERS

.PD 0

.TP

.B buffer_addr 

- initial buffer address (choice)

.PD 1

.PD 0

.TP

.B size 

- buffer size, in bytes (integer)

.PD 1

.SH NOTES

The reason that 

.I MPI_Buffer_detach

returns the address and size of the

buffer being detached is to allow nested libraries to replace and restore

the buffer.  For example, consider

.nf

int size, mysize, idummy;

void *ptr, *myptr, *dummy;

MPI_Buffer_detach(&ptr, &size);

MPI_Buffer_attach(myptr, mysize);

\&.

\&.

\&.

\&.

\&.

\&.

library code ...

\&.

\&.

\&.

MPI_Buffer_detach(&dummy, &idummy);

MPI_Buffer_attach(ptr, size);

.fi

This is much like the action of the Unix signal routine and has the same

strengths (it is simple) and weaknesses (it only works for nested usages).

Note that for this approach to work, MPI_Buffer_detach must return MPI_SUCCESS

even when there is no buffer to detach.  In that case, it returns a size of

zero.  The MPI 1.1 standard for 

.I MPI_BUFFER_DETACH

contains the text

.nf

The statements made in this section describe the behavior of MPI for

buffered-mode sends. When no buffer is currently associated, MPI behaves

as if a zero-sized buffer is associated with the process.

.fi

This could be read as applying only to the various Bsend routines.  This

implementation takes the position that this applies to 

.I MPI_BUFFER_DETACH

as well.

.SH THREAD AND INTERRUPT SAFETY

The user is responsible for ensuring that multiple threads do not try to

update the same MPI object from different threads.  This routine should

not be used from within a signal handler.

The MPI standard defined a thread-safe interface but this does not

mean that all routines may be called without any thread locks.  For

example, two threads must not attempt to change the contents of the

same 

.I MPI_Info

object concurrently.  The user is responsible in this

case for using some mechanism, such as thread locks, to ensure that

only one thread at a time makes use of this routine.

Because the buffer for buffered sends (e.g., 

.I MPI_Bsend

) is shared by all

threads in a process, the user is responsible for ensuring that only

one thread at a time calls this routine or 

.I MPI_Buffer_attach

\&.

.SH NOTES FOR FORTRAN

All MPI routines in Fortran (except for 

.I MPI_WTIME

and 

.I MPI_WTICK

) have

an additional argument 

.I ierr

at the end of the argument list.  

.I ierr

is an integer and has the same meaning as the return value of the routine

in C.  In Fortran, MPI routines are subroutines, and are invoked with the

.I call

statement.

All MPI objects (e.g., 

.I MPI_Datatype

, 

.I MPI_Comm

) are of type 

.I INTEGER

in Fortran.

The Fortran binding for this routine is different.  Because Fortran

does not have pointers, it is impossible to provide a way to use the

output of this routine to exchange buffers.  In this case, only the

size field is set.

.SH NOTES FOR C

Even though the 

.I bufferptr

argument is declared as 

.I void *

, it is

really the address of a void pointer.  See the rationale in the

standard for more details.

.SH SEE ALSO

MPI_Buffer_attach

.br