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

.SH NAME

MPI_Waitsome \-  Waits for some given MPI Requests to complete 

.SH SYNOPSIS

.nf

int MPI_Waitsome(int incount, MPI_Request array_of_requests[],

int *outcount, int array_of_indices[], MPI_Status array_of_statuses[])

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B incount 

- length of array_of_requests (integer)

.PD 1

.PD 0

.TP

.B array_of_requests 

- array of requests (array of handles)

.PD 1

.SH OUTPUT PARAMETERS

.PD 0

.TP

.B outcount 

- number of completed requests (integer)

.PD 1

.PD 0

.TP

.B array_of_indices 

- array of indices of operations that

completed (array of integers)

.PD 1

.PD 0

.TP

.B array_of_statuses 

- array of status objects for

operations that completed (array of Status).  May be 

.I MPI_STATUSES_IGNORE

\&.

.PD 1

.SH NOTES

The array of indicies are in the range 

.I 0

to 

.I incount - 1

for C and

in the range 

.I 1

to 

.I incount

for Fortran.

Null requests are ignored; if all requests are null, then the routine

returns with 

.I outcount

set to 

.I MPI_UNDEFINED

\&.

While it is possible to list a request handle more than once in the

array_of_requests, such an action is considered erroneous and may cause the

program to unexecpectedly terminate or produce incorrect results.

.I MPI_Waitsome

provides an interface much like the Unix 

.I select

or 

.I poll

calls and, in a high qualilty implementation, indicates all of the requests

that have completed when 

.I MPI_Waitsome

is called.

However, 

.I MPI_Waitsome

only guarantees that at least one

request has completed; there is no guarantee that 

.B all

completed requests

will be returned, or that the entries in 

.I array_of_indices

will be in

increasing order. Also, requests that are completed while 

.I MPI_Waitsome

is

executing may or may not be returned, depending on the timing of the

completion of the message.

.SH NOTES ON THE MPI_STATUS ARGUMENT

The 

.I MPI_ERROR

field of the status return is only set if

the return from the MPI routine is 

.I MPI_ERR_IN_STATUS

\&.

That error class

is only returned by the routines that take an array of status arguments

(

.I MPI_Testall

, 

.I MPI_Testsome

, 

.I MPI_Waitall

, and 

.I MPI_Waitsome

).  In

all other cases, the value of the 

.I MPI_ERROR

field in the status is

unchanged.  See section 3.2.5 in the MPI-1.1 specification for the

exact text.

For send operations, the only use of status is for 

.I MPI_Test_cancelled

or

in the case that there is an error in one of the four routines that

may return the error class 

.I MPI_ERR_IN_STATUS

, in which case the

.I MPI_ERROR

field of status will be set.  In that case, the value

will be set to 

.I MPI_SUCCESS

for any send or receive operation that completed

successfully, or 

.I MPI_ERR_PENDING

for any operation which has neither

failed nor completed.

.SH THREAD AND INTERRUPT SAFETY

This routine is thread-safe.  This means that this routine may be

safely used by multiple threads without the need for any user-provided

thread locks.  However, the routine is not interrupt safe.  Typically,

this is due to the use of memory allocation routines such as 

.I malloc

or other non-MPICH runtime routines that are themselves not interrupt-safe.

.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.

.SH ERRORS

All MPI routines (except 

.I MPI_Wtime

and 

.I MPI_Wtick

) return an error value;

C routines as the value of the function and Fortran routines in the last

argument.  Before the value is returned, the current MPI error handler is

called.  By default, this error handler aborts the MPI job.  The error handler

may be changed with 

.I MPI_Comm_set_errhandler

(for communicators),

.I MPI_File_set_errhandler

(for files), and 

.I MPI_Win_set_errhandler

(for

RMA windows).  The MPI-1 routine 

.I MPI_Errhandler_set

may be used but

its use is deprecated.  The predefined error handler

.I MPI_ERRORS_RETURN

may be used to cause error values to be returned.

Note that MPI does 

.B not

guarentee that an MPI program can continue past

an error; however, MPI implementations will attempt to continue whenever

possible.

.PD 0

.TP

.B MPI_SUCCESS 

- No error; MPI routine completed successfully.

.PD 1

.PD 0

.TP

.B MPI_ERR_REQUEST 

- Invalid 

.I MPI_Request

\&.

Either null or, in the case of a

.I MPI_Start

or 

.I MPI_Startall

, not a persistent request.

.PD 1

.PD 0

.TP

.B MPI_ERR_ARG 

- Invalid argument.  Some argument is invalid and is not

identified by a specific error class (e.g., 

.I MPI_ERR_RANK

).

.PD 1

.PD 0

.TP

.B MPI_ERR_IN_STATUS 

- The actual error value is in the 

.I MPI_Status

argument.

This error class is returned only from the multiple-completion routines

(

.I MPI_Testall

, 

.I MPI_Testany

, 

.I MPI_Testsome

, 

.I MPI_Waitall

, 

.I MPI_Waitany

,

and 

.I MPI_Waitsome

).  The field 

.I MPI_ERROR

in the status argument

contains the error value or 

.I MPI_SUCCESS

(no error and complete) or

.I MPI_ERR_PENDING

to indicate that the request has not completed.

.PD 1

The MPI Standard does not specify what the result of the multiple

completion routines is when an error occurs.  For example, in an

.I MPI_WAITALL

, does the routine wait for all requests to either fail or

complete, or does it return immediately (with the MPI definition of

immediately, which means independent of actions of other MPI processes)?

MPICH has chosen to make the return immediate (alternately, local in MPI

terms), and to use the error class 

.I MPI_ERR_PENDING

(introduced in MPI 1.1)

to indicate which requests have not completed.  In most cases, only

one request with an error will be detected in each call to an MPI routine

that tests multiple requests.  The requests that have not been processed

(because an error occured in one of the requests) will have their

.I MPI_ERROR

field marked with 

.I MPI_ERR_PENDING

\&.